Methods to assess amino acid requirements in humans

Post-prandial tracer studies of protein and amino acid utilisation: what can they tell us about human amino acid and protein requirements?

Nitrogen balance (NB), the principal methodology used to derive recommendations for human protein and amino acid requirements, has been widely criticised, and calls for increased protein and amino acid requirement recommendations have been made, often on the basis of post-prandial amino acid tracer kinetic studies of muscle protein synthesis, or of amino acid oxidation. This narrative review considers our knowledge of the homeostatic regulation of the FFM throughout the diurnal cycle of feeding and fasting and what can and has been learnt from post-prandial amino acid tracer studies, about amino acid and protein requirements. Within the FFM, muscle mass in well fed weight-stable adults with healthy lifestyles appears fixed at a phenotypic level within a wide range of habitual protein intakes. However homoeostatic regulation occurs in response to variation in habitual protein intake, with adaptive changes in amino acid oxidation which influence the magnitude of diurnal losses and gains of body protein. Post-prandial indicator amino acid oxidation (IAAO) studies have been introduced as an alternative to NB and to the logistically complex 24 h [13C-1] amino acid balance studies, for assessment of protein and amino acid requirements. However, a detailed examination of IAAO studies shows both a lack of concern for homeostatic regulation of amino acid oxidation and  major flaws in their design and analytical interpretation, which seriously constrain their ability to provide reliable values. New ideas and a much more critical approach to existing work is needed if real progress is to be made in the area.

Is It Time to Reconsider the U.S. Recommendations for Dietary Protein and Amino Acid Intake?

Since the U.S. Institute of Medicine's recommendations on protein and amino acid intake in 2005, new information supports the need to re-evaluate these recommendations. New lines of evidence include: (1) re-analysis/re-interpretation of nitrogen balance data; (2) results from indicator amino acid oxidation studies; (3) studies of positive functional outcomes associated with protein intakes higher than recommended; (4) dietary guidance and protein recommendations from some professional nutrition societies; and (5) recognition that the synthesis of certain dispensable amino acids may be insufficient to meet physiological requirements more often than previously understood. The empirical estimates, theoretical calculations and clinical functional outcomes converge on a similar theme, that recommendations for intake of protein and some amino acids may be too low in several populations, including for older adults (≥65 years), pregnant and lactating women, and healthy children older than 3 years. Additional influential factors that should be considered are protein quality that meets operational sufficiency (adequate intake to support healthy functional outcomes), interactions between protein and energy intake, and functional roles of amino acids which could impact the pool of available amino acids for use in protein synthesis. Going forward, the definition of "adequacy" as it pertains to protein and amino acid intake recommendations must take into consideration these critical factors.

Plasma and Urinary Amino Acid-Derived Catabolites as Potential Biomarkers of Protein and Amino Acid Deficiency in Rats

OBJECTIVE

Dietary intakes must cover protein and essential amino acid (EAA) requirements. For this purpose, different methods have been developed such as the nitrogen balance method, factorial method, or AA tracer studies. However, these methods are either invasive or imprecise, and the Food and Agriculture Organization of the United Nations (FAO, 2013) recommends new methods and, in particular, metabolomics. The aim of this study is to determine total protein/EAA requirement in the plasma and urine of growing rats.

METHODS

36 weanling rats were fed with diets containing 3, 5, 8, 12, 15, and 20% protein for 3 weeks. During experimentation, urine was collected using metabolic cages, and blood from the portal vein and vena was taken at the end of the experiment. Metabolomics analyses were performed using LC-MS, and the data were analyzed with a multivariate analysis model, partial least Squares (PLS) regression, and independent component-discriminant analysis (ICDA). Each discriminant metabolite identified by PLS or ICDA was tested by one-way ANOVA to evaluate the effect of diet.

RESULTS

PLS and ICDA allowed us to identify discriminating metabolites between different diet groups. Protein deficiency led to an increase in the AA catabolism enzyme systems inducing the production of breakdown metabolites in the plasma and urine.

CONCLUSION

These results indicate that metabolites are specific for the state of EAA deficiency and sufficiency. Some types of biomarkers such as AA degradation metabolites appear to be specific candidates for protein/EAA requirement.

Protein intakes of pregnant women and children in India-protein quality implications

The recent National Family Health Survey (NFHS-4, 2016) reports a national average of 18% for low birthweight (LBW) and 38% for stunting in children <5 years. Nutrition and environmental influences (chronic enteric pathogenic exposure through poor water, sanitation, and hygiene) are two critical factors that impact the health outcomes of the populxation. This is particularly relevant for vulnerable age groups such as pregnant women and children <5 years, who bear long-lasting and intergenerational consequences of impoverished nutrition and suboptimal living conditions. The present review provides, for the first time, an analysis of indispensable amino acid (IAA) requirements for pregnant women, separately for the second and third trimesters, using protein accretion data from a recent Indian study. Furthermore, using these estimates for pregnancy, and the current IAA requirements for young children, the quality of protein was assessed in Indian diets consumed by pregnant women and children (1-3 and 4-6 years) from national representative rural National Nutrition Monitoring Bureau survey. The assessment was considered in the context of an adverse environment and in relation to outcomes such as LBW, stunting, and underweight. Finally, an assessment was made of the proportion of the surveyed population at risk of dietary quality protein inadequacy and implications for planning nutrition intervention programmes. Specifically, state-wise estimates of the risk of quality protein inadequacy are provided, in addition to evaluations of additional dietary supplementation, which could inform the policy of supplementary nutrition programmes to improve health outcomes.

Should Competitive Bodybuilders Ingest More Protein than Current Evidence-Based Recommendations?

Bodybuilding is an aesthetic sport whereby competitors aspire to achieve a combination of high levels of muscularity combined with low levels of body fat. Protein is an important macronutrient for promoting muscle growth, and meeting daily needs is necessary to optimize the accretion of lean mass. Current recommendations for muscle hypertrophy suggest a relative protein intake ranging from 1.4 g/kg/day up to 2.0 g/kg/day is required for those involved in resistance training. However, research indicates that the actual ingestion of protein in competitive bodybuilders is usually greater than advocated in guidelines. The purpose of this current opinion article is to critically evaluate the evidence on whether higher intakes of protein are warranted in competitive bodybuilders. We conclude that competitive bodybuilders may benefit from consuming a higher protein intake than what is generally prescribed for recreationally trained lifters; however, the paucity of direct research in this population makes it difficult to draw strong conclusions on the topic.

Dietary protein intake and human health

A protein consists of amino acids (AA) linked by peptide bonds. Dietary protein is hydrolyzed by proteases and peptidases to generate AA, dipeptides, and tripeptides in the lumen of the gastrointestinal tract. These digestion products are utilized by bacteria in the small intestine or absorbed into enterocytes. AA that are not degraded by the small intestine enter the portal vein for protein synthesis in skeletal muscle and other tissues. AA are also used for cell-specific production of low-molecular-weight metabolites with enormous physiological importance. Thus, protein undernutrition results in stunting, anemia, physical weakness, edema, vascular dysfunction, and impaired immunity. Based on short-term nitrogen balance studies, the Recommended Dietary Allowance of protein for a healthy adult with minimal physical activity is currently 0.8 g protein per kg body weight (BW) per day. To meet the functional needs such as promoting skeletal-muscle protein accretion and physical strength, dietary intake of 1.0, 1.3, and 1.6 g protein per kg BW per day is recommended for individuals with minimal, moderate, and intense physical activity, respectively. Long-term consumption of protein at 2 g per kg BW per day is safe for healthy adults, and the tolerable upper limit is 3.5 g per kg BW per day for well-adapted subjects. Chronic high protein intake (>2 g per kg BW per day for adults) may result in digestive, renal, and vascular abnormalities and should be avoided. The quantity and quality of protein are the determinants of its nutritional values. Therefore, adequate consumption of high-quality proteins from animal products (e.g., lean meat and milk) is essential for optimal growth, development, and health of humans.

Controlled trial of whole body protein synthesis and plasma amino acid concentrations in yearling horses fed graded amounts of lysine

Lysine has been reported as the first limiting amino acid in typical equine diets. Indicator amino acid oxidation (IAAO) has become the standard method for determining amino acid requirements in other species, but prior to this study, it has not been used to determine equine requirements. The aim of this study was to evaluate whole body protein synthesis and plasma and muscle amino acid concentrations in response to graded levels of lysine intake in yearling horses. Six Thoroughbred colts (358 ± 5 kg) were fed each of six treatment lysine intakes ranging from 76 to 136 mg/kg body weight/day. Blood samples were taken before and 90 min after the morning concentrate meal. Gluteal muscle biopsies were taken ~100 min after the morning concentrate meal. The next day, whole body phenylalanine kinetics were determined using a 2 h primed, constant infusion of [(13)C] sodium bicarbonate followed by a 6 h primed, constant infusion of [1-(13)C] phenylalanine. Plasma lysine concentrations increased linearly (P <0.05) at both the 0 and 90 min time points with increasing lysine intakes. Free muscle asparagine, aspartate, arginine, glutamine, lysine, taurine and tryptophan concentrations responded quadratically to lysine intake (P <0.05). Phenylalanine kinetics did not differ between treatment intakes (P > 0.10). A broken line analysis of lysine intake and phenylalanine oxidation failed to yield a breakpoint from which to determine a lysine requirement. These diets may have been limiting in an amino acid other than lysine, underscoring the lack of data concerning amino acid requirements and bioavailability data in the horse.

Human Protein and Amino Acid Requirements

Human protein and amino acid nutrition encompasses a wide, complex, frequently misunderstood, and often contentious area of clinical research and practice. This tutorial explains the basic biochemical and physiologic principles that underlie our current understanding of protein and amino acid nutrition. The following topics are discussed: (1) the identity, measurement, and essentiality of nutritional proteins; (2) the definition and determination of minimum requirements; (3) nutrition adaptation; (4) obligatory nitrogen excretion and the minimum protein requirement; (5) minimum versus optimum protein intakes; (6) metabolic responses to surfeit and deficient protein intakes; (7) body composition and protein requirements; (8) labile protein; (9) N balance; (10) the principles of protein and amino acid turnover, including an analysis of the controversial indicator amino acid oxidation technique; (11) general guidelines for evaluating protein turnover articles; (12) amino acid turnover versus clearance; (13) the protein content of hydrated amino acid solutions; (14) protein requirements in special situations, including protein-catabolic critical illness; (15) amino acid supplements and additives, including monosodium glutamate and glutamine; and (16) a perspective on the future of protein and amino acid nutrition research. In addition to providing practical information, this tutorial aims to demonstrate the importance of rigorous physiologic reasoning, stimulate intellectual curiosity, and encourage fresh ideas in this dynamic area of human nutrition. In general, references are provided only for topics that are not well covered in modern textbooks.

Methionine requirement of the enterally fed term infant in the first month of life in the presence of cysteine

BACKGROUND

The essential amino acid methionine can be used for protein synthesis but also serves as a precursor for homocysteine and cysteine.

OBJECTIVE

The objective of this study was to determine the minimal obligatory methionine requirement of infants in the presence of excess cysteine (91 mg ⋅ kg(-1) ⋅ d(-1)) by using the indicator amino acid oxidation (IAAO) method with l-[1-(13)C]phenylalanine as the indicator.

DESIGN

Fully enterally fed term infants <1 mo of age were randomly assigned to methionine intakes that ranged from 3 to 59 mg ⋅ kg(-1) ⋅ d(-1) as part of an elemental formula. After 1 d of adaptation to the test diet, [(13)C]bicarbonate and l-[1-(13)C]phenylalanine tracers were given enterally. Breath samples were collected at baseline and during isotopic plateaus. The mean methionine requirement was determined by using biphasic linear regression crossover analysis on the fraction of (13)CO(2) recovery from l-[1-(13)C]phenylalanine oxidation (F(13)CO(2)). Data are presented as means ± SDs.

RESULTS

Thirty-three neonates (gestational age: 39 ± 1 wk) were studied at 13 ± 6 d. With increasing methionine intakes, F(13)CO(2) decreased until a methionine intake of 38 mg ⋅ kg(-1) ⋅ d(-1); additional increases in methionine intake did not affect F(13)CO(2). The mean methionine requirement was determined at 38 mg ⋅ kg(-1) ⋅ d(-1), and the upper and lower CIs were 48 and 27 mg ⋅ kg(-1) ⋅ d(-11), respectively (P < 0.0001, r(2) = 0.59).

CONCLUSIONS

Although the current recommended methionine intake of 28 mg ⋅ kg(-1) ⋅ d(-1) is within the CIs of our study, the estimated mean requirement is substantially higher. However, most of the infant formulas provide a methionine intake of 49-80 mg ⋅ kg(-1) ⋅ d(-1), which is above the upper CI of our study. This trial was registered at www.trialregister.nl as NTR1610.