It is important
to understand just what is meant by "N balance" or "Nitrogen
balance" while reading these abstracts and the nutritional literature,
as this term is misleading, perhaps intentionally so, as are other misuses
of the word "balance" in the pseudoscience
of orthodox nutrition.
Input - Output = Accumulation
In "N balance" studies, the nitrogen 'input' (which exists only in proteins, not in carbohydrates or fats) consumed in "foods", is measured, as is the 'output' (which is the nitrogen lost in feces, urine, sweat, hair and nail growth, ...) and the net accumulation calculated. If this net accumulation is positive (input > output) this condition is referred to as "positive N Balance"; and conversely, if the net accumulation is negative (input < output) this condition is referred to as "negative N balance".
So, in the growing child, (and the severely protein-malnourished patient who is recovering) new tissue is being created daily, thus the N balance should be positive, as this means that net new protein is being accumulated.
However, in the adult, no longer growing, the N balance should be zero, as input and output should be equal under weight-static, no-growth conditions. Positive N balance in the adult means that excessive protein is being eaten and proteinaceous wastes are being stored in the body, thus the body is gaining excess weight; a condition of pandemic obesity exists among cultural-diet eaters. Thus, a positive N balance is not a healthy state for the adult; however, you will see that most abstracts suggest that a positive N balance is desirable.
Further, N balance should be negative if the body is detoxing from the excessive protein consumption, common in cultural diets, including most vegetarian/vegan ones, since nitrogenous wastes are being excreted.
Therefore, a positive N balance does not mean 'positive' in the sense of being "good", and a negative N balance does not mean 'negative' in the sense of being "bad". Nor does a zero N balance mean 'bad', or impoverished, in the sense of having nothing.
The major problem caused by the N balance method of estimating protein needs is manifest in short-term dietary studies that try to determine human protein needs by restricting or eliminating dietary proteins, measuring "losses", and regressing the results to zero N balance, without the researchers being familiar enough with the dynamics of dietary changes sufficient to understand that a reduction in protein consumed relative to a 'normal' excessive intake will produce cleansing reactions, the excretion of stored proteinaceous wastes, and that this increased excretion is not indicative of protein needs, but rather of bodily toxicity. For these studies to be meaningful, the low-protein diet would have to be maintained for several weeks, or even months, to allow the body to excrete all excessive protein wastes and come to relative equilibrium, and only then present true and constant "obligatory nitrogen losses".
As a logical and unavoidable result of this fundamental error and ignorance of the dynamics of human diet, protein "needs" and the resulting RDA's are highly overestimated.
Nutr Rev. 2003 Sep;61(9):295-305. Related Articles, Links
Nutritional and physiologic significance of alpha-lactalbumin in infants.
Lonnerdal B, Lien EL. Department of Nutrition, University of California, Davis, CA 95616, USA.
alpha-Lactalbumin is the major protein in breast milk (20-25% of total protein) and has been described to have several physiologic functions in the neonatal period. In the mammary gland, it participates in lactose synthesis, thereby creating an osmotic "drag" to facilitate milk production and secretion. alpha-Lactalbumin binds divalent cations (Ca, Zn) and may facilitate the absorption of essential minerals, and it provides a well-balanced supply of essential amino acids to the growing infant. During its digestion, peptides appear to be transiently formed that have antibacterial and immunostimulatory properties, thereby possibly aiding in the protection against infection. A novel folding variant ("molten globule state") of multimeric alpha-lactalbumin has recently been discovered that has anti-infective activity and enhances apoptosis, thus possibly affecting mucosal cell turnover and proliferation. Cow milk also contains alpha-lactalbumin, albeit less than human milk (2-5% of total protein in bovine milk), and protein fractions enriched with alpha-lactalbumin may now be added to infant formula to provide some of the benefits of human alpha-lactalbumin.
Am J Clin Nutr 2003 Jun;77(6):1537S-1543S
Nutritional and physiologic significance of human milk proteins.
Lonnerdal B Department of Nutrition, University of California, Davis, 95616, USA. firstname.lastname@example.org
Human milk contains a wide variety of proteins that contribute to its unique qualities. Many of these proteins are digested and provide a well-balanced source of amino acids to rapidly growing infants. Some proteins, such as bile salt-stimulated lipase, amylase, beta-casein, lactoferrin, haptocorrin, and alpha1-antitrypsin, assist in the digestion and utilization of micronutrients and macronutrients from the milk. Several proteins with antimicrobial activity, such as immunoglobulins, kappa-casein, lysozyme, lactoferrin, haptocorrin, alpha-lactalbumin, and lactoperoxidase, are relatively resistant against proteolysis in the gastrointestinal tract and may, in intact or partially digested form, contribute to the defense of breastfed infants against pathogenic bacteria and viruses. Prebiotic activity, such as the promotion of the growth of beneficial bacteria such as Lactobacilli and Bifidobacteria, may also be provided by human milk proteins. This type of activity can limit the growth of several pathogens by decreasing intestinal pH. Some proteins and peptides have immunomodulatory activities (eg, cytokines and lactoferrin), whereas others (eg, insulin-like growth factor, epidermal growth factor, and lactoferrin) are likely to be involved in the development of the intestinal mucosa and other organs of newborns. In combination, breast-milk proteins assist in providing adequate nutrition to breastfed infants while simultaneously aiding in the defense against infection and facilitating optimal development of important physiologic functions in newborns. [Note: now here's an amazing breakthrough - human milk is best for human infants - ljf.]
J Nutr 2003 Mar;133(3):862S-5S
Dietary animal and plant protein and human bone health: a whole foods approach.
Massey LK. Food Science and Human Nutrition, Washington State University Spokane, Spokane, WA 99210.
Urinary calcium excretion is strongly related to net renal acid excretion. The catabolism of dietary protein generates ammonium ion and sulfates from sulfur-containing amino acids. Bone citrate and carbonate are mobilized to neutralize these acids, so urinary calcium increases when dietary protein increases. Common plant proteins such as soy, corn, wheat and rice have similar total S per g of protein as eggs, milk and muscle from meat, poultry and fish. Therefore increasing intake of purified proteins from either animal or plant sources similarly increases urinary calcium. The effects of a protein on urinary calcium and bone metabolism are modified by other nutrients found in that protein food source. For example, the high amount of calcium in milk compensates for urinary calcium losses generated by milk protein. Similarly, the high potassium levels of plant protein foods, such as legumes and grains, will decrease urinary calcium. The hypocalciuric effect of the high phosphate associated with the amino acids of meat at least partially offsets the hypercalciuric effect of the protein. Other food and dietary constituents such as vitamin D, isoflavones in soy, caffeine and added salt also have effects on bone health. Many of these other components are considered in the potential renal acid load of a food or diet, which predicts its effect on urinary acid and thus calcium. "Excess" dietary protein from either animal or plant proteins may be detrimental to bone health, but its effect will be modified by other nutrients in the food and total diet.
Am J Clin Nutr. 2002 Dec;76(6):1308-16.
Comment in: Am J Clin Nutr. 2003 Oct;78(4):802-3; author reply 803-4.
Estimation of the net acid load of the diet of ancestral preagricultural Homo sapiens and their hominid ancestors.
Sebastian A, Frassetto LA, Sellmeyer DE, Merriam RL, Morris RC Jr.
Department of Medicine and the General Clinical Research Center, University of California, San Francisco, California 94143, USA. email@example.com
selection has had < 1% of hominid evolutionary time to eliminate the inevitable
maladaptations consequent to the profound transformation of the human
diet resulting from the inventions of agriculture and animal husbandry.
J Nutr 2002 Oct;132(10):3225S-7S
Latency, duration and dose response relationships of amino acid effects on human muscle protein synthesis.
Rennie MJ, Bohe J, Wolfe RR Division of Molecular Physiology, School of Life Sciences, University of Dundee, Scotland, United Kingdom. firstname.lastname@example.org
The components of the stimulatory effect of food on net deposition of protein are beginning to be identified and separated. One of the most important of these appears to be the effect of amino acids per se in stimulating muscle anabolism. Amino acids appear to have a linear stimulatory effect within the range of normal diurnal plasma concentrations from postabsorptive to postprandial. Within this range, muscle protein synthesis (measured by incorporation of stable isotope tracers of amino acids into biopsied muscle protein) appears to be stimulated approximately twofold; however, little further increase occurs when very high concentrations of amino acids (>2.5 times the normal postabsorptive plasma concentration) are made available. Amino acids provided in surfeit of the ability of the system to synthesize protein are disposed of by oxidation, ureagenesis and gluconeogenesis. The stimulatory effect of amino acids appears to be time dependent; a square wave increase in the availability of amino acids causes muscle protein synthesis to be stimulated and to fall back to basal values, despite continued amino acid availability. The relationship between muscle protein synthesis and insulin availability suggests that most of the stimulatory effects occur at low insulin concentrations, with large increases having no effect. These findings may have implications for our understanding of the body's requirements for protein. The maximal capacity for storage of amino acids as muscle protein probably sets an upper value on the extent to which amino acids can be stored after a single meal. [Note: this reveals the false belief in body builders that enormous amounts of protein produce more muscle mass.]
A gut feeling
New Scientist vol 159 issue 2146 - 08 August 1998, page 26
If you thought junk food would merely make you fat think again. You could also be fattening a hungry mass of alien gut bacteria that may repay you with bowel disease or even cancer. Gail Vines reports
WHAT'S the difference between the contents of your bowels and the noxious black sludge at the bottom of an estuary? Not a lot perhaps—particularly if you live on a diet of junk food. The same sulphur loving bacteria that give mud in estuaries and ocean sediments their pungent, rotten-egg smell may have invaded your gut. In the sea, they are notorious troublemakers with a penchant for corroding oil pipelines, and their effect on human passageways may be equally devastating.
All these microbes need to flourish in your guts is a good supply of sulphurous compounds. And that's where your diet comes in. Eat large amounts of animal protein and processed food and you could be giving these bad bugs everything they need to triumph at the expense of your natural healthy gut microbes. Over the past decade, the researchers doing this work have steadily accumulated evidence to implicate sulphur bacteria in a range of human diseases from inflammatory bowel diseases to colon cancer.
"It's a potential bombshell," says John Cummings, head of the "gut group"—the team pioneering this type of work at the Dunn Nutrition Unit at Addenbrooke's Hospital in Cambridge. Sulphur-based preservatives are in most processed foods, from instant potato to jams and dried fruit, as well as in most wines, beers and ciders. Sulphur compounds are one of the world's oldest food additives, used by the ancient Greeks and Egyptians to preserve wine, and widely regarded as both versatile and safe. So if these foods do encourage the growth of alien microbes that are linked to disease, the food and drinks industry could face a crisis to rival salmonella or BSE.
The story involves a series of coincidences and begins 500 kilometres north of Cambridge, in the port of Dundee on the east coast of Scotland. There, in the mid-1980s, two PhD students from Dundee University, Glenn Gibson and George Macfarlane, were studying the ecology of the Tay estuary. "As it happens, the results weren't particularly exciting," chuckles Gibson. But the young researchers did learn a lot about sulphur bacteria—knowledge that was to prove useful in a very different quarter.
These mud-loving organisms, officially known as sulphate-reducing bacteria, find plenty to feast on in the oxygen-free (anaerobic) sea sediments. That's because they can exploit both the hydrogen that comes from the fermentation of countless microbes in the stagnant mud, and the plentiful sulphate in seawater. The bugs make their own energy from these raw ingredients, converting sulphate to sulphite and then creating a poisonous waste product: hydrogen sulphide, with its telltale smell of rotten eggs. To humans, the compound is as toxic as cyanide. In water, it rapidly becomes highly corrosive sulphuric acid.
In the late 1980s the oil industry was well aware how caustic by-products of the sulphur-loving organisms could wreck their pipework, but no one yet imagined that they could also be causing trouble in the human gut. This idea was to emerge from multidisciplinary teamwork as Macfarlane and Gibson moved south to Cambridge, to join Cummings and his gut group. The timing was good for the two young microbiologists. "Researchers were discovering just how important the gut bacteria are in health and disease," says Cummings. His own team's decision to pursue this line of enquiry was to lead them eventually to finger the sulphur lovers as the agents of disease.
Something in the wind
The quest began in an unlikely place—with the gases made by gut bacteria that people give off when they belch or fart. The team devised an elegant technique to provide the first accurate measurements of the composition of intestinal gas in healthy people. For 36 hours, volunteers lived in a small airtight room, while researchers controlled the flow of air through it. By measuring the difference in the concentration of gases in the air entering and leaving the room, the investigators could determine which gases were coming from volunteers.
The results, published in 1992, were a surprise. Everyone knew that gut bacteria churn out a variable mix of odourless, mainly harmless gases— hydrogen, nitrogen, carbon dioxide and methane. But the team was surprised by how little hydrogen they found in the air leaving the room—given the chemical composition of the foods the volunteers had eaten. Something in the gut was gobbling up much of the available hydrogen. Another finding was puzzling too: some people produced substantial amounts of methane, while others produced much less, or none at all.
The methane could have come from only one source: methane-producing bacteria, otherwise known as methanogens. These bacteria consume hydrogen, which would explain the low levels of this gas given off by people harbouring methanogens. But breath tests designed to detect methane suggest that only about half of the people living in North America and Northern Europe have methanogens living in their gut. Why do some people have them, while others do not? And what is soaking up the hydrogen if methanogens aren't?
Sulphate-reducing bacteria, first reported in the human gut in the late 1970s, looked like good contenders. Gibson and Macfarlane, recalling their experiences in the Tay estuary, quickly realised that this was not such a preposterous idea. After all, microbial fermentation in the final part of the gut, the distal colon, provides anaerobic conditions on a par with those in marine muds. And sulphate-reducing bacteria predominate in marine sediments where they use up hydrogen as well as sulphurous compounds. What's more, on the seabed these microbes get the better of methane-generating bacteria if sulphate is present.
High levels of sulphur are also present in the typical Western diet. Could sulphate-reducing bacteria be displacing methanogens inside the guts of people who eat large quantities of meat, packed with sulphur-rich amino acids, and processed foods and fermented drinks preserved with the ubiquitous sulphur-based food additives?
To test this idea the team asked volunteers who normally produce methane in their breath to eat a diet rich in sulphate. Ten days on, the breath of half of their subjects no longer showed significant traces of methane. By day 15, sulphide levels in their faeces had shot up. When they stopped eating the added sulphate methanogens returned while the sulphate-reducing bacteria went into sharp decline. In another study, the Dunn team found that rural South Africans, eating a diet low in sulphur, were virtually all methane-producers.
Intrigued, the Dunn researchers next began to wonder if these gut microbes affected human health. They compared the levels of sulphate-reducing bacteria in the faeces of healthy people and in patients suffering from ulcerative colitis, a serious inflammatory bowel disease that afflicts up to one in a thousand people in Britain and the US. Work done in the US during the 1970s showed that "germ-free" lab animals lacking any gut bacteria do not develop colitis-like symptoms, even when exposed to irritants such as sulphated seaweed. Bacteria in general had been implicated in the disease, but could the sulphate reducers be major players?
The team's results were striking. Virtually everyone with colitis—96 per cent of the sufferers tested—played host to the sulphate lovers, but only 50 per cent of the healthy people did. In particular, the gut of someone with colitis was home to large numbers of sulphate-reducing bacteria from the genus Desulfovibrio. "There turned out to be more subtypes of these bacteria in the human gut than we had expected, with some more active or virulent than others," says Cummings. One strain isolated from the colons of people with colitis showed signs of being adapted to life in an inflamed gut, Gibson found. Growing in a continuous culture "gut model" fermenter in the laboratory, the strain can survive high flushing rates that simulate diarrhoea in the colon.
Nevertheless, not everyone harbouring the sulphate-reducing bacteria was ill. And some ill people did not have the bacteria. So, what exactly is their link with colitis? Do they cause it, exacerbate it, or simply take up residence in a diseased colon because they can? Macfarlane points out that pinpointing an individual cause of ulcerative colitis is virtually impossible because it is a chronic inflammatory condition intimately involved with the body's immune response. "It may be that sulphate-reducing bacteria contribute to the maintenance of the disease rather than kick it off," he cautions. "It is difficult to tie gut disease to a particular organism," adds Gibson. The gut is home to at least 400 species of microbes, many of which are difficult or impossible to grow in lab cultures—and the vast majority of which are harmless.
Gibson, who is now at the Institute of Food Research in Reading, is investigating why some sulphate-reducing bacteria are linked to bowel disease but others are not. By studying mutant strains genetically engineered not to make hydrogen sulphide, he hopes to find out whether it is the bacterial invasion of gut cells alone that causes damage, or whether the sulphide by-products are to blame, or indeed both. Gibson hopes this will reveal how sulphate-reducing bacteria can cause colitis.
Meanwhile, an Australian abdominal surgeon has already found one way in which sulphide might damage the gut. In the 1980s Bill Roediger, at the Queen Elizabeth Hospital in Woodville, near Adelaide, first noticed that, in people with ulcerative colitis, the epithelial cells that line their colons don't function normally. These cells lack the ability to oxidise a vital fatty acid called butyrate, which is normally their main nutrient. This metabolic abnormality could be the first step in the development of the disease: it seems to precede the start of obvious colitic changes in the colon. Significantly, in 1993, he showed that exposure to sulphides selectively inhibits the ability of colon cells to use butyrate.
More work is needed to understand the link between diet, bacteria and disease, says Macfarlane. Such research could tell us how to encourage beneficial bacteria and freeze out the harmful ones. One day there might even be a vaccine against harmful gut organisms. But at the moment, the most hopeful strategy is to encourage a process of "natural displacement" through changing what we eat.
Meat and other foods high in protein release sulphur-amino acids as they are digested. Cummings's team believes these feed bacteria in the same way that other sulphur compounds do. A preliminary study at the Dunn shows that as meat consumption rises from 60 to 600 grams per day sulphates in the urine double, and sulphides in faeces increase tenfold. A diet rich in meat has long been implicated in colon cancer, and Cummings suspects that the toxic sulphides released by these microbes might promote cancerous changes in gut cells by damaging their DNA.
But what about vegetarians? Are they off the hook? Vegetable protein—notably in beans and seeds—also contains amino acids with sulphur groups attached, so why are vegetarians at lower risk of colon cancer? The crucial difference could be in the balance of nutrients. In plant foods, protein comes in carbohydrate-rich packages. Cummings suspects that this combination could make the sulphur-amino acids harmless. Carbohydrate fuels the growth of beneficial bacteria which snap up the sulphur amino-acids to incorporate into their own proteins. The end result isn't harmful sulphide, but lots of beneficial "biomass"—bacterial bulk that helps to speed the passage of faeces through the gut. It is possible, he says, that carnivores who eat lots of plant foods and carbohydrates along with their meat could be protected too.
The second major source of sulphur in our diet is a large family of sulphur additives in foods and drinks: sulphur dioxide, sulphites, bisulphites, metabisulphites and sulphates, known in Europe by E number codes E220 to E227, but often collectively called "sulphur dioxide". These sulphur compounds are the major preservative in the Western diet. "They are in hundreds and hundreds of foods," says Cummings, everything from sausages and burgers to jam, dried raisins and instant soup. Even fresh foods may not be sulphur-free—packaged salads are "gassed" with sulphur dioxide to prolong their shelf life. Soft drinks, wines, beers and ciders can contain widely varying levels, which do not have to be listed on the label. It is detoxified by enzymes in the liver and kidneys which makes sulphur dioxide "a very safe additive—about the safest thing we've got that does that job", says Bronik Wedzicha, professor of food science at the University of Leeds. Nonetheless, a shadow of doubt has already been cast on this venerable preservative. Especially lavish use—in American salad bars, for instance—has now been curtailed, after allergic reactions particularly in people with asthma.
Although sulphur additives are in such a huge variety of foods, no one has yet systematically monitored the amount ingested with an average Western diet. In Britain, the Ministry of Agriculture Fisheries and Food recognises an acceptable daily intake for sulphur-based preservatives. But if you eat large amounts of processed food, washed down with beer or wine, your daily consumption could be well above this level. So, with funding from MAFF, Cummings and his colleagues are assessing how much sulphur people typically consume, by measuring their dietary intake and monitoring the amount of sulphate excreted in urine. "The aim is to discover how much sulphur we are getting from protein and how much from sulphur additives," says Cummings.
Although the evidence is not yet in, Cummings suspects that other inflammatory bowel diseases, such as Crohn's disease, as well as the ill-defined irritable bowel syndrome, could also be linked to sulphate-reducing bacteria. If a link between Desulfovibrio bacteria, gut disease and a dietary source of sulphur can be tied down and the mechanism identified, it will mark a major turning point in the way we think about human health. As bacterial warfare is waged in the human gut, our health may yet depend on feeding an army of friendly microbes and starving the foe into submission.
J Nutr 2002 Jun;132(6):1748S-50S
Processing of dietary casein decreases bioavailability of lysine in growing kittens.
Larsen JA, Calvert CC, Rogers QR Department of Molecular Biosciences, School of Veterinary Medicine and. Department of Animal Science, University of California, Davis, CA.
In recent years there has been renewed interest in nutrient bioavailability, and it is an important factor to consider when estimating nutritional requirements. The bioavailability of a nutrient can be adversely affected by many factors, including interaction with other nutrients and processing[cooking - ljf] or storage conditions (1,2). Lysine is an essential amino acid that can be particularly sensitive to the conditions of moist-heat processing (3). These conditions favor the formation of Maillard reaction products, and lysine is commonly involved in these reactions. The epsilon amino group of lysine reacts with the carbonyl groups of reducing sugars and forms a product complex that cannot be digested or absorbed in a form usable by the animal (4). Because of its specific susceptibility to this type of processing damage, lysine shows a decreased bioavailability relative to that of other amino acids (5). Several methods have been developed to estimate amino acid bioavailability in various proteins (6). However, bioavailability values for many nutrients are not known, even for common foodstuffs fed to economically important food animals. Information on the bioavailability of nutrients for cats and dogs is particularly lacking. [also for humans - ljf] Also, the most commonly accepted in vivo methods of determining bioavailability of nutrients have not been validated for many species, including the cat [and human - ljf] (7). The objectives of this study were to examine the relationship between lysine bioavailability and growth response in kittens fed heat-damaged casein and to validate the growth assay method for quantifying amino acid bioavailability in the kitten.
Am J Clin Nutr 2002 Mar;75(3):511-8
Endogenous glycine and tyrosine production is maintained in adults consuming a marginal-protein diet.
Gibson NR, Jahoor F, Ware L, Jackson AA. Institute of Human Nutrition, the University of Southampton, the Clinical Nutrition and Metabolism Unit, Southampton General Hospital, Southampton, United Kingdom.
The adequacy of indispensable amino acid supplies has received much attention
in studies of protein requirements, but the availability of nitrogen for
synthesis and maintenance of the supply of dispensable amino acids has
J Nutr 2002 Feb;132(2):142-4
Prenatal high protein exposure decreases energy expenditure and increases adiposity in young rats.
Daenzer M, Ortmann S, Klaus S, Metges CC. Department Biochemistry and Physiology of Nutrition, German Institute of Human Nutrition, 14558 Bergholz-Rehbrucke, Germany.
Epidemiologic results suggest that protein intake in infancy and later adiposity might be related. We examined whether high dietary protein exposure in utero and/or during postnatal life affects body fatness. Two groups of female rats were mated and pair-fed isocaloric high (40% protein; HP) or adequate protein (20% protein; AP) diets throughout pregnancy. The male offspring were suckled (3 wk) by foster mothers pair-fed HP or AP diets, resulting in 4 pre-/postnatal groups (AP-AP, AP-HP, HP-AP, HP-HP). Subsequently, they were pair-fed the same diets their nurses received during lactation until wk 9. Offspring of HP dams had a lower body weight on d 2 of life than their AP counterparts (7.6 +/- 0.7 vs. 8.3 +/- 0.8 g; P < 0.001). HP-AP rats had a higher body weight than AP-AP controls at wk 3, 5, and 6 (P < 0.05), in contrast to HP-HP which did not differ from controls. Prenatal HP exposure resulted in a greater total and relative fat mass and decreased total energy expenditure at wk 9 (P < 0.05). Postnatal HP alone had no significant effect on body composition or metabolic rate. These results indicate that in utero exposure to a high protein level reprograms body weight and energy homeostasis.
Proc. Natl. Acad. Sci. USA, Vol. 99, Issue 3, 1314-1318, February 5, 2002
Type I collagen is thermally unstable at body temperature
E. Leikina, M. V. Mertts, N. Kuznetsova, and S. Leikin
National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD 20892
by ultra-slow scanning calorimetry and isothermal circular dichroism,
human lung collagen monomers denature at 37°C [98.6F] within a couple
of days. Their unfolding rate decreases exponentially at lower temperature,
but complete unfolding is observed even below 36°C [96.8F]. Refolding
of full-length, native collagen triple helices does occur, but only below
30°C [86F]. Thus, contrary to the widely held belief, the energetically
preferred conformation of the main protein of bone and skin in physiological
solution is a random coil rather than a triple helix. These observations
suggest that once secreted from cells collagen helices would begin to
unfold. We argue that initial microunfolding of their least stable domains
would trigger self-assembly of fibers where the helices are protected
from complete unfolding. Our data support an earlier hypothesis that in
fibers collagen helices may melt and refold locally when needed, giving
fibers their strength and elasticity. Apparently, Nature adjusts collagen
hydroxyproline content to ensure that the melting temperature of triple
helical monomers is several degrees below rather than above body temperature.
Metabolism 1986 Jan;35(1):37-44
Nitrogen conservation in starvation revisited: protein sparing with intravenous fructose.
Gelfand RA, Sherwin RS.
The provision of small amounts of glucose during fasting is known to spare body protein and to attenuate markedly the metabolic response to starvation. These actions, which are not shared by fat, are generally thought to depend on the ability of exogenous glucose to stimulate insulin secretion. To determine whether fructose, a very weak insulin secretagogue, will also conserve nitrogen and alter the response to fasting, we infused small amounts of fructose, 100 g/d (375 kcal), into 7 obese subjects during a 10-day fast: 4 received fructose days 7 to 10, and 3 received fructose days 1 to 7. Fructose virtually abolished (all P less than 0.05-0.01) the fasting induced: (a) fall in glucose and insulin and rise in glucagon, (b) fall in triiodothyronine, (c) ketosis and acidosis, (d) increased ammonia excretion, (e) hyperuricemia (and hypouricosuria), and (f) fall in plasma alanine and rise in branched chain amino acids. Fructose also significantly reduced urinary sodium loss. Moreover, fructose exerted a prominent protein-sparing action, even though plasma insulin concentrations never exceeded postabsorptive levels. Excretion of total nitrogen was reduced by 40% to 50% during periods of fructose infusion, reflecting significant suppression of both urea and ammonia generation (all P less than 0.05-0.01). Most plasma glucogenic amino acids rose significantly during fructose administration. We conclude that low-dose fructose infusion essentially abolishes the entire hormone-substrate response to fasting, and spares body protein without raising insulin above postabsorptive levels.(ABSTRACT TRUNCATED AT 250 WORDS)
J Am Diet Assoc 1990 Jul;90(7):962-7
Metabolic profiles, diet, and health practices of championship male and female bodybuilders.
Kleiner SM, Bazzarre TL, Litchford MD. Sarah W. Stedman Center for Nutritional Studies, Department of Medicine, Duke University Medical Center, Durham, North Carolina 27710.
To obtain a more complete view of their general health and health care habits, 27 bodybuilders (19 men and 8 women) competing at the 1988 National Physique Committee's Junior USA Bodybuilding Championships participated in this study. Data pertaining to demographics and pre-competition nutrition, training, health, and drug abuse practices were collected by self-administered and interview surveys and records. Anthropometric and blood pressure measurements and casual blood samples were collected on -site at the competition registration. Multi-drug abuse was reported by 15% to 40% of the subjects, and 20% to 40% of subjects left the drug use questions unanswered. Severe fluid restrictions and dehydrating practices were reported by all subjects. Eleven men and two women agreed to have blood drawn. Plasma glucose values were at the low end of the normal fasting range. Hemoglobin levels were elevated, indicating hypohydration; magnesium levels were slightly low. Percent body fat, estimated by seven-site skinfold measures, was low for both sexes (men, 6.0 +/- 1.8; women, 9.8 +/- 1.5); 75% of the women reported normal menstrual cycles. The men reported high-protein, low-fat hypocaloric diet patterns. Women had a moderate zinc intake and a remarkably deficient calcium intake despite an adequate energy intake. This research demonstrates that bodybuilders partake in a multitude of practices that may place them in high-risk health categories. It is essential that health care workers in contact with bodybuilding athletes intervene and educate them about healthy dietary and training alternatives.
Br J Nutr 1993 Sep;70(2):439-48
Protein requirement of young adult Nigerian females on habitual Nigerian diet at the usual level of energy intake.
Egun GN, Atinmo T. Department of Human Nutrition, College of Medicine, University of Ibadan, Nigeria.
A short-term N balance study was conducted in twelve healthy female adults aged 21-32 years to determine their protein requirement. Four dietary protein levels (0.3, 0.4, 0.5 and 0.6 g protein/kg per d) were used. Energy intake of the subjects was kept constant at 0.18 MJ/kg per d. All subjects maintained their normal activity throughout the study period. N excretion was determined from the measurements of N in a total collection of urine, faeces, sweat and menstrual fluid for each dietary period. N balance during the four protein levels were -15.15 (SD 5.95), -5.53 (SD 6.71), +6.15 (SD 4.76) and +12.05 (SD 8.63) mg N/kg per d for 0.3, 0.4, 0.5 and 0.6 g protein/kg per d respectively. The calculated average N requirements from regression analysis was 76.0 (SD 3.37) mg N/kg per d (0.48 g protein/kg per d). The estimate of allowance for individual variation to cover the 97.5% population was 95 mg N/kg per d (0.6 g protein/kg per d). The net protein utilization (NPU) of the diet was 0.55. When compared with a similar study with men, there was a significant difference in the protein requirement between sexes. Thus, the unjustifiable sex difference in the protein allowance recommended by the Food and Agriculture Organization/World Health Organization/United Nations University (1985) Expert Consultation group must be reviewed.
Clin Sci (Lond) 1992 Feb;82(2):191-8
Adaptation to a diet low in protein: effect of complex carbohydrate upon urea kinetics in normal man.
Langran M, Moran BJ, Murphy JL, Jackson AA. Department of Human Nutrition, University of Southampton, U.K.
Urea kinetics were measured by using prime/intermittent oral doses of
[15N15N]urea in five healthy men taking formula diets adequate in energy
and containing either 70 or 35 g of protein/day. In some studies the low-protein
diet was supplemented with non-starch polysaccharides in the form of ispaghula
husk or ripe bananas.
Am J Physiol 1995 Jun;268(6 Pt 1):E1143-53
Effects of resistance training and dietary protein intake on protein metabolism in older adults.
Campbell WW, Crim MC, Young VR, Joseph LJ, Evans WJ. Human Physiology Laboratory, US Department of Agriculture Jean Mayer Human Nutrition Research Center on Aging at Tufts University, Boston 02111, USA.
Nitrogen (N) balance, fed-state leucine kinetics, and urinary 3-methylhistidine (3-MeH) excretion were examined in 12 men and women, aged 56-80 yr, before and during 12 wk of resistance training (RT). Subjects were randomized to groups that consumed diets providing either 0.80 +/- 0.02 g protein.kg-1.day-1 (lower protein, LP) or 1.62 +/- 0.02 g protein.kg-1.day-1 (higher protein, HP). At baseline, mean N balance was negative for LP (-4.6 +/- 3.4 mg N.kg-1.day-1) and positive for HP (13.6 +/- 1.0 mg N.kg-1.day-1). N retention increased similarly in LP and HP at the 11th wk of RT by 12.8 and 12.7 mg N.kg-1.day-1, respectively. Thus LP had an increased efficiency of N retention. LP had decreased leucine flux (P < 0.001), oxidation (P < 0.001), and uptake for protein synthesis (P < 0.02), relative to HP, both at baseline and after RT. Leucine flux increased with RT in both diet groups (P < 0.05) and was associated mainly with an increase in protein synthesis in LP (91% of change in flux) and an increase in oxidation in HP (72% of change in flux; RT-diet interaction, P < 0.05). RT increased actomyosin protein breakdown (increased 3-MeH-to-creatinine ratio, P < 0.01). Diet-related differences in protein metabolism did not influence body composition changes with RT. These data show that the efficiency of N retention and protein utilization during RT is higher in older subjects who consume 0.8 vs. 1.6 g protein.kg-1.day-1 dietary protein.
A general introduction to establishment thinking on Dietary Protein and Nitrogen Utilization is given.
The major flaws are:
"Human nitrogen requirements
are usually determined from the nitrogen balance. The usual procedure
is to regress nitrogen balance on intake and to define the requirement
as the intake level that would produce a zero balance, i.e., equality
of dietary N intake and N losses" "Nitrogen losses
occur in different ways. They mainly arise from urinary losses in the
form of urea, ammonia and creatinine but also in the form of fecal and
miscellaneous losses. "Minimum nitrogen losses ["obligatory nitrogen
losses" (ONL)] were measured in subjects fed a protein-free diet for 1
"Because dietary protein
utilization does not achieve 100% efficiency, it has been suggested that
an intake of 0.6 g/kg/d of well-balanced protein will achieve a zero
nitrogen balance. The adequacy of this diet has been reported in studies
conducted over 2- or 3-mo periods (FAO/WHO 1985 ). A safety coefficient
is added to this figure so that the final recommendation for dietary protein
is 0.75 g/kg/d."
Regarding the "nitrogen balance" method: "First, there is a slight difference between large values for N intake and N losses." Thus, this technique both is biased toward excessively large protein intakes, and high protein diets produce large losses, thus high inputs are wasteful and stressful on the body.
Important is the statement "the effects of high protein diets are still poorly understood." As is "The ability of high protein diets to increase nitrogen retention and the protein turnover rate remains unclear."
Metabolism 1998 Sep;47(9):1145-51
Dietary protein restriction alters glucose but not protein metabolism in non-insulin-dependent diabetes mellitus.
Hoffer LJ, Taveroff A, Hamadeh MJ. McGill Nutrition and Food Science Centre and the School of Dietetics and Human Nutrition, McGill University, Montreal, Quebec, Canada.
We determined whether a customary diet high or low in protein (1) influences postabsorptive amino acid catabolism, nitrogen (N) balance, and hepatic glucose output (HGO) in normal subjects or patients with non-insulin-dependent diabetes mellitus (NIDDM) or (2) alters blood glucose levels in NIDDM. Eight normal young adults and five obese middle-aged persons with NIDDM consumed low-protein (0.8 g/kg lean body mass [LBM]) or high-protein (3.0 g/kg LBM) diets at maintenance energy for consecutive 7-day periods. Fasting and average blood glucose and N balance were measured daily. The level of dietary protein had no effect on the basal plasma leucine rate of appearance (Ra) or urinary 3-methylhistidine excretion in either subject group. Basal leucine oxidation (and by inference, whole-body amino acid catabolism) was reduced on the low-protein diet but basal HGO was not, and although exogenous glucose effectively suppressed HGO, it did not reduce leucine oxidation with either diet. After adaptation to the low-protein diet, N balance in both the normal and NIDDM subjects was close to zero. The low-protein diet reduced the fasting and daily blood glucose of the diabetic subjects by approximately 2 mmol/L (P < .05). We conclude that physiologic variation in dietary protein does not affect basal whole-body protein turnover or HGO in either normal young adults or obese middle-aged NIDDM subjects. However, protein restriction to the level of the average daily requirement significantly reduces postabsorptive and average daily blood glucose concentrations in persons with NIDDM.
Arq Gastroenterol 1989 Jul-Sep;26(3):50-4
Bio-utilization of protein, calcium and zinc from diets based on rice and beans, in human beings.
de Angelis RC, Orozco GA, Campos JV. Physiology and Biophysics Department, Sao Paulo University, Brazil.
Nine male healthy adults volunteers of average body weight 69.51 +/- 11.59 kg were submitted to 3 experimental diets: I complete diet, containing rice and beans (RB): II--low protein, low calcium: III--vegetable diet containing RB. The diets were consumed "ad libitum". The total energy intake in each dietary period were: 46.04 +/- 9.18; 37.57 +/- 9.04; 55.27 +/- 7.18 respectively. The average free choice for the proportion of rice/beans was 1.22 and 1.35 in the periods I and III. The protein balance was positive only for the diet I and the balance of calcium was positive in diets I and II. Zinc didn't attained positive balance neither in diet I. It is suggested that the presence of beans in diets I and III plays an important role in decreasing the bio-utilization of the studied nutrients.
Am J Clin Nutr 1999 Jun;69(6):1202-8
Protein pulse feeding improves protein retention in elderly women.
Arnal MA, Mosoni L, Boirie Y, Houlier ML, Morin L, Verdier E, Ritz P, Antoine JM, Prugnaud J, Beaufrere B, Mirand PP. Unite d'Etude du Metabolisme Azote, Institut National de la Recherche Agronomique, Clermont-Ferrand-Theix, France.
BACKGROUND: Adequate protein
nutrition could be used to limit gradual body protein loss and improve
protein anabolism in the elderly.
Br J Nutr 1986 Jul;56(1):17-27
Taurine concentrations in the diet, plasma, urine and breast milk of vegans compared with omnivores.
Rana SK, Sanders TA. Department of Food and Nutritional Sciences, King's College London (KQC), University of London.
The concentration of taurine in the diets, plasma, urine and breast milk
were measured in vegans and age- and sex-matched omnivore controls. Plasma
and urinary amino acid concentrations were also determined.
Am J Physiol 1999 Jun;276(6 Pt 1):E1014-21
Protein kinetics during and after long-duration spaceflight on MIR.
Stein TP, Leskiw MJ, Schluter MD, Donaldson MR, Larina I. Department of Surgery, School of Osteopathic Medicine, University of Medicine and Dentistry of New Jersey, Stratford, New Jersey 08084, USA. email@example.com
Human spaceflight is associated with a loss of body protein. Bed rest studies suggest that the reduction in the whole body protein synthesis (PS) rate should be approximately 15%. The objectives of this experiment were to test two hypotheses on astronauts and cosmonauts during long-duration (>3 mo) flights on MIR: that 1) the whole body PS rate will be reduced and 2) dietary intake and the PS rate should be increased postflight because protein accretion is occurring. The 15N glycine method was used for measuring whole body PS rate before, during, and after long-duration spaceflight on the Russian space station MIR. Dietary intake was measured together with the protein kinetics. Results show that subjects lost weight during flight (4.64 +/- 1.0 kg, P < 0.05). Energy intake was decreased inflight (2,854 +/- 268 vs. 2,145 +/- 190 kcal/day, n = 6, P < 0.05), as was the PS rate (226 +/- 24 vs. 97 +/- 11 g protein/day, n = 6, P < 0.01). The reduction in PS correlated with the reduction in energy intake (r2 = 0.86, P < 0.01, n = 6). Postflight energy intake and PS returned to, but were not increased over, the preflight levels. We conclude that the reduction in PS found was greater than predicted from ground-based bed rest experiments because of the shortfall in dietary intake. The expected postflight anabolic state with increases in dietary intake and PS did not occur during the first 2 wk after landing.
J Appl Physiol 1996 Jul;81(1):82-97
Diet and nitrogen metabolism during spaceflight on the shuttle.
Stein TP, Leskiw MJ, Schluter MD. Department of Surgery, University of Medicine and Dentistry of New Jersey, School of Osteopathic Medicine, Stratford 08084, USA.
Human spaceflight is associated with a loss of body protein. [lack of gravity-induced exercise and stress - ljf] To investigate this problem, dietary intake, nitrogen balance, the whole body protein, and fibrinogen protein synthesis rates were measured on the crews of two Spacelab Life Sciences (SLS) shuttle missions before, during, and after spaceflight. The first mission, SLS-1, lasted 9.5 days, and the second, SLS-2, lasted 15 days. The 15N-glycine method was used for the protein synthesis measurements. The following results were obtained. 1) There was a rapid decline in weight for the first 5 days and then the body weight appeared to stabilize. 2) The mean energy intake preflight was 39.0 +/- 2.5 kcal x kg-1 x day-1 (n = 10). There was a sharp drop in dietary intake on flight day 1, with recovery by the second day, and then energy intake was constant at 30.4 +/- 1.5 kcal x kg-1 x day-1 (n = 12) for the remainder of the flight period (P < 0.05). 3) Nitrogen retention was decreased during flight, with the magnitude of the decrease lessening toward the end of the mission. The daily mean nitrogen balance changed from 58 +/- 9 mg x kg-1 x day-1 (n = 9) preflight to 16 +/- 3 mg N x kg-1 x day-1; P < 0.05; n = 11) in flight, corresponding to a loss of approximately 1 kg of lean body mass over 14 days. 4) Whole body protein synthesis was increased early in flight and on recovery, [stress - ljf] as was fibrinogen synthesis. We conclude that 1) the rapid readjustment and stabilization of energy intake and the improved nitrogen retention with increasing flight duration are consistent with a rapid metabolic accommodation to the novel environment; and that 2) the increased protein turnover indicates that a metabolic stress response is an important factor in this adjustment process. [Could it be that people who "need" lots of protein are reallly under a lot of stress and this "need" is not a true metabolic need, as is common in bodybuilders? - ljf]
J Nutr Sci Vitaminol (Tokyo) 1980;26(3):247-59
A pilot study on protein metabolism in the Papua New Guinea highlanders.
Tanaka N, Kubo K, Shiraki K, Koishi H, Yoshimura H.
In an attempt to clarify the nutritional enigma of the healthy strong physique of Papua New Guinea (PNG) highlanders who have a protein-deficient diet [by whose 'standards'? - ljf] mainly composed of sweet potato, a pilot study was performed as follows with 10 volunteers of PNG highlanders, and with 8 Japanese controls, one group of whom took an experimental protein-deficient diet in Japan. 1. In 4 groups of subjects, i.e. adults of PNG highlanders, their children, Japanese controls who were having standard Japanese food (SPD), and those who were having a low protein diet (0.5 g/kg) for 2 weeks (LPD), urea metabolism was investigated after oral administration of 15N urea. By tracing the cumulative excretion of 15N in urine successively for about 10 days after 15N urea administration, it was found that children of PNG highlanders can retain a large amount of 15N in the body, Japanese controls of LPD fairly well, PNG adult slightly and the Japanese controls of SPD the least of the four groups. It was demonstrated that the 15N atom % excess in the plasma protein of PNG adults, children, and Japanese control of LPD is maintained in the range of 0.02-0.05% fluctuating for 10 days after 15N urea administration. On the other hand, 15N atom % excess in plasma protein of Japanese control of SPD was within the scope of error (0.01%). 15N atom % excess in the lysine fraction of the hydrolysate of plasma protein was found in the range of 0.01-0.05% in a large number of cases of PNG subjects, and Japanese control of LPD, while it was not significantly detectable in Japanese controls of SPD. 15N atom % excess in essential and nonessential amino acids of the hydrolysate was significantly detectable in most cases of PNG subjects and Japanese controls of LPD, while not in the case of Japanese controls of SPD. From discussions on the above findings it is presumed that 15N urea may be utilized in PNG highlanders and Japanese controls of LPD to produce amino acids, especially lysine in the intestine where the bacterial species are changed by a long-continued protein-deficient diet from those of Japanese controls of SPD. The possibility of urea recycling was thus verified.
Br J Nutr 1993 Sep;70(2):449-57
A metabolic nitrogen balance study for 40 d and evaluation of the menstrual cycle on protein requirement in young Nigerian women.
Egun GN, Atinmo T. Department of Human Nutrition, College of Medicine, University of Ibadan, Nigeria.
A long-term N balance study was carried out to determine the adequacy of an estimated protein requirement level recommended for young healthy Nigerian women and the effect of the menstrual cycle on the requirement. Eleven healthy young women, 25 (SD 2.6) years, were fed on a diet providing 0.6 g protein (N x 6.25)/kg per d and an average energy intake of 0.17 (SD 0.012) MJ/kg per d. Urine, faeces, sweat and menstrual fluids were collected for estimation of N balance. Menstrual N loss varied among individuals ranging from 46 to 124 mg N/d with an average of 89 (SD 21.8) mg N/d. Individual N balance was found to vary according to the day of the menstrual cycle. Positive N balances were recorded at about ovulation while negative balances were observed just before the onset of menstruation. [suggesting that 'menstruation' is a cleansing process that rids the body of excess toxins and excess protein. Note that the other ape species do not menstruate! - ljf] The average N balance ranged from +8.49 (SD 5.64) to -430 (SD 7.84) mg N/kg per d. Nevertheless, an overall cumulative positive N balance of +5.7 (SD 6.98) mg N/kg per d which did not change significantly with time was observed for the last 5 d of two consecutive 20 d diet periods, although three subjects were in negative N balance. Blood biochemical measurements were stable except for one subject who had elevated serum aspartate aminotransferase (EC 18.104.22.168) levels. These findings suggest that our estimate of protein requirements was sufficient to achieve N balance equilibrium in a majority (70%) of young women. However, to satisfy 97.5% of the population, slight adjustments might be necessary in the energy intake [not additional protein - ljf] since subjects who were in cumulative negative N balance also lost weight.
J Nutr Sci Vitaminol (Tokyo) 1997 Oct;43(5):505-14
Recommended daily exercise for Japanese does not increase the protein requirement in sedentary young men.
Kido Y, Tsukahara T, Rokutan K, Kishi K. Department of Nutrition, School of Medicine, University of Tokushima, Japan.
In a previous study, we reported that protein intake at the level of dietary protein allowance for Japanese adults, i.e. 1.08 g/kg per day, was enough for recommended daily exercise. However, whether or not recommended daily exercise increases the protein requirement for young adults has not been examined. In this study, we investigated the effect of recommended daily exercise on the protein requirement under an isoenergetic state by a nitrogen balance method. After an adaptation period of 3 days, 12 healthy college students exercised for 10 days with a non-exercise control period of 10 days before or after the exercise period. They were given a maintenance level of energy and 0.64 g/kg per day of high-quality mixed proteins, estimated as the average protein requirement for adults by the Ministry of Health and Welfare of Japan, throughout the experimental period. They performed treadmill running during the exercise period at about 65% of VO2 max for 25 or 40 min/d, which expended 200 or 300 kcal of extra energy, respectively. Although the exercise increased the dermal nitrogen loss, a compensatory decrease in urinary nitrogen excretion was observed. Consequently, the exercises (200 and 300 kcal/d) did not significantly affect the nitrogen balance. These findings indicate that the recommended amount of daily exercise does not change the protein requirement.
Am J Clin Nutr 1984 Jan;39(1):8-15
A long-term metabolic balance study in young men to assess the nutritional quality of an isolated soy protein and beef proteins.
Young VR, Wayler A, Garza C, Steinke FH, Murray E, Rand WM, Scrimshaw NS.
To evaluate the capacity of an isolated soy protein to maintain long-term protein nutritional status in healthy young adult men, an 84-day metabolic balance experiment was conducted in eight subjects. The sole source of protein intake was provided by the isolated soy protein, given at a level of 0.8 g (N X 6.25) per kg per day. In a second and similar study, four young men received 0.8 g protein and three subjects 0.68 g protein per kg per day from beef proteins for 60 to 81 days. Body weight, nitrogen balance, blood chemistries, and body composition (whole body 40K) were monitored throughout each study. Body nitrogen balances were maintained within the range of N equilibrium in both diet groups. Body cell mass, as judged from 40K measurements, did not reveal any deterioration in protein nutritional status. These observations confirm the prediction, derived from previous short-term. N balance studies, that the nutritional quality of isolated soy protein is high and that this plant protein can serve as the sole source of essential amino acids and nitrogen for protein maintenance in adults.
J Nutr Sci Vitaminol (Tokyo) 1985 Jun;31(3):393-402
Utilization of urea nitrogen in Papua New Guinea highlanders.
Rikimaru T, Fujita Y, Okuda T, Kajiwara N, Date C, Heywood PF, Alpers MP, Koishi H.
The utilization of urea nitrogen was examined in 10 healthy adult men from a village near Lufa, in the Eastern Highlands Province of Papua New Guinea. The staple diet of these men was sweet potatoes. [15N]urea was used as tracer for urea released into their intestinal tracts and the utilization of the urea-N was estimated from the trend of 15N. The men were orally given [15N]urea at the beginning of the study and then their daily protein intake, serum protein levels, 15N excretion in the feces and urine, 15N retention in the whole body and 15N incorporation into serum protein were examined. Their daily protein intake (32.2 +/- 8.6 g/day) was low, but their serum protein level (8.05 +/- 0.41 g/100 ml) was within the normal range. 15N retention in the whole body on day 3 was estimated to be 35.4 +/- 20.2% of the total amount administered, calculated from the recoveries in the feces (1.64 +/- 0.85%) and urine (63.0 +/- 20.5%) on days 1-3. The utilization of urea nitrogen in Papua New Guinea highlanders was confirmed from the finding of 15N incorporation into serum proteins on day 3 (0.008 +/- 0.005 atom% excess). This incorporation was negatively correlated with the urinary nitrogen excretion and serum protein level. This correlation suggests that Papua New Guinea highlanders with low urinary nitrogen excretion or a low level in serum protein, who are in a poor state of protein nutrition, tend to utilize more urea nitrogen for the synthesis of serum protein.
J Nutr 1997 Sep;127(9):1788-94
The effect of an increase of protein intake on whole-body protein turnover in elderly women is tracer dependent.
Pannemans DL, Wagenmakers AJ, Westerterp KR, Schaafsma G, Halliday D. Department of Human Biology, Maastricht University, 6200 MD Maastricht, The Netherlands.
To compare the response of whole-body protein turnover with variations in dietary protein level, whole-body protein turnover was measured by different stable isotope methods in six elderly women (69 +/- 5 y) consuming two levels of protein (10 and 20% of total energy, diets A and B, respectively). Protein turnover was measured during 12 h of overnight fasting with 15N-glycine with urea and ammonia as end products. During the last 4 h of the interval, protein turnover was also estimated by -[1-13C]-leucine infusion. Nitrogen balance [diet A, -0.040 +/- 0.015 g/(kg.d); diet B, 0.002 +/- 0.053 g/(kg.d); mean +/- ] did not differ significantly between the diet periods, although all subjects were in negative nitrogen balance at the end of diet A. Protein breakdown, as measured with 15N-glycine, did not differ from results obtained using -[1-13C]-leucine, whereas protein synthesis was found to be significantly lower using the former isotope. The 15N-glycine method indicated that protein turnover (both synthesis and breakdown) was higher in fasting elderly women when they consumed a 20% rather than a 10% protein diet [thus, the high protein diet created more, and unnecessary, metabolic stress - ljf], whereas the -[1-13C]-leucine method did not show significant differences between the diet periods in the last 4 h of the overnight fasting period. However, the relative increase in net protein breakdown when comparing diet B with diet A, was comparable for both tracers. These data indicate that care is needed with the choice of the tracer used in measuring the components of protein turnover in elderly women with the aim of understanding the physiological basis behind the adequacy of the level of protein intake.
Kidney Int 1982 Oct;22(4):392-7
Nitrogen utilization in uremic patients fed by continuous nasogastric infusion.
Abras E, Walser M.
Patients with severe renal failure were continuously fed an electrolyte-free solution containing oligosaccharides plus 35 g of a mixture of six amino acids and four (or, in one study, five) nitrogen-free analogues via a small-bore nasogastric tube attached to a pump. They also ingested three small meals daily. Total caloric intake averaged 34 kcal/kg (of which 76% was infused). Total nitrogen intake averaged only 3.3 g (of which 68% was infused). Nevertheless, nitrogen balance was positive (average + 1.22 g/day). All components of nitrogen excretion fell to unusually low average values: urea nitrogen appearance, 1.14 g/day; non-urea urinary nitrogen. 0.63 g/day; fecal nitrogen, 0.48 g/day. Nitrogen requirement for balance on this regimen, estimated from linear regression, was only 2.0 g/day. [12.5g pro/day - ljf] Body weight did not change significantly. Serum albumin and transferrin remained normal. Thus, this regimen induces positive nitrogen balance despite low nitrogen intake.
Clin Sci (Lond) 1992 Jul;83(1):103-8
Limits of adaptation to a diet low in protein in normal man: urea kinetics.
Danielsen M, Jackson AA. Department of Human Nutrition, University of Southampton, U.K.
1. Urea kinetics were measured using prime/intermittent oral doses of [15N15N]urea in six healthy men taking diets adequate in energy and containing either 74 or 30 g of protein/day. 2. On 74 g of protein/day, urea production (199 mg of N day-1 kg-1) was 121% of intake, with 60% of the urea produced being excreted in the urine and 40% being salvaged in the colon; 69% of the salvaged nitrogen was retained in the metabolic nitrogen pool. 3. Nitrogen balance was not maintained on 30 g of protein/day. There was a significant decrease in the urea production rate (123 mg of N day-1 kg-1) and 54% of production was excreted in urine, with 46% being salvaged. 4. The pattern of urea production and salvaging on 30 g of protein/day was different to that seen in an earlier study on 35 g of protein/day, with a significant decrease in both production (71%) and salvaging (50%). 5. These data reinforce the conclusions drawn from an earlier study, that the salvaging of urea nitrogen by the colon is an integral part of the process of adaptation to low protein diets. The salvage system appears to fail on an intake of 30 g of protein/day and nitrogen is no longer conserved in sufficient amounts for balance to be maintained. [This conclusion was formed without knowing if the 'adaptation' had been complete; that is, was the experiment too short to produce the maximum results? - ljf] 6. The changes seen in urea kinetics reinforce the conclusion based upon nitrogen balance that the minimum physiological requirement for protein in normal adult man lies between 30 and 35 g of protein/day.
Eur J Clin Nutr 1999 Apr;53 Suppl 1:S34-43
Adaptation of protein metabolism in relation to limits to high dietary protein intake.
Garlick PJ, McNurlan MA, Patlak CS. Department of Surgery, Health Sciences Center, State University of New York at Stony Brook, 11794-8191, USA.
Studies of the effects of dietary protein level on human metabolism have usually concentrated on the effects of protein deprivation and on establishing a minimum dietary requirement. By contrast, less is known about the effects of very high protein diets, although general levels of protein intake in the developed world are increasing, and high protein diets have been advocated for maintaining or increasing muscle mass in certain groups of the population. This article, therefore, examines the response of protein metabolism to high dietary protein, studied in adults by nitrogen balance and isotopic tracer techniques, and concentrating on the evidence for increased lean body mass. It is concluded that high protein feeding initially results in protein retention, with greater cycling of body protein in response to meals, but that neither N-balance nor isotopic tracer methods possess sufficient sensitivity to detect whether a long term increase in functional lean tissue ensues. Improved methods of body composition measurement will be needed to establish this. Moreover, the absence of strong evidence that high protein diets confer any advantage in terms of strength or health must be weighed against potentially injurious consequences.
Am J Clin Nutr 1978 Apr;31(4):585-91
Ability of 6 grams of nitrogen from a combination of rice, wheat, and milk to meet protein requirements of young men for 4 weeks.
Clark HE, Kollenkark MA, Halvorson JD.
Young men consumed a diet that provided 6.0 g of nitrogen [37.5g pro - ljf], of which rice, wheat, and milk supplied 33, 38, and 29%, respectively, for 28 days following adjustment. This diet contained amounts of essential amino acids that were at least twice the minimal required amounts reported for men. Mean nitrogen balances in four successive 7-day periods were 0.27 +/- 0.13, 0.25 +/- 0.08, 0.10 +/- 0.07 and 0.24 +/- 0.12 g; and the mean for 28 days was 0.22 +/- 0.05 g. Nitrogen retention did not differ significantly between periods. All men who weighed between 65 and 72 kg retained small amounts of nitrogen but one who weighed 78 kg was in slightly negative balance. The mean balances as reported do not provide an allowance for integumental and other losses. The daily protein intake from these sources, equivalent to 0.5 g/kg of body weight, was a critical level slightly lower than the amount appropriate for long-term maintenance of young men weighing 70 kg.
Hum Nutr Clin Nutr 1983 Jan;37(1):43-51
Long-term nitrogen balance in preschool children fed the safe level of protein from a cereal-legume-milk diet and adequate energy.
Iyengar AK, Narasinga Rao BS.
A long-term N balance study was carried out in five preschool children 3-4 years of age fed a cereal-legume-milk diet providing 1.75 g protein and 100 kcal (418 kJ) energy/kg body weight. This level of protein was found to be safe for preschool children in an earlier short-term N balance study. Continuous N balance was determined for 70 d in three subjects and 45 d in two subjects. All the subjects were found to be in steady positive N balance throughout the experimental period, the mean daily N retention varying between 54.5 and 87.5 mg/d. [Note: this represents protein retention of 0.34 and 0.55 grams/day respectively. Taking an average weight of 25-44lbs = 11-20kg =~ 15.5kg, this average weight child would get ~27 g dietary protein/day of which only ~ 0.44 grams was actually retained and utilized by the body. This means that 0.44/27 = only 1.6% of the ingested protein was actually utilized by the body, again proving that the recommended dietary protein amounts are exceptionally excessive. This highly excessive protein consumption is the cause for the 'diaper rash', colic, frequent crying, mucus discharge, general malaise, and "childhood diseases" common to those babies unfortunate enough to be fed a cultural diet. - ljf] All the children increased their body weight steadily over the experimental period. No tendency of N balance to decrease with time was observed. It is concluded that the safe level of protein for preschool children estimated from a short-term N balance study is quite adequate when tested by a long-term feeding trial.
J Nutr Sci Vitaminol (Tokyo) 1983 Apr;29(2):169-85
Nitrogen requirement of amino acid mixture with maintenance energy in young men.
Komatsu T, Kishi K, Yamamoto T, Inoue G.
The effect of energy on nitrogen balance was examined in young men given amino acid mixture. The minimum amino acid nitrogen requirement for nitrogen equilibrium was determined together with the egg protein requirement. In experiment 1, the nitrogen sparing effect of energy was evaluated in four male students receiving diet containing an amino acid mixture and a constant nitrogen intake of 3.5 g N/day,[P=N*6.25, P=21.9g/day - ljf] which was equivalent to the nitrogen requirement with excess energy intake determined by Rose and Wixom (1). When the dietary energy supply was 45 kcal/kg, which is approximately the maintenance level, the mean nitrogen balance was negative, being -23.9 +/- 9.3 mg N/kg. However, with an excess energy intake of 55 kcal/kg, the nitrogen balance improved significantly, being -6.1 +/- 7.7 mg N/kg. In experiment 2, the nitrogen requirement of egg-pattern amino acid mixture for apparently zero balance was evaluated at maintenance energy intake in 28 Japanese young men and was compared with that of egg protein. After receiving standard diet, the subjects were given a semi-purified experimental diet containing egg-pattern amino acid mixture at nitrogen intake levels of 60, 75, 100, and 130 mg N/kg for two weeks. Then all groups except the 60 mg N/kg group were given isonitrogenous egg protein diet for another week. Energy intake was kept constant at approximately the maintenance level of 44.4 +/- 1.4 kcal/kg throughout the experiment. Nitrogen balance was not significantly different in groups given egg-pattern amino acid mixture and intact egg protein in each nitrogen intake level. From regression analysis, the nitrogen requirement for nitrogen equilibrium of the amino acid mixture was calculated to be 110.1 +/- 50.2 mg N/kg, which was not significantly different from the value of 88.4 +/- 40.6 mg N/kg of egg protein. It was concluded that the total amino acid requirement estimated by Rose and Wixom (1) was too low because they gave excess energy, and that there was no difference between the nitrogen requirement of egg protein and that of the corresponding amino acid mixture.
Postgrad Med J 1984;60 Suppl 3:59-65
Protein requirements with very low calorie diets.
The goal of the dietary treatment of obesity is to reduce the patient's weight with minimum risk. This is accomplished by a dietary regimen which allows a preferential loss of body fat with a preservation of lean body mass. Total fasting leads to a loss of 150 grams of nitrogen in the first month alone. In a study by Hoffer et al. reported below, two levels of dietary protein were compared for their effects on nitrogen balance in 17 obese women on a low calorie (500 cal) weight reduction diet. After three weeks of adaptation to the diets, the group given 0.8 grams protein/kg were in -2 grams nitrogen balance while the group given 1.5 grams protein/kg were at zero nitrogen balance. It was concluded that protein intakes at the level of the recommended dietary allowance (0.8 g/kg) are not compatible with nitrogen equilibrium when the energy intake is severely restricted. While weight loss is the obvious goal for obese persons, a careful examination of the composition of the weight loss (protein, fat, water) is essential in defining the optimal dietary regimen. [These researchers are obviously ignorant of the fact that large amounts of proteinaceous waste products (mucus, etc.) are excreted during healthy weight loss and that this is beneficial; they are misinterpreting this nitrogen excretion to be due only to loss of lean muscle and that is not the case - ljf]
J Nutr 1978 Apr;108(4):658-69
Requirement and utilization of egg protein by Japanese young men with marginal intakes of energy.
Kishi K, Miyatani S, Inoue G.
The effect of marginal intakes of energy on the requirement and utilization of egg protein was evaluated in 46 Japanese young men. The subjects were given a standard diet for 1 week and then low protein diets for 2 weeks. These diets contained about 32, 64, and 80 mgN/kg with whole eggs as the protein source. In the first experiment with excess energy, the energy intakes of 31 subjects were kept constant during the 3 week experiment, the mean intakes being 48.2 +/- 1.5 kcal/kg. The body weight was affected by changing protein intakes while maintaining energy intakes at 48 kcal/kg. From regression analysis, the N requirement for apparent N equilibrium was estimated to be 82.0 +/- 8.0 mgN/kg, where NPU was calculated as 56. In the second experiment with submaintenance energy, 15 subjects received 40 kcal/kg. The N requirement was 124 +/- 21 mgN/kg, where NPU was calculated as 37. From these results and those of previous studies, it was concluded as follows: 1) N balance and NPU were remarkably affected by energy intake changed around maintenance level; and 2) the NPU for egg protein in young men for maintenance intakes of energy and N is about 50 to 55. For estimation of the protein requirement for Japanese adults, a correction factor of 100/55 (about 1.8) was used instead of 1.3 adopted by the 1973 FAO/WHO.
Am J Physiol 1993 May;264(5 Pt 1):E824-8
Effect of spaceflight on human protein metabolism.
Stein TP, Leskiw MJ, Schluter MD. Department of Surgery, University of Medicine and Dentistry of New Jersey, School of Osteopathic Medicine, Camden 08103.
Nitrogen balance and the whole body protein synthesis rate were measured before, during, and after a 9.5-day spaceflight mission on the space shuttle Columbia. Protein synthesis was measured by the single-pulse [15N]glycine method. Determinations were made 56, 26, and 18 days preflight, on flight days 2 and 8, and on days 0, 6, 14, and 45 postflight. We conclude that nitrogen balance was decreased during spaceflight. The decrease in nitrogen balance was greatest on the 1st day when food intake was reduced and again toward the end of the mission. An approximately 30% increase in protein synthesis above the preflight baseline was found for flight day 8 for all 6 subjects (P < 0.05), indicating that the astronauts showed a stress response to spaceflight. [Note: high protein turnover rate is indicative of high levels of stress - ljf]
J Nutr 1978 Mar;108(3):506-13
The minimum cottonseed protein required for nitrogen balance in women.
Alford BB, Onley K.
This study was designed to assess minimum amount of nitrogen from deglanded (LCP) and glandless (GSF) cottonseed protein required to maintain nitrogen balance in young adult women. Fourteen women aged 19 to 25 were divided into two groups, one group was fed LCP and the other GSF in a purified formula diet for the entire study. The protein level constituted approximately 15% of the total energy intake initially. After reaching and maintaining equilibrium, the protein level for each subject was decreased until that lowest level was found that would maintain the individual in nitrogen balance. Nitrogen status was calculated using the formula: Nitrogen Status = N Intake--N Output (Urinary + Fecal + other Obligatory Loss). Regression data for each subject was used to define nitrogen intake required to maintain equilibrium. The nitrogen intakes showed similar results for the two forms of cottonseed with a mean value of 0.106 g N per kg body weight [0.66g/kg - ljf] to maintain equilibrium. The quality of cottonseed protein was between reported values in men for soybean and wheat protein.
Int J Vitam Nutr Res 1983;53(3):338-44
Studies on the protein requirement of Brazilian rural workers ("boias frias") given a rice and bean diet.
Vannucchi H, Duarte RM, Dutra de Oliveira JE.
The present study was conducted to evaluate the nutritive value of a multiple level rice and bean diet fed to migrant workers. Nine healthy males, 18 to 28 years old were admitted to our metabolic unit for a three period metabolic balance study. The rice and beans based diet was fed at levels to provide 0.4, 0.6 or 0.8 g of protein/kg body weight. Mean energy intake for the three levels of intake and for all subjects was 46.9 +/- 2.9 kcal/kg Bwt/day. Each nitrogen balance period consisted of a) one day on a nitrogen free diet b) five days on an adaptation period, and c) five days on the balance period. "True digestibility', "true nitrogen balance", biologic value, and net protein utilization (NPU) were calculated. Mean protein requirements were estimated by regression analysis of pooled data of balances at different levels of intake. These results showed values of 103.8 mg N/kg Bwt/day. [P=N*6.25, so P = 0.1038g*6.25 = 0.65 grams protein/kg bodyweight, considerably less than the RDA of 0.8g/kg which is excessive- ljf] Mean and standard deviation for protein digestibility at each level of intake were 69.2 +/- 17.0, 75.5 +/- 5.3, and 74.9 +/- 10.6% respectively. Mean and standard deviation for NPU were 49.9 +/- 26.3, 55.6 +/- 10.6, and 57.8 +/- 14.4 respectively. The data support the conclusion that a rice and bean diet is a well balanced food combination and can serve as a fairly good source of protein for the adult human.
Hum Nutr Clin Nutr 1983 Dec;37(6):433-46
Whole-body protein turnover and nitrogen balance in young children at intakes of protein and energy in the region of maintenance.
Jackson AA, Golden MH, Byfield R, Jahoor F, Royes J, Soutter L.
Nitrogen balance and whole-body protein turnover were measured in children aged about one year taking diets which provided 1.7 or 0.7 g milk protein/kg/d at three levels of metabolizable energy, 80, 90 and 100 kcal/kg/d. All the children were in positive nitrogen balance at all levels of energy intake on 1.7 g protein/kg/d. Nitrogen equilibrium was maintained on 0.7 g protein/kg/d when the energy intake exceeded 90 kcal/kg/d, but on 80 kcal/kg/d nitrogen balance was negative. Whole-body protein turnover was measured from the enrichment in urinary ammonia following a continuous infusion of 15N-glycine. The variation between individuals on the same diet was significantly greater than the variation within individuals at different levels of energy intake. For the group as a whole protein synthesis on 1.7 g protein/kg/d was 0.74, 0.75 and 0.87 g N/kg/d on 100, 90 and 80 kcal/kg/d respectively; whereas on 0.7 g protein/kg/d it was 0.37, 0.38 and 0.40 g N/kg/d. These results show that over this range of intakes protein synthesis decreased as dietary protein fell, but tended to increase as energy intake fell. [again, this shows that excessive energy intake depletes the body of protein - ljf]
Am J Clin Nutr 1988 Oct;48(4):1015-22
Excess energy and nitrogen balance at protein intakes above the requirement level in young men.
Chiang AN, Huang PC. Department of Biochemistry, College of Medicine, National Taiwan University, Republic of China.
Effects of excess energy on nitrogen balance at above the safe level of protein intake were studied in six young men. They were given test diets with a fixed protein intake of 1.2 g.kg-1.d-1 but with three successive energy (E) levels: at maintenance (1.0 E), 15% above maintenance (1.15 E) and 30% above maintenance (1.3 E), both in ascending (1.0 E, 1.15 E, 1.3 E) and descending (1.3 E, 1.15 E, 1.0 E) sequences. Duration of each dietary period was 10 d. N balance increased from 7.2 to 23.8 to 33.3 mg N.kg-1.d-1 in the ascending series and decreased from 27.8 to 17.6 to 4.8 mg N.kg-1.d-1 in the descending series. [This indicates that excessive energy consumption, quite common in cultural diets, depletes the body of protein - ljf] The changes in N balance per 100 kcal change in energy intake ranged between 144 and 243 mg with smaller changes at higher energy levels. Biological value and net protein utilization of the dietary protein were positively correlated with energy intake in both the ascending series and the descending series (p less than 0.001).
Ann Physiol Anthropol 1994 Nov;13(6):393-401
[Protein metabolism in vegans] [Article in Japanese]
Okuda T, Miyoshi-Nishimura H, Makita T, Sugawa-Katayama Y, Hazama T, Simizu T, Yamaguchi Y. Faculty of Human Life Science, Osaka City University.
To elucidate the mechanisms of adaptation to a low-energy and low-protein vegan diet, we carried out dietary surveys and nitrogen balance studies five times during one year on two women and a man who ate raw brown rice, raw green vegetables, three kinds of raw roots, fruit and salt daily. Individual subjects modified this vegan diet slightly. The mean daily energy intake of the subjects was 18, 14, and 32 kcal/kg, of body weight. The loss of body weight was about 10% of the initial level. The daily nitrogen balance was -32, -33, and -11 mg N/kg of body weight. In spite of the negative nitrogen balance, the results of routine clinical tests, initially normal, did not change with the vegan diet. Ten months after the start of the vegan diet, the subjects were given 15N urea orally. The incorporation of 15N into serum proteins suggested that these subjects could utilize urea nitrogen for body protein synthesis. The level of 15N in serum proteins was close to the level in other normal adult men on a low-protein diet with adequate energy for 2 weeks.
Vnitr Lek 1990 May;36(5):417-25
The Czechoslovak low-energy protein diet contains 1559 kj per day, incl. 33.0 g protein, 50 g carbohydrate, 4.0 g fat, 6.0 g fibre and recommended vitamin and mineral allowances. The diet was administered to 75 subjects with a mean age of 39.88 +/- 1.21 years and a body weight of 112.57 +/- 2.89 kg, and a body weight index (BMI) of 39.44 +/- 0.94, for 28 days. During treatment the body weight declined by 9.67 +/- 0.41 kg and the BMI by 3.74 +/- 0.20. At the same time there was a significant decline of the total cholesterol (p less than 0.001), LDL cholesterol (p less than 0.001), HDL cholesterol (p less than 0.01) and triacylglycerols (p less than 0.05). In 20 subjects with hyperlipidaemia similar changes of the total cholesterol and LDL cholesterol were recorded, while the decrease of HDL cholesterol (p less than 0.001) and triacylglycerols (p less than 0.001) was more marked. The nitrogen balance became during the fourth week on the diet positive (+0.64 g), there was a transitory decline of the total protein concentration. The serum amino acid concentration did not change significantly with the exception of isoleucine, the level of which increased. The serum immunoglobulin concentration did not change significantly. It may be concluded that the low-energy protein diet has a favourable impact on the lipid metabolism, without a marked negative effect on the protein metabolism.