Human Amino Acid and Protein Requirements: Current Dilemmas and Uncertainties

Development and the art of nutritional maintenance

Development from early conceptus to a complex, multi-cellular organism is a highly ordered process that is dependent on an adequate supply of nutrients. During this process, the pattern of organ growth is robust, driven by a genetic blueprint and matched to anticipated body mass with high precision and with built-in physiological reserve capacity. This apparent canalisation of the developmental process is particularly sensitive to variation in environmental stimuli, such as inappropriate drug or hormone exposure, or pattern of nutrient delivery. Significant variation in any of these factors can profoundly affect fetal and neonatal growth patterns, with later detriment for physiological function and/or reserve capacity of the resultant adult, with potential health impact. This paradigm shift in science has become known as the Developmental Origins of Health and Disease (DOHaD). Over the last 30 years, many animal and clinical studies have vastly expanded our fundamental knowledge of developmental biology, particularly in the context of later effects on health. In this horizons article, we discuss DOHaD through the lens of nutritional quality (e.g. micronutrient, amino acid, NSP intake). The concept of ‘Quality’ was considered undefinable by Robert Persig in his book, ‘Zen and the Art of Motorcycle Maintenance’. Here, development and the art of nutritional maintenance will define quality in terms of the pattern of nutrient intake, the quality of development and how each interact to influence later health outcomes.

Dispensable Amino Acids, except Glutamine and Proline, Are Ideal Nitrogen Sources for Protein Synthesis in the Presence of Adequate Indispensable Amino Acids in Adult Men

BACKGROUND

Nutritionally, there is a dietary requirement for indispensable amino acids (IAAs) but also a requirement for nitrogen (N) intake for the de novo synthesis of the dispensable amino acids (DAAs). It has been suggested that there might be a dietary requirement for specific DAAs.

OBJECTIVES

Experiment 1 tested whether 9 of the DAAs (Ala, Arg, Asn, Asp, Gln, Glu, Gly, Pro, Ser) are ideal N sources using the indicator amino acid oxidation (IAAO) technique. Experiment 2 examined whether there is a dietary requirement for Glu in adult men.

METHODS

Seven healthy men (aged 20-24 y) participated in 11 or 2 test diet intakes, in experiment 1 and 2, respectively, in a repeated measures design. In experiment 1, a base diet consisting of the IAA provided at the RDA was compared with test intakes with the base diet plus addition of individual DAAs to meet a 50:50 ratio of IAA:DAA on an N basis. In experiment 2, the diets corresponded to the amino acid pattern present in egg protein, in which all Glu and Gln was present as Glu, or removed, with Ser used to make the diets isonitrogenous. On each study day the IAAO protocol with l-[1-13C]phenylalanine was used to measure whole-body protein synthesis.

RESULTS

In experiment 1, repeated measures ANOVA with post hoc multiple comparisons showed that 7 of the 9 DAAs (Ala, Arg, Asn, Asp, Glu, Gly, Ser) decreased IAAO significantly (P < 0.05) compared with base IAA diet, the exceptions being Gln and Pro. In experiment 2, a paired t test did not find significant (P > 0.05) differences in the IAAO in response to removal and replacement of Glu intake.

CONCLUSIONS

The results suggest that in healthy men most DAAs are ideal N sources for protein synthesis, in the presence of adequate IAAs, and that endogenous synthesis of Glu is sufficient.Registered clinicaltrials.gov identifier: NCT02009917.

Human Amino Acid Requirements: A Re-Evaluation

The most recent internationally stated estimates of the amino acid requirements in adult humans are those given in the 1985 report of the Joint FAO/WHO/UNU Expert Consultation on Energy and Protein Requirements. In this review we present, in brief a number of scientific concerns and problems that lead us to conclude that these current recommendations for amino acid requirements are not valid and that the appropriate values are likely to be considerably higher. Following a short review of the C-labelled amino acid tracer studies carried out at the Massachusetts of Technology (MIT) and designed to reassess the requirements for specific indispensable amino acids, we focus particular attention on the lysine requirement in adults. When various criteria and methods are used to estimate this requirement, it appears that a cohesive body of data indicates the mean requirement value for lysine in healthy adults to be about 30 mg/kg/day or 50 mg/g protein. Although this value contrasts with the FAO/WHO/UNU value of 12 mg/kg/day or 16 ma/g protein, this new, tentative requirement value is consistent with findings from studies carried out earlier at MIT on the nutritional quality of wheat proteins. We propose that it would be prudent to apply the MIT amino acid requirement pattern (see Food and Nutrition Bulletin 1990;12:298–300), rather than the 1985 FAO/WHO/UNU adult amino acid requirement pattern, in the design and implementation of sound nutrition policies and programmes that include considerations of the amount and quality of the protein component of national and regional diets.

Protein digestibility-corrected amino acid scores and digestible indispensable amino acid scores differentially describe protein quality in growing male rats

BACKGROUND

The FAO has recommended replacing the protein digestibility-corrected amino acid score (PDCAAS) with the digestible indispensable amino acid score (DIAAS).

OBJECTIVE

The objective of this study was to compare aspects underlying the calculation of the DIAAS and PDCAAS, including 1) fecal digestibility vs. ileal digestibility, 2) using a single nitrogen digestibility value for all amino acids, and 3) the effect of truncation. Truncated PDCAAS and untruncated DIAAS values calculated as formally defined were also compared and DIAAS data presented for 14 dietary protein sources.

METHODS

Semisynthetic wheat starch-based diets were formulated to contain the test protein (as consumed by humans) source (whey- and soy-protein isolates, milk-, whey-, rice- and pea- protein concentrates, cooked kidney beans, roasted peanuts, cooked peas, corn-based breakfast cereal, cooked rice, cooked rolled oats, and wheat bran) as the sole nitrogen source and with an indigestible marker (titanium dioxide). Growing male rats (∼250 g bodyweight) were given a basal casein-based diet from day 1 to day 7 and then allocated (n = 6) to the test diets for day 8 to day 14 before ileal digesta were collected after the rats were killed. Total feces were collected from day 11 to day 14.

RESULTS

True fecal nitrogen digestibility was different (P < 0.05; 10% difference on average) from true ileal nitrogen digestibility for 11 of the 14 protein sources. True ileal nitrogen digestibility was different (P < 0.05) from true ileal amino acid digestibility for almost half of the indispensable and conditionally indispensable amino acids (differences ranged from 0.9% to 400%). DIAAS values ranged from 0.01 for a corn-based cereal to 1.18 for milk protein concentrate.

CONCLUSION

Untruncated PDCAAS values were generally higher than a DIAAS values, especially for the poorer quality proteins; therefore, the reported differences in the scores are of potential practical importance for populations in which dietary protein intake may be marginal.

Protein/energy ratios of current diets in developed and developing countries compared with a safe protein/energy ratio: implications for recommended protein and amino acid intakes

Revised estimates of protein and amino acid requirements are under discussion by the Food and Agriculture Organization (FAO)/World Health Organizaion (WHO), and have been proposed in a recent report on Dietary Reference Intakes (DRIs) from the USA. The nature and magnitude of these requirements are not entirely resolved, and no consideration has been given to the potential influence of metabolic adaptation on dietary requirements. We have examined the implications of these new values, and of the conceptual metabolic framework in which they are used, for defining the nutritional adequacy of protein intakes in developed and developing countries. We have expressed proposed values for protein requirements in relation to energy requirements, predicted for physical activity levels of 1.5, 1.75 and 2.0 times basal metabolic rate, in order to generate reference ratios for protein energy/total energy (reference P/E ratio) as a function of age, body weight, gender and physical activity level. Proposed values for amino acid requirements have been used to adjust the available digestible P/E ratio of foods and diets for protein quality. Focusing on the diets of UK omnivores and vegetarians and on diets in India, the risk of protein deficiency is evaluated from a comparison of P/E ratios of metabolic requirements with protein-quality-adjusted P/E ratios of intakes. A qualitative and conservative estimate of risk of deficiency is made by comparing the adjusted P/E ratio of the intake with a reference P/E ratio calculated for age, body weight, gender and physical activity according to FAO/WHO/United Nations University. A semi-quantitative estimate of risk of deficiency has also been made by the cut point approach, calculated as the proportion of the intake distribution below the mean P/E ratio of the requirement. Values for the quality-adjusted P/E ratio of the diet range from 0.126 for the UK omnivore diet to 0.054 for a rice-based diet of adults in West Bengal, which is lysine-limited, falling to 0.050 for 1-year-old children. The reference P/E ratio for men and women increases with age, is higher for females than males, is higher for small compared with large adults at any age and decreases with physical activity. Thus if a particular diet is potentially limiting in protein, protein deficiency is most likely in large, elderly sedentary women followed by the adolescent female and least likely in moderately active young children, the opposite of what has usually been assumed. Within the currently accepted framework, the diets do not meet the protein needs of the entire population of the UK, have a significant risk of deficiency throughout India for all except extremely active small adults, and are grossly inadequate for all population groups, apart from physically active young children in West Bengal, regardless of body weight or level of food intake. The lysine limitation of the cereal-based Indian diets is dependent on the choice of lysine requirement values from the published range. We consider that the value selected is too high, because of uncertainties and inconsistencies in the approaches used. A more appropriate choice from the lower end of the range would remove the lysine limitation of cereal-based diets, and reduce some of the perceived risk of deficiency. However, diets remain limited by the amount of digestible protein for many population groups, especially in West Bengal. In the context of risk management, one option would be to accept the current values and the conceptual metabolic framework within which they have been derived. This would have major implications for the supplies of high-quality protein to the developing countries. An alternative option would be to re-evaluate the currently proposed values for the requirements for protein and amino acids. We conclude that the choice of values for the adult lysine requirement should be re-evaluated and that serious consideration should be given to the extent to which adaptive mechanisms might enable the metabolic requirement for protein to be met from current intakes. This will entail a better understanding of the relationships between dietary protein and health.

What are the essential elements needed for the determination of amino acid requirements in humans?

The aims of this introductory article are to survey and critically evaluate the concepts and approaches that have been used to assess amino acid adequacy and to hypothesize about possible future directions of research. The issue in question is extensive, consequently this article will be limited to: 1) definitions of amino acid requirements; 2) available techniques to assess amino acid requirements; 3) actual recommendations for healthy adults; 4) factors influencing requirements; and 5) requirements in acute and chronic wasting diseases. Recommendations for amino acid intakes for healthy adults were proposed by the FAO/WHO expert committee in 2001. They have not yet been published. The major factors affecting amino acid requirements are the stage of development, reproductive state, environmental factors, digestibility of dietary proteins, genotype of the individual, and pathological conditions. Remarkably, there are no conclusive data relative to changes in requirements induced by infection, injury, trauma, and renal or liver failure. Future research using modern methods to evaluate requirements must thus receive a high priority. Wasting diseases are associated with deficiencies and imbalances of particular amino acids causing specific changes in requirements. Consequently, a new approach has been used to categorize amino acids as conditionally indispensable according to their functional and physiological properties. Kinetic measurements of plasma amino acids might help to estimate qualitative requirements. Measurement of tissue intracellular free amino acid deficiencies or excesses is another method to estimate qualitative requirements. Based on these measurements tentative values for conditionally indispensable amino acids during disease are given in the article.

An adaptive metabolic demand model for protein and amino acid requirements

The shortcomings of the metabolic implications of the current protein requirements model are reviewed, and an alternative model, validated with [1-(13)C]leucine balance results in human adults, is presented and evaluated in the context of defining protein requirements. The model identifies metabolic demands for amino acids as comprising a small fixed component and a variable adaptive component that is relatively insensitive to acute food or protein intake, but which changes slowly with a sustained change in intake, enabling N equilibrium to be achieved. The model accounts for the apparent low efficiency of utilisation of animal proteins in N balance studies and enables more realistic efficiency values to be measured within an experimental framework that takes account of the adaptive metabolic demand. However, the complex relationship between the adaptive metabolic demand and habitual level and quality of protein intake prevents prediction of protein quality by amino acid scoring, which can markedly underestimate actual values. In contrast to the current model, for fully adapted individuals risk of deficiency (i.e. negative N balance after complete adaptation) will only start to increase when intakes fall below the range of the true minimum requirements, i.e. a value that is currently unknown, but likely to be between 0.40 and 0.50 g/kg per d at the lower end of the reported distribution of requirements. At intakes greater than this with additional metabolic demands varying directly with intake, deficiency is only likely as a short-term response to a change to a lower intake within the adaptive range. Thus, for adults satisfying energy needs on most mixed human diets, intakes will be within the adaptive range, and N equilibrium ceases to be a useful indicator of nutritional adequacy of protein. In the context of prescriptive dietary guidelines it may be expedient to retain current values until the benefits (and any risks) of protein intakes within the adaptive range can be quantified. However, from a diagnostic perspective, indicators other than N balance need to be identified, since maintenance of N balance can no longer be used as a surrogate of adequate protein-related health.

Protein and amino acid requirements of adults: current controversies

Protein intakes vary widely but costs and benefits of such variation is a long standing unresolved issue. The wide range of reported values for the minimum protein intake for N equilibrium in adults, i.e. 0.39 to 1.09 g/kg is best explained by an Adaptive Metabolic Demands model in which metabolic demands include amino acid oxidation at a rate varying with habitual protein intake and which changes slowly with dietary change. Thus within the reported data the true minimum requirement intake, the lowest values in the range at intakes approaching the Obligatory Nitrogen Loss, allows only fully adapted subjects to achieve N equilibrium. The higher values reflect incomplete adaptation. (13)C-1 leucine tracer balance studies of this model show (a) a fall with age in apparent protein requirements, (b) better than predicted efficiency of wheat protein utilization, and (c) controversially, lower lysine requirements than other workers, consistent with new evidence of de novo synthesis of lysine from urea salvaged by large bowel microflora. The main implication of the requirements model for athletes on high protein diets is increased exercise induced amino acid oxidation and risk of loss of body N when such high intakes are not maintained.

Threonine requirement of healthy adults, derived with a 24-h indicator amino acid balance technique

BACKGROUND

Because we question the validity of the 1985 FAO/ WHO/UNU upper requirement for threonine of 7 mg x kg(-1) x d(-1), we proposed a tentative mean requirement of 15 mg x kg(-1) x d(-1).

OBJECTIVE

Our goal was to assess threonine adequacy at 3 test intakes and the consequences of a 6-d compared with a 13-d dietary adaptation phase.

DESIGN

We used a 24-h indicator amino acid balance technique ([1-(13)C]leucine as indicator) to assess the threonine requirement. Fifteen healthy adults were randomly assigned to receive 7, 15, or 46 mg threonine x kg(-1) x d(-1) and were studied after 6 and 13 d of adaptation to the experimental diets. Diets were based on an L-amino acid mixture in which the threonine content was varied. At 1700 on days 6 and 13, a 24-h intravenous [(13)C]leucine tracer infusion protocol was begun to assess leucine oxidation and daily leucine balances.

RESULTS

There was no detectable effect of duration of dietary adaptation in leucine oxidation or balance, but the 24-h leucine oxidation and balances differed significantly between the 7-mg intake and each of the 2 higher intakes (P < 0.05). The latter were not significantly different. The 24-h leucine oxidation rate decreased across threonine intakes (P < 0.01 for main effect of diet, independent of infusion day). Leucine oxidation was highly correlated (r = 0.80) between the 2 dietary adaptation phases across all test intakes.

CONCLUSION

The 1985 FAO/WHO/UNU threonine recommendation is inadequate, and 15 mg x kg(-1) x d(-1) is sufficient to achieve mean indicator (leucine) amino acid balance.

Tyrosine requirement of healthy men receiving a fixed phenylalanine intake determined by using indicator amino acid oxidation

BACKGROUND

The currently accepted total aromatic amino acid requirement for adults is based on nitrogen balance measurements in individuals who received their intake of aromatic amino acids solely as phenylalanine.

OBJECTIVE

The objective of this study was to determine the requirement for the amino acid tyrosine in healthy men receiving an adequate, but not excessive, intake of phenylalanine (9 mg x kg(-1) x d(-1)).

DESIGN

The effect of a graded intake of tyrosine was determined in 6 healthy men consuming energy-sufficient diets containing 1 g protein x kg(-1) x d(-1). The tyrosine requirement was determined by using indicator amino acid oxidation methodology with L-[1-13C]lysine as the indicator. Subjects were studied at each of 7 tyrosine intakes.

RESULTS

A graded intake of tyrosine had no effect on lysine flux. The mean tyrosine requirement was determined from the response of the oxidation of L-[1-13C]lysine to breath 13CO2. A 2-phase linear regression crossover analysis of breath 13CO2 identified the breakpoint and upper 95% confidence limit, which represents the mean and safe intakes, to be 6.0 and 7.0 mg x kg(-1) x d(-1), respectively.

CONCLUSIONS

The safe intake of total aromatic amino acids calculated from the present results for tyrosine and our previous estimate for phenylalanine is estimated to be 21 mg x kg(-1) x d(-1). This intake is 1.5 times the currently recommended total aromatic amino acid intake of the FAO/WHO/UNU (1985), 14 mg x kg(-1) x d(-1). Furthermore, the absolute aromatic amino acid requirement may be dependent on the proportional balance of these amino acids in the diet.

Statistical analysis of nitrogen balance data with reference to the lysine requirement in adults

Statistical analysis of nitrogen balance data is an important approach to the estimation of human nutrient requirement. The usual procedure is to regress nitrogen (N) balance on intake and to define the requirement as intake that would produce zero balance. In the actual application of this methodology, there are a number of options, and in the present study we explore the sensitivity of the regression approach to those options. To put this problem into a realistic context, we examine the current controversy over the lysine requirements of healthy adults. From early N balance studies, investigators concluded that the mean requirement was generally less than 10 mg. kg(-1). d(-1), whereas based on recent (13)C-tracer and metabolic studies, we propose a tentative mean requirement of approximately 30 mg. kg(-1). d(-1). Jones et al. (1956) conducted careful N balance studies from which they derived an estimate of lysine requirement of less than about 8 mg. kg(-1). d(-1). We reanalyzed these data with different choices of modes of analysis, mathematical models, and different assumptions concerning the magnitude of miscellaneous N losses. We find that for these data the choice of a specific mathematical model has only a small effect on resultant estimates of requirement, while estimated requirements are very sensitive to amount of unmeasured losses that are assumed and how the model is applied (whether the aggregate data are fitted in one pass to a single model, or the data for each individual subject are fitted to that individual's unique model). Moreover, our reanalysis suggests that the population requirement for lysine is in the range of 17 to 36 mg. kg(-1). d(-1) and strongly supports a lysine requirement value of about 30 mg. kg(-1). d(-1). In general, our results indicate that whenever possible, N balance data should be analyzed using a square root model fitted to individual data and that the median of the individual requirements so derived be used as the estimate of population requirement. Moreover, clearly any statistical analyses of N balance data should include a sensitivity analysis to determine the influence of underlying assumptions. Finally, the finding that these estimates are highly dependent on the assumed amount of N miscellaneous losses recommends that further studies on these losses and of the factors that influence them are essential.

A tracer investigation of obligatory oxidative amino acid losses in healthy, young adults

BACKGROUND

Estimation of the minimum requirement for indispensable amino acids (IAAs) has been attempted by assuming that obligatory oxidative losses (OOLs) of IAAs can be approximated from nitrogen losses and that the efficiency of utilization of IAAs at requirement intakes is approximately 70%.

OBJECTIVE

We wished to determine the rates of OOLs in healthy adults, using L-[1-(13)C]leucine and L-[1-(13)C, methyl-(2)H(3)]methio-nine as tracers, after adjustment to a protein-free diet and how these rates compare with those when either sulfur amino acids (SAAs: methionine and cyst(e)ine) or leucine were removed from an otherwise adequate diet.

DESIGN

Eleven subjects were randomly assigned to a 5-d protein-free diet or a 5-d diet providing adequate nitrogen and amino acids except for the SAAs or leucine. A 24-h constant intravenous infusion of [(15)N, (15)N]urea and L-[1-(13)C]leucine (Leu group; n = 5) or L-[1-(13)C, methyl-(2)H(3)]methionine (Met group; n = 6 ) began at 1800 on day 5 and rates of amino acid oxidation were determined.

RESULTS

Mean (+/-SD) oxidation rates (mg kg(-)(1) d(-)(1)) of methionine and leucine were 6.4 +/- 1.4 and 24.7 +/- 3.6, respectively, with the protein-free diet; rates were significantly lower (3.9 +/- 2.2 and 7. 2 +/- 3.4, respectively) after the SAA- and leucine-free diets. Urea production was significantly lower (P < 0.01) with the protein-free than with the SAA- or leucine-free diet.

CONCLUSIONS

Isotopically determined OOLs for methionine and leucine are consistent with losses predicted from nitrogen excretion, and consistent with our previous measurements of cysteine oxidation as an index of total SAA losses. The data further support our earlier conclusions regarding methionine sparing by cysteine and tentative recommended SAA requirements in adults.

The nutritional value of plant-based diets in relation to human amino acid and protein requirements

The adequacy of plant-based diets in developed and developing countries as sources of protein and amino acids for human subjects of all ages is examined. Protein quantity is shown not to be an issue. Digestibility is identified as a problem for some cereals (millet (Panicum miliaceum) and sorghum (Sorghum sp.)) and generally is poorly understood. Direct measurements of biological value in children are reviewed and scoring is considered. Various existing requirement values for amino acids and especially lysine are reviewed, and it is concluded that stable-isotope studies do not yet provide adequate alternative values of N balance data, which for lysine are robust after recalculation and adjustment. A new maintenance requirement pattern is developed, with higher values than those of Food and Agriculture Organization/World Health Organization/United Nations University (1985) but lower values than the Massachusetts Institute of Technology pattern (Young et al. 1989). Calculations of age-related amino acid requirements are based on most recent estimates of human growth and maintenance protein requirements, a tissue amino acid pattern and the new maintenance amino acid pattern. These values appear valid when used to score plant proteins, since they indicate values similar to or less than the biological value measured directly in young children. When used to score plant-based diets in India, no marked deficiencies are identified. All regions score > 1 for adults, whilst for children scores range from > 1, (Tamil Nadhu) from 6 months of age to 0.78 (West Bengal), rising to 0.9 in the 2-5 year old, consistent with reports that high-lysine maize supports similar weight and height growth to that of casein. Inadequate amino acid supply is not an issue with most cereal-based diets.

Adult human amino acid requirements

This brief review is concerned with the status of our current understanding of the quantitative needs for dietary amino acids in healthy adults. The nutritional significance of the conditionally indispensable amino acids is assessed and the requirements for the indispensable amino acids are discussed. Recent research involving the use of the tracer balance approach is reviewed, and the importance of resolving the current uncertainties regarding adult human amino acid needs is highlighted.

Metabolic demands for amino acids and the human dietary requirement: Millward and rRvers (1988) revisited

In 1988, Millward and Rivers reappraised existing metabolic models for amino acid requirements. The metabolic demand for amino acids was reviewed in relation to both obligatory metabolic consumption and adaptive pathways of amino acid oxidation. The obligatory demand pattern was deemed unknowable from first principles except that the level of one amino acid would be similar to its concentration in an amount of tissue protein equivalent to the obligatory nitrogen loss. The adaptive demand pattern was predicted to vary in relation to the amount and the periodicity of food protein intake that influenced the amplitude of the diurnal cycle of gains and losses. A regulatory influence of protein intake on anabolism, the anabolic drive, was identified in animal studies; benefit appeared to derive from intakes in excess of the minimum for balance, which could facilitate definition of an optimal requirement. The inherent and design-related limitations of both nitrogen and stable isotope balance studies of requirement were recognized as a major problem in identifying secure values for indispensable amino acid requirements. A decade of research of increasing methodological sophistication has generated much new information, confirming the adaptive diurnal model of balance regulation and allowing development of the anabolic drive into a general protein-stat theory for coordinated control of growth and maintenance of the lean body mass. However, notwithstanding several new estimates of amino acid requirement values, definition of a widely accepted human amino acid requirement pattern remains unresolved. Although a case can be made for an adjusted 1985 FAO adult requirement pattern being a reasonable estimate of the obligatory indispensable amino acid requirements for human maintenance, the problems posed by adaptation, methodological inadequacies and lack of independent measures of adequacy mean that assessment of the adequacy of the human diet to satisfy amino acid needs remains inherently difficult.

[15N]-labeled pea flour protein nitrogen exhibits good ileal digestibility and postprandial retention in humans

The aim of the present study was to evaluate postprandial absorption of pea protein as well as exogenous nitrogen retention in humans. For this purpose, after fasting overnight, seven healthy adults (4 males and 3 females) ingested [15N]-labeled pea protein (195 mmol N). Ileal effluents were collected for 8 h at 30-min intervals using a nasointestinal intubation technique. Urine and plasma samples were collected for 24 h. The [15N]-enrichment was determined in the intestinal samples, in the plasma amino acids and urea as well as in the urinary urea and ammonia fractions. The true gastroileal absorption of pea protein was 89.4 +/- 1.1%. This absorption was correlated with a significant increase (P < 0.05) in [15N]-enrichment in the plasma amino acids and in the nitrogen incorporated into the body urea pool for 1 h following pea ingestion. The enrichment remained significantly higher than the basal values in these pools 24 h after pea ingestion. The recovery of total urinary exogenous nitrogen after 22 h was 31.1 +/- 9.3 mmol N. Moreover, the kinetics of [15N]-labeled pea amino acids deamination reached a plateau of 39 mmol. Under these conditions, pea nitrogen retention represented 78% of the absorbed dietary nitrogen in healthy humans. The present results demonstrate the good true nitrogen digestibility and retention of pea protein in humans.

Protein requirements and ageing: metabolic demand and efficiency of utilization

The protein requirements of the elderly were investigated with [13C]leucine balance studies of metabolic demand, the efficiency of postprandial protein utilization (PPU) and the consequent apparent protein requirement. Ten elderly subjects aged 68-91 years (five men and five women) and ten young adult subjects aged 21-31 years (five men and five women) were infused with L-[1-13C]leucine for 9 h commencing in the postabsorptive state (0-3 h), continuing during the half-hourly feeding of low-protein meals (LP; protein 3% energy, 3-6 h), and during similar feeding of isoenergetic higher protein meals (HP; protein 15% energy, 6-9 h). Leucine oxidation and balance were determined from plasma [1-13C]-alpha-ketoisocaproate enrichment and expired 13CO2 excretion measured during the 3rd hour of each 3 h period. The protein intake during the HP phase was similar to the habitual intake estimated in the subjects from 24 h urinary N excretion. Metabolic demand was defined as equal to twice the body-protein equivalent of measured postabsorptive leucine oxidation. The efficiency of PPU was calculated from the increased leucine oxidation observed during feeding, and the apparent protein requirement was defined as metabolic demand/PPU and calculated in relation to both body weight (BW) and fat-free mass (FFM) determined by densitometry or bioimpedance. Metabolic demand in the young adults was 0.83 g protein/kg per d; in both elderly groups it was 36% lower when expressed per kg BW and 30% lower when expressed per kg FFM. The apparent protein requirement calculated from metabolic demand and PPU was 0.99 g protein/kg per d in the young adults and this was also lower in the elderly, although this was only significant in the men (0.66 g per kg BW, P = 0.013; 0.79 g per kg FFM, P = 0.02). The results show that in this group of healthy elderly adults protein requirements as assessed from leucine balance studies were either similar to or less than those of younger adults.

Growth hormone modulates amino acid oxidation in the surgical patient: leucine kinetics during the fasted and fed state using moderate nitrogenous and caloric diet and recombinant human growth hormone

Twelve patients (aged 70 +/- 9 years) who were scheduled for resection of rectosigmoid colon adenocarcinoma but were otherwise healthy were randomly allocated after surgery to receive either peripheral parenteral nutrition alone ([PPN] n = 6) or in combination with recombinant human growth hormone (rGH) at a daily dose of 0.15 U x kg(-1) x d(-1)(PPN + rGH, n = 6). The daily nutritional regimen was 0.1 g nitrogen x kg(-1) x d(-1) and 20 kcal x kg(-1) x d(-1) (nonprotein energy was supplied as 60% lipid and 40% carbohydrate), and it was maintained for 6 days before and 6 days after surgery. Protein kinetics were studied in all 12 patients during the fasted and fed states before and 6 days after surgery using an 8-hour 13C-leucine tracer infusion. Daily urinary nitrogen, gaseous exchange, and plasma insulin, growth hormone, and insulin-like growth factor-I (IGF-I) were determined before and after surgery. Surgery was responsible for significant increases in postabsorptive whole-body protein flux and synthesis and leucine oxidation (P < .01). Supplementation of PPN with rGH contributed to a significant attenuation of the postoperative increase in leucine oxidation (P = .02), with a significant increase in whole-body protein synthesis (P = .02) and no effect on protein breakdown (P = .40). During the fed state, leucine oxidation increased significantly (P = .005), with the greatest change occurring in the PPN group. Feeding was associated with a significant decrease in whole-body protein breakdown before and after surgery in both groups (P = .001). Postoperative urinary nitrogen excretion was lower but was not statistically significant in the PPN + rGH group compared with the PPN group. There was a significant increase in oxygen consumption (VO2) and carbon dioxide production (VCO2) as a result of feeding and surgery (P < .01). Supplementation with rGH caused a decrease in the respiratory quotient (RQ) (P = .04), particularly after surgery, indicating a direct effect of rGH on fatty acid oxidation. Circulating plasma insulin increased significantly in both groups with feeding and rGH supplementation (P < .05). This was enhanced after surgery, particularly in the rGH group (P < .05). Plasma growth hormone decreased after surgery in the PPN group (P < .05), but did not change as a result of feeding. The circulating levels increased in the PPN + rGH group following subcutaneous administration before or after surgery. Plasma IGF-I decreased after surgery in the PPN group (P < .05), and no changes occurred in the PPN + rGH group with feeding. The present findings suggest a distinct positive effect of rGH on protein synthesis in catabolic patients receiving a moderate intake of nitrogen and calories. This is achieved by modulation of amino acid oxidation. The acute effect of intravenous (IV) nutrients on protein metabolism during the catabolic phase of surgical stress caused a direct decrease in protein breakdown with no effect on protein synthesis.

Urine nitrogen as an independent validatory measure of dietary intake: potential errors due to variation in magnitude and type of protein intake

In work aimed at developing methodologies for validation of estimates of the dietary intake of free-living individuals, Bingham and colleagues have examined the use of urinary nitrogen (UN) excretion as an index of protein intake (Bingham & Cummings, 1985; Bingham, 1994; Bingham et al. 1995). The basis of this approach is that in subjects in N equilibrium, N intake is assumed equal to N excretion. Thus, if N excretion is measured it should indicate N and dietary protein intakes. In practice, since most N excretion occurs via the urine, which is relatively simple to collect, Bingham and colleagues examined the relationship between 24 h UN and N intake (DN). They argued that in subjects on typical UK diets UN should bear a fixed relationship to DN, and measured it carefully in a group of subjects. They showed that in a group of eight individuals in which intake and N excretion were measured meticulously, UN was 81% DN (SD 2, range 78–83%). They argued that measurement of this ratio could be used to assess the validity of food intake measurement and concluded ‘In healthy individuals eating normal western diets, 24 h urine N from an 8 day collection verified for its completeness by the PABA check method, should establish urine N to within 81(SD 5)% of the habitual dietary intake, range 70–90%. If the dietary assessment from 18 days of records or 24 h recalls or the diet history falls within these limits, it can be stated that there is no evidence of interference with normal dietary habits, or of reporting errors….’ (Bingham & Cummings, 1985).

Protein requirement of young adult Nigerian females on habitual Nigerian diet at the usual level of energy intake

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.

The nutritional sensitivity of the diurnal cycling of body protein enables protein deposition to be measured in subjects at nitrogen equilibrium

In adults in overall protein balance on intakes of protein equal to or greater than minimum requirements, the diurnal pattern of feeding and fasting results in cycling of body protein, with fasted losses and fed state gains of increasing amplitude with increasing habitual protein intake. Measurement of the slope of the fed state gain-intake relationship enables investigation of the ability of various patient groups to utilise dietary protein, without the need to impose negative nitrogen balance due to sub-maintenance protein intakes.

Protein and energy interactions throughout life. Metabolic basis and nutritional implications

We review selected aspects of the interactions between protein and energy in human metabolism and nutrition. Following a short account of the underlying metabolic basis for the effects of energy on protein metabolism, the contribution made by whole body protein turnover to the metabolic rate is discussed, including the relationship between protein turnover and energy metabolism at different phases of life. The effects of changes in energy metabolism and intake on the nitrogen economy of the host are also reviewed briefly and we explore the relationship between amino acid oxidation and requirements for indispensable amino acids. Interactions between energy and protein metabolism need to be investigated in greater detail and also they must be considered in relation to further attempts to establish more precisely energy and amino acid requirements of people under various circumstances.

Effects of exaggerated amino acid and protein supply in man

A general update review of the dynamic aspect of protein metabolism is presented. The effect of excess protein level on protein metabolism has been the object of a limited number of studies in man. From the information available, it appears that the primary regulatory pathway for body protein homeostasis is the process of amino acid (protein) oxidation.