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Background: We proposed previously that the mean lysine requirement value is approximately 30 mg * kg(-)(1) * d(-)(1) rather than the proposed 1985 FAO/WHO/UNU estimate of the upper range of the requirement, which is 12 mg * kg(-)(1) * d(-)(1)., Objective: Our objective was to explore the 24-h pattern and rate of whole-body lysine [l-(13)C]oxidation and status of whole-body lysine balance in healthy, young adults given an L-amino acid diet supplying either a low lysine intake (14-15 mg * kg(-)(1) * d(-)(1)) or an intermediate lysine intake (29 mg * kg(-)(1) * d(-)(1)) for 6 d before a continuous tracer study with L-[1-(13)C]lysine., Design: Five subjects received the low lysine intake, 6 subjects received the intermediate intake, and all were studied by using a standard 24-h oral tracer protocol that was described earlier for studies at a generous lysine intake., Results: The rate of lysine oxidation was not significantly different between the 12-h fasted and 12-h fed states. The daily oxidation rate (f1.gif" BORDER="0"> +/- SD) was 27. 9 +/- 8.8 and 27.3 +/- 17.6 mg lysine * kg(-)(1) * d(-)(1) for the low- and intermediate-intake groups, respectively (NS). Daily lysine balance was -12.4 +/- 92 and 1.8 +/- 17.7 mg * kg(-)(1) * d(-)(1), respectively (P < 0.025), for the low and intermediate intakes. The balance was significantly less than zero (P < 0.001) for the low intake., Conclusion: The FAO/WHO/UNU lysine requirement value is not sufficient to maintain lysine homeostasis in healthy adults. From the results of this and tracer studies done by others, the mean lysine requirement of healthy adults was determined to be 30 mg * kg(-)(1) * d(-)(1).

In two groups of five adults, each adapted to two different dietary regimens for 6 days, the metabolic fate of dietary [1-(13)C]leucine was examined when ingested either together with a mixture of free amino acids simulating casein (extrinsically labeled; condition A), along with the intact casein (extrinsically labeled; condition B), or bound to casein (intrinsically labeled; condition C). Fed state leucine oxidation (Ox), nonoxidative leucine disposal (NOLD), protein breakdown, and splanchnic uptake have been compared using an 8-h oral [1-(13)C]leucine and intravenous [(2)H(3)]leucine tracer protocol while giving eight equal hourly mixed meals. Lower leucine Ox, increased NOLD, and net protein synthesis were found with condition C compared with condition A (19.3 vs. 24.9; 77 vs. 55.8; 18.9 vs. 12.3 micromol. kg(-1). 30 min(-1); P < 0.05). Ox and NOLD did not differ between conditions B and C. Splanchnic leucine uptake calculated from [1-(13)C]- and [(2)H(3)]leucine plasma enrichments was between 24 and 35%. These findings indicate that the form in which leucine is consumed affects its immediate metabolic fate and retention by the body; the implications of these findings for the tracer balance technique and estimation of amino acid requirements are discussed.

The effect of a "normal" (n = 8) and "high" (n = 6) protein intake (1 and 2.5 g x kg(-1) x day(-1), respectively) and of exercise on plasma amino acid (AA) concentrations, insulin, and glucagon concentrations was followed throughout a continuous 24-h period in adult male subjects at energy balance after six days on a standardized diet and exercise program. Subjects were fasting from 2100 on day 6 to 1200 on day 7 and then fed 10 identical meals hourly until 2100. Physical exercise was performed (46% maximal oxygen uptake) between 0830 and 1000 (fasting) and in a fed state (1600-1730) on each day. The normal-protein group showed fasting plasma AA concentrations that were higher (P < 0.05) than those for the high-protein group, except for leucine, methionine, and tyrosine. Glutamine, glycine, alanine, taurine, and threonine concentrations were distinctly higher ( approximately 30% or greater) throughout the 24-h period in subjects consuming the normal- vs. the high-protein diets. Exercise appeared to increase, although not profoundly, the plasma concentrations of amino acids except for glutamate, histidine, ornithine, and tryptophan. The profound diet-related differences in plasma AA concentrations are only partially explained by differences in the renal clearance of the amino acids. We speculate on the possible metabolic basis for these findings.

The quantitative relationships between nitrogen (N) intake, urea production, excretion and amino acid oxidation are currently a matter of debate. Some investigators have proposed that urea production is essentially constant over a wide range of N intakes and that urea hydrolysis is regulated according to the N needs of the organism. We have assessed this proposal by compiling results from four separate experiments in healthy young adults (n = 34) carried out in our laboratories and all at the end of the respective diet periods using an identical 24-h continuous intravenous infusion of [(15)N, (15)N]urea and L-[1-(13)C]leucine. The N intakes were: expt. 1; protein-free diet for 5 d; expt. 2; N at 44 mg N. kg(-1). d(-1) from a balanced L-amino acid mixture for 13 d; expt. 3; N at 161 mg. kg(-1). d(-1) from egg protein for 6 d; expt. 4 -one group received 157 mg. kg(-1). d(-1) and the other 392 mg. kg(-1). d(-1) from milk-protein-based diets for 6 d. Urea production and excretion were linearly correlated with N intake (r = 0.98 and 0.94, respectively; P < 0.01). Urea hydrolysis increased linearly with N intake (r = 0.7; P < 0.05), with considerable variation in the rate among individuals, especially at the N intake of approximately 160 mg N. kg(-1)d(-1). These findings are consistent with the generally accepted view that a control of body N balance is via a regulation of urea production. They do not support the concept that urea hydrolysis is the more important site in the control of body N loss.

Effects of moderate physical activity (90 min at 45-50% of maximal O2 uptake 2 times daily) and "high" (2.5 g protein. kg-1. day-1, n = 6) or "normal" protein intake (1.0 g protein. kg-1. day-1, n = 8) on the pattern and rate of 24-h macronutrient utilization in healthy adult men were compared after a diet-exercise-adjustment period of 6 days. Energy turnover (ET) was determined by indirect and direct (suit) calorimetry, and "protein oxidation" was determined by a 24-h continuous intravenous infusion of [1-13C]leucine. Subjects were in slight positive energy balance during both studies. Protein contributed to a higher (22 vs. 10%) and carbohydrate (CHO) a lower (33 vs. 58%) proportion of total 24-h ET on the high- vs. normal-protein intake. The highest contribution of fat to ET was seen postexercise during fasting (73 and 61% of ET for high and normal, respectively). With the high-protein diet the subjects were in a positive protein (P < 0.001) and CHO balance (P < 0.05) and a negative fat balance (P < 0.05). The increased ET postexercise was not explained by increased rates of urea production and/or protein synthesis.

Essential amino acids, now called indispensable amino acids (IAA) are not synthesized by the body, and have therefore to be supplied from an exogenous source. IAA requirements as set out in 1985 by the FAO/WHO/UNU have been challenged by the data derived from the application of obligatory nitrogen loss measurement technique and the stable isotope tracer amino acid technique. These measurements suggest that IAA requirement in adults may be between two to three times the requirements set out in the 1985 FAQ/WHO/UNU recommendations.

In healthy adult men adapted to a diet/exercise regimen for 6 days, the effects of small, frequent meals supplying daily protein intakes of 1 (n = 8) or 2.5 g . kg-1 . day-1 (n = 6) on leucine oxidation, urea production, and whole body protein synthesis (PS) and degradation (PD) have been compared with the use of a 24-h continuous intravenous [13C]leucine and [15N,15N]urea infusion protocol. Two 90-min periods of exercise (approximately 50% maximal O2 consumption) were included during the fasting and the fed periods of the 24-h day. Subjects were determined to be at approximate energy, nitrogen, and leucine balances on both diets. Increased protein intake raised the urea production rate; the absolute rate of urea hydrolysis was the same on both diets. When the first-pass splanchnic uptake of leucine was taken to be 25% of intake, PS was stimulated by feeding (after an overnight fast) at both protein intake levels (P < 0.05 and P < 0.01), whereas PD declined significantly (P < 0.01) at both protein levels. Protein gain at a high protein intake appears to be the result of both a stimulation of PS and a marked decline in PD, whereas at a less generous intake, the gain appears to be a result of a fall in PD with a less evident change in PS. Exercise moderately decreased PS during and/or immediately after exercise at each protein level, and there was a postexercise-induced increase (P < 0.01) in PD, which was more dramatic when feeding was at the higher protein intake level.

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.

Six patients with chronic renal failure (glomerular filtration rate 18 +/- 2 ml/min) underwent two 10-day admissions separated by at least 1 yr of outpatient therapy with a very low-protein diet (VLPD) providing 0.28 g protein.kg-1.day-1 plus an amino acid-ketoacid supplement. During each Clinical Research Center admission, subjects completed a 5-day nitrogen balance (BN), and whole body protein turnover was measured during fasting and feeding using intravenous [1-13C]leucine and intragastric [5,5,5-2H3]leucine. Outpatient dietary protein compliance was very good (25 vs. 20 g protein/day or 125% goal), whereas energy intake was only 69% of goal (24 vs. 35 kcal.kg-1.day-1). During the 16 +/- 2 mo of dietary therapy, there were no changes in serum proteins or anthropometrics. BN after > or = 1 yr of dietary therapy was neutral and did not differ from initial values (+0.46 +/- 0.20 vs. +0.55 +/- 0.19 g N/day). Similarly, rates of whole body protein synthesis, degradation, and leucine oxidation after long-term therapy with the VLPD regimen did not differ from baseline values, and neutral BN was maintained by a marked suppression of amino acid oxidation and postprandial inhibition of protein degradation. This is the first evidence that the compensatory changes in whole body protein turnover activated in response to dietary protein restriction are sustained during long-term therapy.

Dietary protein requirements of elderly people were determined by short-term nitrogen-balance techniques and using calculations recommended by the 1985 Joint FAO/WHO/UNU Expert Consultation. Twelve men and women aged 56-80 y were randomly assigned to groups that consumed either 0.80 +/- 0.01 or 1.62 +/- 0.02 g protein.kg-1.d-1 (mean +/- SEM). Net nitrogen balance was negative for the lower-protein group (-4.6 +/- 3.4 mg N.kg-1.d-1) and positive for the higher-protein group (13.6 +/- 1.0 mg N.kg-1.d-1); the intake required for nitrogen equilibrium was estimated to be 1.00 g.kg-1.d-1. Nitrogen-balance data from three previous protein requirement studies in elderly people were recalculated by using the same balance formula and combined with the current study data to provide an overall weighted mean protein requirement estimate of 0.91 +/- 0.043 g.kg-1.d-1. Together, the current and retrospective nitrogen-balance data suggest that the mean protein requirement in elderly adults is considerably greater than the 0.60 g.kg-1.d-1 established by the 1985 Joint FAO/WHO/UNU Expert Consultation. A safe protein intake for elderly adults would be 1.0-1.25 g.kg-1.d-1 of high-quality protein.

We investigated plasma methionine and cysteine kinetics in eight healthy adult men receiving for 6 d each of five L-amino acid diets supplying 13 mg methionine.kg-1.d-1 without cystine or 6.5 mg methionine.kg-1.d-1 plus 0, 5.2, 10.5, or 20.9 mg cystine.kg-1.d-1. On the morning of day 7, primed, constant intravenous infusions of L-[2H3-methyl, 1-13C]methionine and L-[3,3-2H]cysteine were given for 8 h (for the first 3 h subjects remained in a fasted state and for the next 5 h received small, equal meals at hourly intervals to achieve a fed state). Methionine and cysteine fluxes and rate of methionine oxidation were estimated from plasma methionine and cysteine labeling and 13C in expired air. Methionine oxidation declined (P < 0.05) with lowered methionine intake. Cysteine flux was similar across diets and dietary cystine did not affect tracer methionine oxidation. If there is a sparing effect of dietary cystine on the methionine requirement in adults, it probably takes place during the "first-pass" removal of these amino acids within the splanchnic region.

The effects of removing proline from the diet on plasma leucine and proline kinetics were investigated. After a 1-wk control period, during which young adult men received a diet containing a complete L-amino acid mixture, seven subjects were given for 4 wk a diet devoid of proline (group 1); six received a diet devoid of proline, arginine, aspartate, glutamate, and serine (group 2); and seven continued with the complete diet (group 3). At the end of the control and 4-wk periods subjects were given a continuous, (3-h fast, 5-h fed) intravenous infusion of L-[1-13C]leucine and L-[5,5-2H]proline. Plasma proline was reduced significantly, especially during the fed state, in groups 1 and 2 after the 4-wk diet periods. Small but statistically significant (P < 0.05) reductions occurred in nonoxidative leucine disappearance and leucine appearance during the fasted state in group 2. Proline fluxes decreased by approximately 50% in fasted and fed states in groups 1 and 2. Mean de novo proline synthesis during the fasted state declined markedly (P < 0.05) after 4 wk in groups 1 and 2.

Plant protein foods contribute approximately 65% of the per capita supply of protein on a worldwide basis and approximately 32% in the North American region. These sources of protein are discussed in relation to their amino acid content, human amino acid requirements, and dietary protein quality. Mixtures of plant proteins can serve as a complete and well-balanced source of amino acids for meeting human physiological requirements. This short review ends with a list of series of myths and realities concerning the relationship between plant protein and human nutrition and a list of some nutritional issues of concern to the health professional and informed consumer.

The validity of tracer-derived estimates of whole-body leucine balance was investigated. Seven healthy young adult subjects received an adequate protein diet for 6 d; at 1800 on the last day, L-[1-13C]leucine and [15N-15N]urea were given as primed, continuous intravenous infusions for 24 h. Subjects were in a fasting state for the first 12 h and at 0600 on day 7 they then received hourly 10 equal meals to achieve a fed state. Total leucine intake (diet plus tracer) was 89.4 mg.kg-1.d-1. Mean daily leucine oxidation was equivalent to 89.5 +/- 3.3 mg leucine/kg. The predicted daily oxidation rate, from measurements made during the last hour of the fast and the fifth hour of the fed period, was 91.2 +/- 5.8 mg/kg (P = 0.25 from measured). Measured and predicted whole-body leucine balances were 0.76 +/- 2.99 and -0.98 +/- 5.54 mg/kg, respectively (P = 0.25). Urea production exceeded urea excretion by 20%; daily protein oxidation was the same when estimated from leucine oxidation or nitrogen excretion. Thus, the tracer-balance concept is valid, and reliable predictions of total daily leucine oxidation and whole-body leucine balance can be obtained from short-term measurements of leucine oxidation during fasted and fed states.

Eight patients with chronic renal failure (GFR 18.8 +/- 2.7 ml/min) were randomized to a crossover comparison of a very low protein diet (VLPD) containing 0.28 g protein and 35 kcal per kg per day, plus an isosmolar mixture of either ketoacids (KA) or essential amino acids (EAA). Subjects initiated the diets 14 days before hospital admission and following a four-day equilibration, a five-day nitrogen balance (BN) was performed. Whole-body protein turnover (WBPT) was measured during fasting and feeding using intravenous [1-13C]leucine and intragastric [5,5,5-2H3]leucine. Even though the VLPD/KA regimen contained 15% less nitrogen, BN was neutral and did not differ between the regimens. Nitrogen conservation with KA was due to a reduction in urea nitrogen appearance. Rates of WBPT measured during fasting and feeding did not differ between the KA or EAA regimens. During both regimens, feeding decreased protein degradation, whereas protein synthesis was unchanged. Although feeding stimulated leucine oxidation, rates were 50 to 100% lower than reported in CRF patients consuming 0.6 or 1.0 g protein/kg/day. Thus, neutral Bn with the VLPD regimen is achieved by a marked reduction in amino acid oxidation and a postprandial inhibition of protein degradation.

The effects of dietary protein restriction on protein synthesis were investigated in perfused rat hindlimb. The fractional rate of protein synthesis was measured with [3H]phenylalanine in young adult (7-week-old) rats fed a low protein (5% casein) diet and a protein-free diet for 3 weeks. The low protein diet (LPD) allowed a moderate gain in body weight. The fractional rate of protein synthesis fell to 70% of the control value in LPD group and further fell to less than a half in the protein-free diet (PFD) group. Thus, the protein synthesis rate decreased as the dietary protein content was reduced. The fall of protein synthesis was mainly accompanied by the reduction of RNA activity (mg protein/mg RNA/day) rather than RNA concentration (RNA/protein). The rate of protein breakdown was calculated by subtracting growth rate from protein synthesis rate. The breakdown rate was decreased in LPD group and increased slightly in PFD group. From the low rates of protein synthesis and breakdown, it appears that dietary protein restriction, at least allowing a gain in body weight, makes the turnover rate slow down. The overall changes in protein synthesis obtained in the perfused hindlimb are consistent with the reported results in vivo.

To explore the effects of altered protein intake on the uptake and utilization of dietary amino acids in human subjects, six healthy male Massachusetts Institute of Technology students were studied during three dietary periods each of 9 days, with tracer infusion studies conducted on days 8 and 9. During one period the diet provided a generous protein intake (1.5 g.kg-1.day-1) and during the other two a low-protein intake (0.1 g.kg-1.day-1). Tracer infusions (fed state) were given while subjects received either a liquid formula, supplying the prior protein intake or (on day 9) a generous intake. Combinations of [2H3]Leu and [13C]Leu and of [15N]Lys and [2H4]Lys were given via continuous intravenous or intragastric infusion for 4 h to estimate leucine kinetics and the first-pass splanchnic uptake of tracer. Dietary-induced changes in leucine flux and oxidation were similar irrespective of the tracer and/or route of administration and sampled pool (leucine or KIC) used for calculation. Estimates of absolute changes in the rates of leucine disappearance via nonoxidative metabolism (protein synthesis) and of appearance via protein breakdown depended on the route of [13C]Leu administration. However, it appears that both altered rates of protein synthesis and breakdown contribute to the adaptations required in the body nitrogen economy when protein intakes are restricted and subsequently resupplied. From the intragastric [13C]Leu protocol, the first-pass splanchnic uptake of tracer was the same (approximately 30%) for generous, low-protein, and refed conditions.(ABSTRACT TRUNCATED AT 250 WORDS)

Because the regulation of protein and energy balance is of major research interest in the nutrition and physiology of humans and animals, a selected account of interactions between protein and energy is given here, with particular emphasis on studies in human subjects. The discussion begins with reference to the relations between protein and energy intakes and nitrogen balance; selected aspects of the relations between protein dynamics and energy metabolism among the various mammalian species are then considered. This leads to a brief account of oxidative amino acid catabolism and its relevance to the assessment of amino acid requirements, particularly in adult man. It is concluded that obligatory oxidative losses of amino acids can be used to predict or approximate amino acid requirements in children and adults. The nitrogen-sparing properties of carbohydrate and lipid-derived fuels are then considered. Despite the well-known and profound, yet differential, impacts of dietary protein and energy sources, and their interactions on body protein balance, there remain wide gaps in our understanding of the mechanisms responsible for their effects, such as the quantitative and mechanistic involvement of hormones, including insulin and the counter-regulatory hormones, and the roles played by the major amino acids responsible for the interorgan transport of nitrogen and the regulation of urea production. Additional studies focusing on metabolic nitrogen trafficking would significantly enhance an understanding of how protein and energy interact to achieve the efficient utilization of dietary protein for maintenance and promotion of lean body gain.

The nutritional value of processed soy protein (isolated soy proteins and soy-protein concentrates) in human protein and amino acid nutrition is evaluated on the basis of a review of studies of growth and nitrogen balance in infants, children, adolescents, and adults. Findings show that well-processed soy-protein isolates and soy-protein concentrates can serve as the major, or even sole, source of protein intake and that their protein value is essentially equivalent to that of food proteins of animal origin. The importance of the sulfur amino acid content of soy protein for practical human nutrition is also examined from nitrogen-balance data. Under conditions of an anticipated normal usage of soy protein, methionine supplementation is not only unnecessary but may even be undesirable for young children and adults. However, for newborns, the data suggest that modest supplementation of soy-based formulas with methionine may be beneficial. Soy proteins have also been found to be of good quality to include in hypocaloric diets for weight reduction in obese subjects. Finally, the data indicate that soy proteins are well-tolerated and of good acceptability.

Recent technological developments have permitted the large-scale preparation of ingredients enriched with microparticulated proteins (MPP) that can be used to replace much or essentially all of the fat in a variety of traditional, full-fat foods. These include ice cream, salad dressings, mayonnaise and cheese spreads. In this paper we consider the nutritional implications of MPP, beginning with an assessment of the need for a fat-substitute and then the potential impact of MPP on the fat content of applicable foods and on the diet as a whole. Under projected conditions of maximum use of these MPP-containing foods by the consumer, the mean reduction in the fat content of the "current U.S. diet" would be about 14%. Although not dramatic, this represents a significant move in the direction of dietary changes that have been recommended by national and international authorities concerned with diet-health relationships. The risk/benefit ratio associated with inclusion of MPP in the diet is considered to be very high. This new development by the food industry is of considerable scientific interest and of potential practical importance for the nutrition and well-being of our society.

In normal subjects, short to intermediate responses to dietary protein restriction include decreased amino acid oxidation and protein degradation plus increased utilization of amino acids for protein synthesis; these responses are activated to improve amino acid utilization and nitrogen balance. To assess whether chronic renal failure (CRF) impairs the adaptive responses to a low-protein diet, we measured nitrogen balance and the kinetics of infused L-(15N,1-13C)leucine during fasting and feeding. In six adult CRF and four control (C) subjects, 0.6 (LP) and 1.0 (HP) g protein kg-1 day-1 diets were compared. LP reduced feeding stimulated oxidation of leucine by 26% in CRF and 33% in C (P = NS). During fasting, oxidation was unaffected by diet or CRF. For both groups, feeding suppressed protein degradation to the same extent; leucine incorporation into protein did not change. Nitrogen balance during the two diets was the same with C and CRF, as was protein balance estimated from results of measured leucine kinetics. Thus, patients with CRF can activate appropriate adaptive responses to LP inasmuch as reduced amino acid oxidation occurring with feeding and estimates of protein balance did not differ from control.

In this short review some aspects of body protein and amino acid metabolism during ageing in human subjects have been explored. The picture that emerges is a progressive diminution of total body protein with ageing, due largely to a decline in the size of the skeletal muscle mass. These changes are accompanied by a shift in the overall pattern of whole body protein synthesis and breakdown, with muscle mass estimated to account for about 30% of whole body protein turnover in the young adult, as compared with a lower value of 20% or less in the elderly subject. The metabolic significance and possible functional implications of this alteration in the quantitative contribution by muscle to whole body amino acid and protein dynamics have been considered. The determination of requirements for individual essential amino acids and for total protein has been discussed, and it is evident that the data are limited and often contradictory. However, elderly individuals are more likely to be influenced by various biological, environmental and social factors, the effects of which would be generally to increase protein needs above those for younger adults. Thus, in practice, the protein needs of elderly people are probably higher than for the young. The decline in energy intake, together with its possible consequences for reduced dietary protein utilization, will also tend to increase the protein need of elderly subjects, compared with that for physically more active young adults. Until more data become available, it is recommended for food planning purposes that an appropriate protein allowance could be 12-14% of the total energy intake, for mixed protein sources characteristic of the diets of industrialized countries or the more affluent sectors of populations in developing countries. Energy intake should be at a level that meets the estimates proposed by FAO/WHO/UNU for older persons. Tentative recommendations are made herein that intakes of specific indispensable (essential) amino acids, per unit of protein need, should be similar to those for the young school-age child and they should be higher than those currently judged by international authorities (i.e. FAO/WHO/UNU) to be sufficient for maintenance of protein nutritional status in the adult. In view of (i) the increasing proportion of older individuals within technically advanced populations together with the need of this group for health care and (ii) the important role played by diet and food habits in health maintenance, and in the aetiology or progression of degenerative disease, it is vitally necessary to improve upon the current state of knowledge concerning protein and amino acid metabolism and nutrition during the later phases of our lives.

Altered intakes of protein and amino acids modulate the rates of the major systems (protein synthesis, protein degradation, and amino acid oxidation) responsible for the maintenance of organ and whole-body protein and amino acid homeostasis. The cellular mechanisms responsible for such changes at low intakes are discussed. For oxidation amino acid availability is a primary determinant and protein synthesis is affected particularly at the initiation phase. Much remains to be learned about amino acid-dependent changes in mRNA synthesis, processing, turnover, and translation. The relationships between protein and amino acid intake and components of whole-body protein and amino acid kinetics are considered with reference to nutritional adaptation and accommodation. The limit of adaptation to protein intake cannot be lowered substantially beyond that for healthy adults whose habitual protein intake is generous. Metabolic control theory should be considered in the interpretation of results of studies dealing with amino acid requirement estimations.

Whole-body leucine kinetics and rectus muscle synthetic rates were evaluated in postabsorptive rats fed semipurified diets that varied in the casein content. Rats were allowed to consume ad libitum a 2% casein diet or were pair-fed or ad libitum-fed 6, 20, or 40% casein diets for 14 days. After overnight starvation, rates of whole-body leucine kinetics and rectus muscle synthetic rates were determined with a 2-h constant intravenous infusion of L-[1-14C]leucine. The postabsorptive response to inadequate protein intakes included a significant reduction in the release of leucine from whole-body protein degradation as well as subsequent reutilization for protein synthesis. In contrast, dietary protein intake at levels greater than required for maximal growth were not associated with any increases in leucine incorporation into whole-body protein or muscle fractional synthetic rates. Rates of whole-body leucine oxidation based on plasma leucine specific radioactivities underestimated total oxidation by 22-27%, and this was relatively constant as the protein component of the diet was varied. In addition, the muscle acid-soluble leucine specific radioactivity was similar to the plasma alpha-ketoisocaproate enrichment, regardless of dietary protein intake.

The protein nutritional value of meat, poultry, and their products must be considered in relation to humans. Because direct assessment of protein nutritional value in human subjects is impractical for regulatory purposes, methods based on in vitro (chemical) and animal bioassays for assessment of protein quality have been developed. Herein, a case is made for the use of amino acid data as a basis for regulation of meat and poultry products, where considerations of protein nutritional value are included in the regulation. Current data on the protein nutritional quality of these products are based almost entirely on the use of the rat PER assay procedure, supplemented by data on the amino acid content of these products. Because of the inadequacy of the PER assay, the available data base provides an inadequate body of knowledge from which to draw final recommendations concerning policies for regulation of the protein nutritional quality of meat and poultry products. However, if it is necessary to include a measure of protein nutritional quality in a regulation, these data provide a reasonably sound basis for tentatively recommending use of amino acid composition data rather than the standard rat bioassay procedure. In such an approach based on amino acid composition data, consideration might be given to the contents of hydroxyproline (or possibly proline), 3-methylhistidine, lysine, and nitrogen, with a view to establishing a composite or minimum value for the noncollagen muscle protein content of these products.

The effects of thyroid status and of dietary protein and energy content on muscle protein breakdown have been compared in young rats, using urinary N tau-methylhistidine (3-methylhistidine; 3-Mehis) output as a measure of myofibrillar protein breakdown. Recently, thyroidectomized young rats receiving an adequate diet grew slowly and showed a reduction in 3-Mehis output, relative to the weight of their leg muscles. When the thyroidectomized rats were given 2-5 microgram thyroxine (T4) per 100 g body weight, output of 3-Mehis increased, even when growth was prevented by feeding a diet low in protein. This demonstrates that thyroid-status affects muscle protein breakdown through a mechanism other than via a change in growth rate. In confirmation of previous studies, administration of a diet low in protein to young rats prevented growth and reduced output of 3-Mehis relative to leg muscle weight. The reduction in protein breakdown is not likely to be due to reduced thyroid function, since the diet low in protein increased plasma total triiodothyronine (T3) levels. Furthermore, thyroidectomized rats fed the low-protein diet also underwent a decrease in output of 3-Mehis. In contrast to protein deficiency, a moderate dietary energy deficiency allowed some growth to occur, and output of 3-Mehis, relative to muscle mass, was not depressed. Nevertheless, total T3 concentration in the plasma of these animals was considerably reduced, indicating once more that total circulating thyroid hormone levels are not the only determinants of muscle protein breakdown. Finally, reduced plasma total T3 levels occurred in energy-deficient diets even in thyroidectomized rats receiving exogenous T4. This suggests that diet affects metabolism of the hormone independently of changes in secretory output from the thyroid gland.

1. Crude polyribosomes from skeletal muscle of the hind leg of rats fed on a low-protein diet for 10 days are less active in cell-free protein synthesis than are polyribosomes obtained from well-nourished control rats. 2. The polyribosomes were salt-washed (0.5m-NH(4)Cl) and the wash extract was examined for its amino acid incorporating activity and for EF (elongation factor) 1 and EF 2 activities. 3. Compared with preparations from control rats, the salt-wash fraction from protein-depleted rats was less active and showed lower EF 1 and EF 2 activity. 4. The ribosomes were rendered equal in activity by salt-washing, but no inhibitor was detected in the salt wash. 5. Differences in the incorporating activity of crude polyribosomes from the diet groups persisted in the presence of saturating amounts of partially purified EF 1 and EF 2. 6. It is concluded that the lowered protein-synthetic activity of crude polyribosomes caused by restricted protein intake is not causally related to the lower activities of EF 1 and EF 2 in the polyribosome preparations. 7. The possible nature of the change in crude polyribosome activity due to low-protein feeding is discussed.

The protein nutritional value of two isolated soy proteins was compared with that of dried skim milk proteins in healthy young men. Eight subjects received one of the isolated soy proteins and six subjects received the dried skim milk as the test protein source. Each protein was tested at intakes of 0.35, 0.45, 0.55 and 0.65 g protein (N X 6.25) per kilogram per day during 10-day diet periods separated by break periods of 3 days and a 1-day protein-free period. Nitrogen balances were determined for the final 5 days of each experimental diet period. From regression analysis of nitrogen intake minus nitrogen output data, intakes of each protein source to meet mean nitrogen requirements were 124, 146 and 144 mg N/kg per day for the two isolated soy proteins and skim milk protein, respectively. It is concluded that well-processed isolated soy proteins are indistinguishable from milk as a protein source for maintenance of short-term N balance in adult human nutrition.

The temporal pattern of daily urinary nitrogen (UN) excretion was investigated in 21 young adult men and women who participated in two metabolic studies involving constant diets and lasting 8 to 11 weeks. For one group (16 subjects), nitrogen (N) intake was about 280 mg N/kg per day (mixed animal and plant protein sources) for 8 weeks; for the second group (five subjects), N intake was about 90 mg N/kg per day (egg protein) for 11 weeks. Two mathematical models were examined to determine how well they described the variations in UN. The first modeled only random variability while the second added a component of serial correlation (correlation between successive daily observations). After correcting the UN data for linear trend, to minimize effects of possible alterations in body composition, only two of the 21 subjects showed significant serial correlation of daily UN. Moreover, it is shown that any serial correlation undetectable in these data would not be of practical importance in estimating UN levels in short-term dietary experiments. It is concluded that the interpretation of metabolic N balance measurements involving constant N intakes over relatively short dietary periods in not complicated by cyclic, time-dependent variations in UN.

The protein nutritional value of an isolated soy protein was examined in healthy young men during 10-day N-balance periods. In the first study (expt 1), 10 young men received a diet providing a test protein (N X 6.25) intake of 0.6 g protein per kilogram per day with nitrogen from either lean beef, isolated soy protein or various combinations of the two sources. No differences in N balance, digestibility or net protein utilization were observed when the soy protein replaced beef. In the second study (expt 2), three separate groups of seven young men each received graded intakes of either beef or an isonitrogenous mixture of beef and the isolated soy protein or dried skim milk. An estimate was made of the N intake from each source required for N equilibrium. These values were 116, 106 and 103 mg N/kg per day, respectively. No statistically significant differences were found among the three dietary groups. Thus, the protein quality of the isolated soy protein appears to be comparable to that of animal protein sources such as milk and beef.

A diet supplying 0.5 or 1 g egg and milk protein/kg per day and adequate energy for maintenance of body weight was given to seven healthy men aged 18 to 21. After 2 weeks for adaptation to the diet they participated in 75 min of daily isometric exercises for 4 to 6 weeks. Two men on each protein intake continued to exercise for 4 to 5 additional weeks after undergoing changes in their daily protein intake from 0.5 to 1 g/kg or vice versa. A progressive decrease in total body potassium became significant by the fourth week of exercises in all men who began on the 0.5 g protein diet. Two men on that diet who remained as nonexercising controls lost no body K. These findings indicated that 0.5 g of egg and milk protein/kg per day was insufficient for men performing isometric exercises without prior training, whereas 1 g/kg per day appeared to be adequate. Two other nonexercising men on the 0.5 g diet who lost weight showed a marked negative nitrogen balance and loss of total body potassium which improved after their dietary energy intake increased. The changes in nitrogen balance and body weight observed in this investigation support the concept that protein requirements vary with energy intakes. Conversely, they suggest that energy requirements are influenced by the level of protein intake.

Fasting plasma glucose turnover, urinary 3-methylhistidine excretion, and fasting plasma protein profiles were compared in a 4-week randomized clinical trial of two very low-calorie weight-reduction diets. Diet A (360 kcal) provided 1.5 g egg protein per kg ideal body weight (IBW) but no carbohydrate. Diet B (340 kcal) provided 0.8 g egg protein per kg IBW plus 0.7 g carbohydrate per kg IBW. Eleven moderately obese healthy young women were studied. After 3 weeks of dieting, fasting plasma glucose appearance and oxidation decreased by equal amounts (20% and 30%, respectively) for both diets. 3-methylhistidine excretion remained at control rates for the first week on the diets, then fell by equal amounts (25% to 30%) with both diets. Similar declines were observed for both diets in serum prealbumin and retinol-binding protein concentrations. Mean serum transferrin declined with both diets, but the changes were not statistically significant. Serum albumin was unchanged by either diet. Thus, there were no significant differences between the two diets with regard to any of the measured parameters.

The purpose of this paper is to attempt to define the upper limit of the safe range of protein intake, with particular reference to the protein content of prepared fixed-formulas used for feeding healthy, term infants. For discussion purposes we use the current upper limit proposed by the U.S. Food and Drug Administration (FDA), namely, 4.5 g protein per 100 kcal, as our initial reference level. To help reach a conclusion, the concept and definitions of nutritional adaptation and accommodation are considered, followed by a brief review of selected studies in full-term infants fed varying levels of protein intake. Based on growth and blood biochemical data, principally plasma free amino acid levels, we conclude that the currently proposed FDA upper limit is probably too high. The available data support a recommendation for lowering the value to about 3.5 g of protein per 100 kcal. Concerns for renal solute load (RSL) may require a further, desirable refinement in the value proposed. Indeed RSL should perhaps serve as the primary basis for establishing a rational and safe upper limit for the protein content of prepared fixed-formula diets for the very young, healthy infant.

In this short review, some aspects of body protein and amino-acid metabolism during aging in human subjects have been explored. There is a progressive diminution of total body protein with aging, due largely to a decline in the size of the skeletal muscle mass. These changes are accompanied by a shift in the overall pattern of whole body protein synthesis and breakdown, with muscle mass estimated to account for about 30 per cent of whole body protein turnover in the young adult, as compared with a lower value of 20 per cent or less in the elderly subject. Because skeletal muscle mass plays an important role in the response of body protein and amino-acid metabolism to stress, such as infection and trauma, this decline in the contribution of muscle to total body protein metabolism might be a factor responsible for the reduced ability of older people to withstand unfavorable circumstances. The determination of the dietary requirements for individual essential amino acids and for total protein has been discussed, and it is evident that the data are limited and often contradictory. However, elderly individuals are more likely to be affected by various biological, environmental, and social factors, which would generally increase protein needs above those for younger adults. Thus, in practice, the protein needs in the elderly are likely to be higher than those in the young. The decline in energy intake, together with its possible consequences for reducing the efficiency of dietary protein utilization, also will tend to increase the protein need for elderly subjects, relative to that for physically more active young adults. Until more data become available, it is recommended, for food planning purposes, that an appropriate protein allowance would be 12 to 14 per cent of the total energy intake, for mixed protein sources characteristic of the diets of industrialized countries or the more affluent sector of populations in developing countries. Energy intake should be at a level that meets the estimates proposed by FAO/WHO/UNU8 for older persons. Higher levels of protein intake would be required to achieve tissue protein repletion following periods of undernutrition or increased body protein (nitrogen) losses due to stressful stimuli of physical or psychological origin.

Five young, mildly obese females consumed a formula diet providing total calories at 1.2 X basal energy expenditure and egg white protein, 1.5 gm./kg. ideal body weight (IBW), for 1 week (period 1). During period 2, lasting 3 weeks, a protein-sparing modified fast (PSMF) consisted of similar amounts of egg white without nonprotein calories followed by a 1-week total fast (period 3). In the final period (4), a PSMF with the use of meat protein, 1.5 gm./kg. IBW, alone was given for 1 week. Nitrogen balance (Nbal) measurements were made in all periods, and periods 2, 3, and 4 were compared with period 1. No significant difference existed between period 1 and period 2, although Nbal improved weekly during period 2. Nitrogen excretion in period 3 was similar to that found after 3 weeks of total fasting and was significantly negative, whereas positive balances occurred in period 4 with both periods being significantly different from period 1. Thus nitrogen equilibrium in the PSMF can be achieved with the use of protein free from fat, confirming earlier experience with meat protein. The metabolic adaption which occurs during toal fasting and results in reduced nitrogen loss develops in the course of PSMF. After a deficit in lean body mass is produced, net protein anabolism can be achieved by a PSMF despite insufficinet dietary energy.

The protein nutritional value of an isolated soy protein (Supro-620) was evaluated in a series of nitrogen balance studies in healthy young male MIT students. Experiment 1 involved giving eight subjects graded intakes of the isolated soy protein while seven additional subjects received egg protein. Mean nitrogen intake required for N balance for the isolated soy protein and the egg protein were not significantly different (p greater than 0.1). In experiment 2, nine subjects received soy and the effects on N balance at various levels of L-methionine supplementation were studied. In experiment 3 eight subjects each were studied at two nitrogen intake levels of isolated soy protein/kg/day with methionine supplementation, and an unsupplemented egg protein period included. Results from experiment 2 and 3 at an 82 mg N/kg/day test level showed that N balance for 1.6% supplementation was significantly below that of unsupplemented egg while 1.1% and unsupplemented soy were indistinguishable from egg. No beneficial effects of methionine supplementation were observed when the test nitrogen intake level was 128 mg N/kg/day. These results indicate that for healthy adults, the isolated soy protein is of high nutritional quality, comparable to that of animal protein sources, and that the methionine content is not limiting for adult protein maintenance.

Estimated rates of total body protein synthesis breakdown, and amino nitrogen flux were determined in five obese females using [15N]glycine. The subjects were fed a baseline diet of 1.5 g protein/kg ideal BW and 1602 +/- 146 Cal total for at least 3 days, followed by 3 weeks where the sole caloric intake was a mixed diet of 0.8 g meat protein and 0.7 g carbohydrate/kg ideal BW and approximately 437 +/- 44 Cal. Amino nitrogen flux, total body protein synthesis, and breakdown values were 289 +/- 56, 198 +/- 37, and 202 +/- 47 g protein/day (187 +/- 24, 129 +/- 19, and 131 +/- 21 g protein/g creatinine) initially and fell significantly (P less than 0.01) to 192 +/- 30 (P less than 0.005), 138 +/- 27 (P less than 0.01), and 146 +/- 25 (P less than 0.01) g protein/day [125 +/- 13 (P less than 0.001), 90 +/- 14 (P less than 0.005), and 95 +/- 11 (P less than 0.005) g protein/g creatinine] by the final 60 h of the third week. Nitrogen balance remained significantly negative (P less than 0.01) during each week of the mixed diet and overall (--3.3 +/- 0.9 g N/day; P less than 0.01). Thus, total body protein synthesis and the net balance between synthesis and catabolism are not maintained with this diet.