Nitrite and nitrate are formed by endogenous synthesis in the human intestine.

Studies of nitrate balance in humans and analyses of fecal and ileostomy samples indicate that nitrite and nitrate are formed de novo in the intestine, possibly by heterotrophic nitrification. These findings significantly alter our previous conceptions of human exposure to nitrite and suggest an even wider role for nitrite in the etiology of human cancer.

Mechanisms permitting nephrotic patients to achieve nitrogen equilibrium with a protein-restricted diet.

UNLABELLED: Clinical experience suggests nephrotic patients are at risk for malnutrition. To determine if nephrotic patients can adapt successfully to a protein-restricted diet, nephrotic (glomerular filtration rate, 52+/-15 ml/min; urinary protein [Uprot.], 7.2+/-2.2 grams/d) and control subjects completed a crossover comparison of diets providing 0.8 or 1.6 grams protein (plus 1 gram protein/gram Uprot.) and 35 kcal per kg per day. Nitrogen balance (BN) was determined and whole body protein turnover measured during fasting and feeding using intravenous -[1-13C]leucine and intragastric -[5,5, 5- 2H3]leucine. BN was positive in both nephrotic and control subjects consuming either diet and rates of whole-body protein synthesis, protein degradation, and leucine oxidation did not differ between groups. In both nephrotic and control subjects anabolism was due to a suppression of whole-body protein degradation and stimulation of protein synthesis during feeding. The principal compensatory response to dietary protein restriction was a decrease in amino acid oxidation and this response was the same in both groups. With the low protein diet leucine oxidation rates during feeding correlated inversely with Uprot. losses (r = -0.83; P < 0. 05). CONCLUSIONS: (a) a diet providing 0.8 gram protein (plus 1 gram protein/gram Uprot.) and 35 kcal per kg per day maintains BN in nephrotic patients; (b) nephrotic patients activate normal anabolic responses to dietary protein restriction (suppression of amino acid oxidation) and feeding (stimulation of protein synthesis and inhibition of protein degradation); (c) the inverse correlation between leucine oxidation and Uprot. losses suggests that proteinuria is a stimulus to conserve dietary essential amino acids.

Palmitate and glycerol kinetics during brief starvation in normal weight young adult and elderly subjects.

Data obtained in vitro suggest that the ability to mobilize fat decreases with age. We determined lipolytic rates in vivo in normal weight young adult (22-33 yr) and elderly (65-77 yr) subjects using a simultaneous infusion of [1,2-13C2]palmitate and [2H5]glycerol. The subjects were studied after a 12-h fast and again after 60-82 h of fasting. When lipolysis was expressed per unit of adipose tissue the values for the young adults were more than double those for the elderly (P less than 0.05). However, the amount of body fat in the elderly was twice that of the young adults, so that lipolysis per unit of body weight was similar in both groups. These results demonstrate that lipolysis per unit of adipose tissue is lower in elderly subjects. This may be due to their increase in body fat, however, since the total amount of potential energy mobilized from adipose tissue was similar to that of the young adults.

Metabolic effects of very low calorie weight reduction diets.

A randomized comparison trial of two very low calorie weight reduction diets was carried out for 5 or 8 wk in 17 healthy obese women. One diet provided 1.5 g protein/kg ideal body weight; the other provided 0.8 g protein/kg ideal body weight plus 0.7 g carbohydrate/kg ideal body weight. The diets were isocaloric (500 kcal). Amino acid metabolism was studied by means of tracer infusions of L-[1-13C]leucine and L-[15N]alanine. After 3 wk of adaptation to the diets, nitrogen balance was zero for the 1.5 g protein diet but -2 g N/d for the 0.8 g protein diet. Postabsorptive plasma leucine and alanine flux decreased from base line by an equal extent with both diets by approximately 20 and 40%, respectively. 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, and that nitrogen balance is improved by increasing the protein intake above that level. Basal rates of whole body nitrogen turnover are relatively well maintained, compared with total fasting, at both protein intakes. However, turnover in the peripheral compartment, as evidenced by alanine flux, may be markedly diminished with either diet.

Arginine, citrulline, and nitric oxide metabolism in end-stage renal disease patients.

The kidneys are thought to be a major site of net de novo arginine synthesis, but the quantitative status of arginine metabolism and its substrate precursor relationship to nitric oxide (NO) synthesis in end stage renal disease (ESRD) patients have not been characterized. We have investigated kinetic aspects of whole body arginine metabolism in six patients with ESRD. They received two pre- and two post-hemodialysis intravenous tracer infusion studies with L-[guanidino-(15)N(2)]arginine and L-[(13)C]leucine during the first study, and L-[5-(13)C]arginine and L-[5-(13)C-ureido,5,5, (2)H(2)]citrulline during the second study. Arginine homeostasis in ESRD patients was found to be associated with a lower rate of arginine oxidation, and despite the decrease in renal function, the rate of de novo arginine synthesis appeared to be preserved. Plasma citrulline concentrations and flux were also elevated in these subjects compared with healthy adults. The rate of whole body NO synthesis was increased in the ESRD patients, but apparently not different pre- and post-hemodialysis therapy. The anatomic site(s) responsible for the maintenance of net de novo arginine synthesis and for the elevated NO synthesis and its pathophysiological importance in ESRD remain to be established.

Insulin-mediated reduction of whole body protein breakdown. Dose-response effects on leucine metabolism in postabsorptive men.

In vivo effects of insulin on plasma leucine and alanine kinetics were determined in healthy postabsorptive young men (n = 5) employing 360-min primed, constant infusions of L-[1-13C]leucine and L-[15N]alanine during separate single rate euglycemic insulin infusions. Serum insulin concentrations of 16.4 +/- 0.8, 29.1 +/- 2.7, 75.3 +/- 5.0, and 2,407 +/- 56 microU/ml were achieved. Changes in plasma 3-methyl-histidine (3-MeHis) were obtained as an independent qualitative indicator of insulin-mediated reduction in proteolysis. Hepatic glucose output was evaluated at the lowest insulin level using D-[6,6-2H2]glucose. The data demonstrate a dose-response effect of insulin to reduce leucine flux, from basal values of 77 +/- 1 to 70 +/- 2, 64 +/- 3, 57 +/- 3, and 52 +/- 4 mumol(kg X h)-1 at the 16, 29, 75, and 2,407 microU/ml insulin levels, respectively (P less than 0.01). A parallel, progressive reduction in 3-MeHis from 5.8 +/- 0.3 to 4.3 +/- 0.3 microM was revealed. Leucine oxidation estimated from the 13C-enrichment of expired CO2 and plasma leucine (12 +/- 1 mumol[kg X h]-1) and from the 13C-enrichment of CO2 and plasma alpha-ketoisocaproate (19 +/- 2 mumol[kg X h]-1) increased at the 16 microU/ml insulin level to 16 +/- 1 and 24 +/- 2 mumol(kg X h)-1, respectively (P less than 0.05 for each), but did not increase at higher insulin levels. Alanine flux (206 +/- 13 mumol(kg X h)-1) did not increase during the clamp, but alanine de novo synthesis increased in all studies from basal rates of 150 +/- 13 to 168 +/- 23, 185 +/- 21, 213 +/- 29, and 187 +/- 15 mumol(kg X h)-1 at 16, 29, 75, and 2,407 microU/ml insulin levels, respectively (P less than 0.05). These data indicate the presence of insulin-dependent suppression of leucine entry into the plasma compartment in man secondary to a reduction in proteolysis and the stimulation of alanine synthesis during euglycemic hyperinsulinemia.

Development of a nutrient data system for international use: INFOODS (International Network of Food Data Systems).

The current interest in the area of nutrition, diet, and cancer has called attention to some major deficiencies in the data base available to support a variety of research activities in the field. These deficiencies have been especially evident in epidemiologic studies where attempts are being made to characterize the dietary differences existing among various populations who have markedly different incidences of chronic diseases, including cancers, thought to be associated with diet. Such studies have been able to characterize international differences in diet by broad, nutrient-food categorizations, but they suffer from a dearth of detailed information on the nutrient content of the enormous variety of foodstuffs consumed by different populations and by the subgroup populations within each of the countries or geographic areas. At the present time, there is no internationally agreed upon food composition data system that provides a continuum of information on food composition. Current diet and disease studies require data on the human requirements or allowances for essential nutrients and quantified data on the ability of the food supply to provide these factors. In addition, information on other chemical components of foods that represent protection for or risk to good health is needed. Such a comprehensive data base would be helpful in providing criteria for formulation of public policy on diet-related public health issues. The Diet and Cancer Branch of the Division of Cancer Prevention and Control, National Cancer Institute, together with other groups support the development of an International Food Composition Data System (INFOODS) as a response to the need for more complete, reliable, and comprehensive information on the nutrient and nonnutrient composition of foods and beverages and their ingredients.

Metabolic fate of an oral dose of 15N-labeled nitrate in humans: effect of diet supplementation with ascorbic acid.

The metabolic fate of a p.o. dose of 3.5 mmol 15N-labeled nitrate has been investigated in 12 healthy young adults. Samples of urine, saliva, plasma, and feces were collected over a period of 48 hr following administration of the dose. Subjects received either 60 mg of ascorbic acid, 2 g of ascorbic acid, or 2 g of sodium ascorbate per day. An average of 60% of the 15NO3- dose appeared in the urine as nitrate within 48 hr. Less than 0.1% appeared in the feces. The 15N label of nitrate was also found in the urine (3%) and feces (0.2%) in the form of ammonia or urea. The fate of the remaining 35% of the 15NO3- dose administered is unknown. No effect of ascorbic acid or sodium ascorbate on the nitrate and nitrite levels of plasma, saliva, urine, or feces was observed. A one-compartment pharmacokinetic model was used to describe the relationships between intake, plasma concentration, and urinary excretion of nitrate. The half-life of nitrate in the body was found to be approximately 5 hr, and its volume of distribution was about 30% of body weight. Daily endogenous biosynthesis of nitrate was estimated to be about 1 mmol/day.

Effect of vitamins C and E on endogenous synthesis of N-nitrosamino acids in humans: precursor-product studies with [15N]nitrate.

The endogenous formation of nitrosoproline (NPRO) following administration of nitrate and proline is reported in ten healthy young adults. There was a relatively constant basal excretion of NPRO, 26 +/- 10 (SD) nmol/day, in excess of amounts found in the diet. This basal synthesis of NPRO was not reduced by ascorbic acid (2 g/day) or alpha-tocopherol (400 mg/day). A significant rise in the excretion of NPRO was observed following the administration of nitrate and proline, ranging from 29 to 318 nmol/24 h with a mean of 100 nmol/24 h. [15N]Nitrate was used as a tracer to study the observed excess excretion of NPRO in urine. The data revealed that urinary NPRO excretion as a result of endogenous synthesis is not totally derived from ingested nitrate as its precursor. The ingestion of ascorbic acid and alpha-tocopherol inhibited the incorporation of [15N]nitrate into NPRO by 81 and 59%, respectively. An additional nitrosamino acid, N-nitrosothiazolidine-4-carboxylic acid, was present in the urine. It was found that N-nitrosothiazolidine-4-carboxylic acid increased 6-fold upon ingestion of nitrate. Ascorbic acid and alpha-tocopherol blocked this nitrate induced synthesis.

Synergism of triiodothyronine and corticosterone on muscle protein breakdown.

The concerted effect of triiodothyronine (T3) and corticosterone on muscle protein synthesis and breakdown was studied. Thyroidectomized young male rats were treated with T3 (1.5 microgram/100 g body weight per day), corticosterone (10 mg/100 g body weight per day) and both T3 and corticosterone for 4 days. On the 3rd day of the experiment urine was collected to measure N tau-methylhistidine excretion as an index of muscle protein breakdown. On the last day of the experiment, the rates of protein synthesis in skeletal muscles were measured by the large-dose [3H]phenylalanine method. N tau-Methylhistidine excretion was slightly increased by T3 treatment and it was increased about 3-times by corticosterone treatment. When both T3 and corticosterone were administered, it was increased about 6-fold. The rate of muscle protein breakdown calculated from the difference between the rate of protein synthesis and the growth rate was consistent with these findings. The rate of muscle protein synthesis was increased by T3, and it was decreased by corticosterone. The rate was the same as that of the thyroidectomized control group when the animals were given T3 and corticosterone, showing that T3 restrained the inhibiting effect of corticosterone on muscle protein synthesis. The results indicate that a physiological level of T3 enhances the catabolic action of pharmacological doses of glucocorticoids on muscle protein breakdown.

Ntau-methylhistidine content of mixed proteins in various rat tissues.

In order to use Ntau-methylhistidine (3-methylhistidine) excretion in the urine as a measure of muscle protein breakdown, it is necessary to demonstrate that other tissues are not important sources of this protein constituent. Accordingly, the concentration of Ntau-methylhistidine in blood serum and in the mixed proteins of heart, brain, lung, kidney, diaphragm, spleen, testis, stomach, liver and hind leg skeletal muscle was measured in male rats of approx. 400 g body weight. The free Ntau-methylhistidine concentration of rat serum was less than 2 nmol per ml. In contrast, measurable amounts of Ntau-methylhistidine were found in the mixed proteins of all tissues and organs examined. The highest concentration was found in skeletal muscle (658 nmol/g tissue). Assuming muscle mass to be 45% of body weight, it has been estimated that the muscle contains more than ten times the total amount of this amino acid present in all of the other organs analyzed, which together account for about 20% of total body weight. These findings indicate that skeletal muscle is likely to be the major source of urinary Ntau-methylhistidine and the latter is, in consequence, a reflection of myofibrillar protein breakdown in skeletal muscle.

L-arginine transport in the human coronary and peripheral circulation.

BACKGROUND: Nitric oxide synthase (NOS) uses arginine for the production of nitric oxide (NO). High intracellular concentrations of arginine suggest that NOS activity should be independent of plasma arginine supply. However, under certain conditions, increased plasma arginine concentrations appear to be associated with increased NOS activity. The purpose of this study was to explore arginine transport within the human coronary and peripheral circulation METHODS AND RESULTS: Mass-labeled 15N2-arginine was infused to steady state before cardiac catheterization in 31 patients. After diagnostic angiography, a catheter was placed in the coronary sinus. The transcardiac concentration gradient (aorta-coronary sinus) of 15N2-arginine was used as a measure of arginine transport at baseline and during infusions of acetylcholine and N(G)-monomethyl-L-arginine (L-NMMA). No gradient was detected at rest. During the infusion of acetylcholine, a significant gradient was detected (2.5+/-1.2 micromol/L, P=0.01) corresponding to a fractional extraction of 11.7+/-7.5%. This is consistent with in vitro studies that suggest that stimulation of NOS induces arginine transport. During the infusion of L-NMMA, the concentration of 15N2-arginine increased in the coronary sinus, producing a gradient of -3.9+/-1.3 micromol/L (P=0.0002), corresponding to a fractional production of 20.5+/-5.0%. This is consistent with in vitro studies that suggest that L-NMMA induces the efflux of arginine from the cell to the extracellular space via transporter-mediated transstimulation. CONCLUSIONS: The use of steady-state 15N2-arginine to examine transorgan L-arginine gradients represents a novel tool for the study of L-arginine transport and the mechanisms of endothelial and NOS dysfunction.

Quantitative aspects of glucose production and metabolism in healthy elderly subjects.

The metabolic basis for the reduced glucose tolerance that occurs during aging in humans has been explored with the aid of a primed constant intravenous infusion method of labeled glucose (6-3H; 6,6,2H- and U-13C-glucose). Healthy young adult men and women (24 +/- 3 yr) and elderly men and women (75 +/- 4 yr) participated in a series of studies designed to quantify rates of plasma glucose appearance, oxidation, and recycling while subjects were in the postabsorptive (basal) state and to determine rates of hepatic glucose production and glucose disappearance in response to intravenous glucose at approximately 1 and 2 mg x kg-1min-1 and also 4 mg x kg-1min-1 without or with a simultaneous infusion of insulin to maintain normoglycemia. Basal rates of glucose production were 2.41 +/- 0.06 and 2.18 /+/- 0.05 mg x kg-1min-1 in the young adults and elderly, respectively (P less than 0.05). Recycling of glucose carbon and glucose oxidation rates did not differ significantly between the two age groups. Infusion of unlabeled glucose reduced hepatic glucose production to the same extent in the two groups, indicating that the mechanisms responsible for altered hepatic glucose production with intravenous glucose administration remain intact during human aging. Plasma insulin changes were similar in young adult and elderly subjects receiving 4 mg x kg-1min-1 unlabeled glucose except that the higher plasma glucose levels in the elderly were associated with higher insulin levels. For elderly subjects, the amount of exogenous insulin required to maintain normoglycemia at the 4 mg x kg-1min-1 glucose infusion rate was about twice that necessary in young adults.

Whole body de novo amino acid synthesis in type I (insulin-dependent) diabetes studied with stable isotope-labeled leucine, alanine, and glycine.

Dynamic aspects of whole body alanine and glycine metabolism have been explored in insulin-dependent (type I) diabetic subjects. Using a primed, continuous intravenous (i.v.) infusion of [2H3]alanine and [15N]glycine given simultaneously with [1-13C]leucine, whole body alanine and glycine fluxes and their rates of de novo synthesis were measured in 6 diabetic young men. Subjects were studied in the postabsorptive state, after blood glucose was clamped overnight at 15.2 +/- 0.3 mM, and then, on the following night, at 5.9 +/- 0.2 mM (insulin infusion rates of 0.24 +/- 0.09 and 1.65 +/- 0.20 U/h, respectively). In the normoglycemic state, leucine, alanine, and glycine fluxes averaged 88 +/- 4, 378 +/- 39, and 155 +/- 8 mumol X kg-1 X h-1, respectively. Based on the leucine flux, alanine and glycine de novo synthesis rates were 264 +/- 36 and 67 +/- 8 mumol X kg-1 X h-1. In the hyperglycemic state, leucine flux increased 23% (P less than 0.01), alanine flux rose slightly (+5%) but significantly (P less than 0.05), while alanine de novo synthesis and glycine flux remained unchanged and glycine de novo synthesis decreased by 33% (P less than 0.001). These results show that small alterations in peripheral alanine inflow in the hyperglycemic state reflect increased proteolysis and suggest that increased circulating plasma glucose does not contribute to de novo alanine synthesis in the absence of adequate insulin effect and/or augmented glucose tissue uptake. These observations also reveal the importance of insulin in the maintenance of whole body leucine economy, since a lower rate of insulin administration was associated with an increased rate of leucine oxidation.

1987 McCollum award lecture. Kinetics of human amino acid metabolism: nutritional implications and some lessons.

Clinical nutrition is an integrative science with the ultimate purpose of defining in quantitative terms the characteristics of an optimum nutritional intake in relation to a defined level of nutritional health. Thus, to achieve major progress in our field of clinical nutrition, data from the molecular, subcellular, cellular, and organ levels need to be exploited and considered in reference to the whole organism; this requires that we identify important unanswered questions for this latter and more complex, hierarchical level of biological organization and then pursue the answers with the aid of techniques and approaches used in and concepts emerging from all areas of modern biology. In relation to this, the present overview of some of the studies that my colleagues, my students, and I have conducted was meant to emphasize that there is considerable merit in attempting to explore the integrative aspects of the physiology and biochemistry of human nutrient metabolism, specifically of amino acids, with the aid of stable-isotope probes. Recognition of the importance of the phosphorylation and dephosphorylation of cellular proteins as a major regulatory process and of the regulation of leucine oxidation through changes in the activity of the branched-chain 2-Oxo acid dehydrogenase complex via a reversible phosphorylation catalyzed by a specific branched-chain dehydrogenase kinase and phosphatase is indeed exciting new knowledge. Following from this, Espinal et al state: "The activity of the complex determines the rate of degradation and the dietary requirement for branched-chain amino acids." However, the physiological situation cannot be appreciated simply in these terms because we showed that the rate of oxidation of leucine depends upon the tissue availability of the amino acid. Furthermore, our studies revealed that the regulation of leucine oxidation in the intact human appears to be achieved through biochemical mechanisms that are linked to the host's nutritional requirements. These observations and interpretations would not have emerged by considering only the enzymology of branched-chain amino acid metabolism; this underscores the value of exploring, through use of safe noninvasive tracer techniques, the communication of amino acid metabolism among different systems and how these systems might interplay to influence the nutritional needs of the individual. This recalls Fishman's advice: "Physiology has a special role to play here, for after probing the submicroscopic, life is left behind. It is physiology's responsibility to put together the lifeless pieces of the molecular biologist into living systems."(ABSTRACT TRUNCATED AT 400 WORDS)

Protein intake and requirements with reference to diet and health.

We have reviewed, in brief, various aspects of human protein and amino-acid metabolism with particular reference to the assessment of protein and amino acid needs. The current requirements for protein and amino acids in various groups of healthy people were then summarized. Emerging from this discussion was the view that some plant protein foods, such as soy for example, are of much better nutritional value for humans than has been commonly appreciated. The figures for requirements were then compared with intakes characteristic of populations in the developed regions of the world. It is evident that these intakes greatly exceed the minimum physiologic need for most healthy individuals but little is known about the possible health benefit or otherwise of these intake levels of protein. Furthermore, also it was pointed out that consideration of different food proteins in relation to amino acid and nitrogen requirements constitutes an important, but only an initial, basis for evaluating the role of dietary protein and of various food protein sources in human nutrition and health. Different food protein sources influence, in various ways, the utilization of and possibly requirements for other nutrients. This complicates determination of requirements and the setting of rational and safe dietary allowances as well as of dietary guidelines for individuals and population groups. A further assessment of the role and impact of food proteins and their interrelationships with and interactions among other foods and essential nutrients, particularly with respect to the nutritional and metabolic status of free-living individuals and their long-term health, presents a difficult but exciting research challenge to nutrition and other health professionals.

Energy costs of fat and protein deposition in the human infant.

The energy costs of depositing fat and protein in the low-birth-weight infant were determined by multiple-regression analysis from published information on metabolizable-energy intake and fat and protein deposition in groups of subjects fed different dietary regimes. The total energy requirement for deposition was 1.17 kJ/kJ deposited for fat (ie, 1 kJ deposited and 0.17 kJ expended for deposition, and 2.38 kJ/kJ for protein. These values are similar to published determinations for animal species with simple stomachs. The metabolizable-energy requirement for weight gain during infancy was calculated (range, 12.2-25.1 kJ/g, or 2.9-6.0 kcal/g; means, 18.7 kJ/g, or 4.5 kcal/g) from the energy costs of fat and protein deposition and published information on changes in body composition during the first year of life.

Use of stable isotopes to determine bioavailability of minerals in human diets using the method of fecal monitoring.

Use of stable isotopes to determine bioavailability of minerals, by the method of fecal monitoring, in human diets is discussed. Analytical expressions are developed to permit prediction of the accuracy of the method as a function of several variables involved. The effects of extent of absorption, natural abundance of the selected isotope, and the enrichment ratio of administered diet on the accuracy of the estimate of absorption are examined. the method of neutron activation analysis as applied to the isotopic measurement of trace minerals including iron, zinc, copper, chromium, and nickel is briefly discussed. The method of fecal monitoring for zinc and iron is illustrated with data obtained in a healthy adult subject receiving a diet enriched with 70Zn and 58Fe.

Urinary excretion of N gamma-methylihistidine (3-methylihistidine): a tool to study metabolic responses in relation to nutrient and hormonal status in health and disease of man.

Some of the histidine residues of actin and myosin are methylated after synthesis of these contractile muscle proteins. During breakdown of muscle protein in the course of protein turnover, the product of methylation, N gamma-methylhistidine, is released and quantitatively excreted in the urine both of rat and of man. Since most of the N gamma-methylhistidine in the body occurs in muscle, the rate of its excretion becomes a convenient measure of muscle protein breakdown. Output per kilogram of body weight is highest in the infant, especially when related to creatinine, and is reduced in the elderly as a result of loss of muscle mass with aging. A diet deficient in protein causes the young rat to have a reduced output of methylhistidine (reduced rate of muscle protein breakdown) which increases again during repletion on an adequate diet. Fasting obese human subjects also show a progressive fall in output of this metabolite. Thyroidectomy in rats reduces N gamma-methylhistidine excretion, which is only restored by giving large doses of thyroxine. On the other hand, studies on growing rats show that adrenalectomy and moderate doses of cortecosterone have no appreciable effect on the N gamma-methylhistidine output, which is only elevated by steriod administered in amounts large enough to raise plasma corticosteroid levels several-fold. These various observations show that N gamma-methylhistidine provides a useful tool in the study of muscle protein metabolic responses under a variety of nutritional and hormonal circumstances in the intact human.

Genetic and biological variability in human nutrient requirements.

Multiple factors, associated with the host and environment, as well as the chemical form of the nutrient, its availability, chemical and metabolic interactions among nutrients, result in a variability in nutrient requirements that is probably well beyond that observed in controlled laboratory studies of small groups of "normal" healthy subjects. Although many factors are recognized as contributing to the variation in human nutrient requirements, current knowledge is largely of qualitative significance: Garn (40) state that this level of knowledge and investigation "has a long past but no foreseeable future." It is sad commentary that neither do we know adequately the quantitative extent of biological variation in requirements among individuals for any of the essential nutrients nor the quantitative importance of most of the factors which affect requirements in population groups. We believe that the potential contributions of nutrition toward solving problems of human disease and for maintaining health in populations depend as much on a better definition of the quantitative aspects of human nutrient requirements as on improved understanding of the utilization, function and metabolism of nutrients at the cellular, organ, and whole body level.