Aromatic amino acid requirements in healthy men measured by indicator amino acid oxidation

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.

Evaluation of protein requirements using the indicator amino acid oxidation method: a scoping review

BACKGROUND

The indicator amino acid oxidation (IAAO) method has been accepted as an approach to evaluate habitual protein requirements under free-living conditions.

OBJECTIVES

This scoping review reports on literature that evaluated protein requirements in humans using the IAAO methods.

METHODS

Three databases (PubMed/Medline, Web of Science, and ProQuest) were systematically searched to identify studies that evaluated protein requirements using the IAAO method published in English until 5 June, 2023. We evaluated the study quality using previously developed criteria. We extracted the characteristics of the study design and the results of protein requirements. Two reviewers conducted both reviews and quality assessments independently; any differences among them were resolved by consensus or agreement of all team members.

RESULTS

We extracted 16 articles targeting children, young adults (including pregnant women, resistance training athletes, endurance-training athletes, and team sports), and older adults. In quality assessment, 14 studies were evaluated "strong," but the remaining 2 were "moderate." These studies were conducted in only 3 countries and did not include all sexes or life stages. The range of the estimated average protein requirements of each life stage was 1.30 g/kg body weight/d for children, 0.87 to 2.10 (0.87-0.93 for general young adults, 1.22-1.52 for pregnant women, 1.49-2.00 for resistance-trained athletes, 1.65-2.10 for endurance athletes, and 1.20-1.41 for team sports athletes) g/kg body weight/d for young adults, and 0.85 to 0.96 g/kg body weight/d for older adults.

CONCLUSIONS

Protein requirements in 14 studies were higher than the current reference for each sex, life stage, and physical activity that are related to protein requirements. In the future, protein requirements of various populations including sex and life stage could be assessed using the IAAO methods worldwide.

Determination of a steady-state isotope dilution protocol for carbon oxidation studies in the domestic cat

The present study aimed to develop an isotope protocol to achieve equilibrium of ¹³CO₂ in breath of cats during carbon oxidation studies using L-[1-¹³C]-Phenylalanine (L-[1-¹³C]-Phe), provided orally in repeated meals. One adult male cat was used in two experiments. In each experiment, three isotope protocols were tested in triplicate using the same cat. During carbon oxidation study days, the cat was offered thirteen small meals to achieve and maintain a physiological fed state. In experiment 1, the isotope protocols tested (A, B and C) had a similar priming dose of NaH¹³CO₃ (0⋅176 mg/kg; offered in meal 6), but different priming [4⋅8 mg/kg (A) or 9⋅4 mg/kg (B and C); provided in meal 6] and constant [1⋅04 mg/kg (A and B) or 2⋅4 mg/kg (C); offered in meals 6-13] doses of L-[1-¹³C]-Phe. In experiment 2, the isotope protocols tested (D, E and F) had similar priming (4⋅8 mg/kg; provided in meal 5) and constant (1⋅04 mg/kg; provided in meals 5-13) doses of L-[1-¹³C]-Phe, but increasing priming doses of NaH¹³CO₃ (D: 0⋅264, E: 0⋅352, F: 0⋅44 mg/kg; provided in meal 4). Breath samples were collected using respiration chambers (25-min intervals) and CO₂ trapping to determine ¹³CO₂:¹²CO₂. Isotopic steady state was defined as the enrichment of ¹³CO₂, above background samples, remaining constant in at least the last three samples. Treatment F resulted in the earliest achievement of ¹³CO₂ steady state in the cat's breath. This feeding and isotope protocol can be used in future studies aiming to study amino acid metabolism in cats.

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.

Amino acid oxidation methods to determine amino acid requirements: do we require lengthy adaptation periods?

Determination of indispensable amino acid (IAA) requirements necessitates a range of intakes of the test IAA and monitoring of the physiological response. Short-term methods are the most feasible for studying multiple intake levels in the same individual. Carbon oxidation methods measure the excretion of 13CO2 in breath from a labelled amino acid (AA) in response to varying intakes of the test AA following a period of adaptation. However, the length of adaptation to each AA intake level has been a source of debate and disagreement among researchers. The assertion of the minimally invasive indicator amino acid oxidation (IAAO) technique is that IAA requirements can be estimated after only a few hours (8 h) of adaptation to each test AA intake, suggesting that adaptation occurs rapidly in response to dietary adjustments. On the contrary, the assertion of most other techniques is that 6–7 d of adaptation is required when determining IAA needs. It has even been argued that a minimum of two weeks is needed to achieve complete adaptation. This review explores evidence regarding AA oxidation methods and whether long periods of adaptation to test IAA levels are necessary when estimating IAA requirements. It was found that the consumption of experimental diets containing lower test IAA intake for greater than 7 d violates the terms of a successful adaptive response. While there is some evidence that short-term 8 h IAAO is not different among different test amino acid intakes up to 7 d, it is unclear whether it impacts assessment of IAA requirements.

Dietary Aromatic Amino Acid Requirements During Early and Late Gestation in Healthy Pregnant Women

BACKGROUND

Phenylalanine and tyrosine (referred to as total aromatic amino acids; TAAs) are essential for protein synthesis, and are precursors for important catecholamines. Current estimated average requirement (EAR) recommendations for TAA during pregnancy are 36 mg·kg-1·d-1, and has not been experimentally determined.

OBJECTIVES

The aim was to determine TAA requirements (dietary phenylalanine in the absence of tyrosine) during early and late gestation using the indicator amino acid oxidation (IAAO, with L-[1-13C]leucine) technique.

METHODS

Nineteen healthy pregnant women (age 22-38 y) were studied at a range of phenylalanine intakes (5 to 100 mg·kg-1·d-1) in early (13-19 wk) and/or late (33-39 wk) pregnancy for a total of 51 study days. Graded test intakes were provided as 8 hourly isonitrogenous and isocaloric meals. Breath samples were collected for 13C enrichment analysis on an isotope ratio mass spectrometer. A plasma sample was collected and analyzed for phenylalanine and tyrosine concentrations on an amino acid analyzer. The TAA requirement in early and late pregnancy was calculated using 2-phase linear regression crossover analysis that identified breakpoints in 13CO2 production (the requirement) in response to phenylalanine intakes.

RESULTS

TAA requirement during early pregnancy was 44 mg·kg-1·d-1 (95% CI: 28.3, 58.8) and during late pregnancy was 50 mg·kg-1·d-1 (95% CI: 36.1, 63.1). In early and late pregnancy, plasma phenylalanine and tyrosine concentrations rose linearly in response to graded phenylalanine intakes.

CONCLUSIONS

Our results suggest that the current EAR of 36 mg·kg-1·d-1 for TAAs is underestimated. When compared with results previously determined in nonpregnant adults, early pregnancy requirements were similar (43 compared with 44 mg·kg-1·d-1, respectively). During late pregnancy, a 14% higher TAA requirement was observed when compared with early pregnancy. The results from this study have potential implications for creating gestation stage-specific TAA recommendations.

Adult dogs of different breed sizes have similar threonine requirements as determined by the indicator amino acid oxidation technique

Threonine (Thr) requirements for immature (growing) Beagles have been determined, but little knowledge is available on Thr requirements for maintenance in mature dogs. Moreover, differences of Thr requirements among different breeds or sizes of adult dogs have not been investigated. The objective of the present study was to determine Thr requirements in adult dogs of three different breeds using the indicator amino acid oxidation (IAAO) technique. In total, 13 adult dogs were used, 4 Miniature Dachshunds (5.8 ± 0.4 kg body weight [BW]; 3 spayed and 1 neutered), 4 spayed Beagles (9.3 ± 0.6 kg BW), and 5 neutered Labrador Retrievers (30.5 ± 1.7 kg BW). Dogs were fed a Thr-deficient diet (Thr = 0.23%) and randomly allocated to receiving one of seven concentrations of Thr supplementation (final Thr concentration in experimental diets was 0.23%, 0.33%, 0.43%, 0.53%, 0.63%, 0.73%, and 0.83%; as fed basis) for 2 d. After 2 d of adaptation to the experimental diets, dogs underwent individual IAAO studies. During the IAAO studies, total daily feed was divided into 13 equal meals; at the sixth meal, dogs were fed a bolus of l-[1-13C]-Phenylalanine (Phe) (9.40 mg/kg BW), and thereafter, l-[1-13C]-Phe (2.4 mg/kg BW) was supplied with every meal. Before feeding the next experimental diet, dogs were fed a Thr-adequate basal diet for 4 d (Thr = 0.80% as fed basis) in known amounts that maintained individual dog BW. Total production of 13CO2 during isotopic steady state was determined by enrichment of 13CO2 in breath samples and total production of CO2 measured using indirect calorimetry. The mean requirements for Thr, defined as the breakpoint, and the 95% confidence interval (CI) were determined using a two-phase linear regression model. For Miniature Dachshunds, the two-phase model was not significant, and Thr requirements could not be determined. Mean Thr requirements for Beagles and Labradors were 72.2 and 64.1 mg/kg BW on an as-fed basis, respectively. The requirement for Thr between these two dog breeds was not different (P > 0.10). Thus, the data for Beagles and Labradors were pooled and a mean requirement for Thr was determined at 66.9 mg/kg BW, and the 95% CI was estimated at 84.3 mg/kg BW. In conclusion, estimated Thr requirements for Beagles and Labradors did not differ, and these recommendations are higher than those suggested by NRC (2006) and AAFCO (2014) for adult dogs at maintenance.

Dietary phenylalanine requirements during early and late gestation in healthy pregnant women

BACKGROUND

Phenylalanine is an indispensable amino acid and, via tyrosine, is the precursor for the neurotransmitters dopamine, norepinephrine, and epinephrine. Currently, dietary requirements for phenylalanine during pregnancy are unknown.

OBJECTIVES

This study's aim was to determine phenylalanine requirements (in the presence of excess tyrosine) during early and late gestation using direct amino acid oxidation (DAAO; with l-[1-13C]phenylalanine) and indicator amino acid oxidation (IAAO; with l-[1-13C]leucine).

METHODS

Twenty-three healthy women (age: 30.4 ± 3.1 y, mean ± SD) were studied at a range of phenylalanine intakes (5.5-30.5 mg · kg-1 · d-1 in early and late pregnancy using DAAO, and 2.5-30.5 mg · kg-1 · d-1 in late pregnancy using IAAO) for a total of 76 study days. Test intakes were provided as 8 isocaloric and isonitrogenous meals with 1.5 g · kg-1 · d-1 protein and energy at 1.7 times the measured resting energy expenditure. Breath samples were analyzed on an isotope ratio mass spectrometer for 13C enrichment. Phenylalanine requirement was determined using a 2-phase linear regression crossover model to identify a breakpoint in 13CO2 production (representing the mean requirement) in response to phenylalanine intakes.

RESULTS

Phenylalanine requirement during early pregnancy was determined to be 15 mg · kg-1 · d-1 (95% CI: 10.4, 19.9 mg · kg-1 · d-1); during late pregnancy, it was determined to be 21 mg · kg-1 · d-1 by DAAO (95% CI: 17.4, 24.7 mg · kg-1 · d-1) and IAAO (95% CI: 10.5, 32.2 mg · kg-1 · d-1).

CONCLUSIONS

Our results suggest a higher requirement (40%) for phenylalanine during late pregnancy than during early pregnancy. Moreover, the early pregnancy requirements are higher than the previous adult male requirement (9.1 mg · kg-1 · d-1; 95% CI: 4.6, 13.6 mg · kg-1 · d-1), although the 95% CIs overlap. Both DAAO and IAAO methods provided similar breakpoints in late pregnancy, showing that the DAAO method was appropriate even though low phenylalanine intakes could not be tested. These results have potential implications for gestation stage-specific dietary phenylalanine recommendations in future.This trial was registered at clinicaltrials.gov as NCT02669381.

The Phenylalanine Requirement of Elderly Men and Women Measured by Direct 13C Carbon Oxidation Method Is Similar to That of Young Adults

BACKGROUND

The phenylalanine requirement of the elderly is not known. Current recommendations are based on studies in young adults and are derived from a combined estimate of the total aromatic amino acids, phenylalanine, and tyrosine.

OBJECTIVES

The purpose of this study was to determine the dietary phenylalanine requirement of adults aged >65 y, using the direct amino acid oxidation method, by measuring the oxidation of l-[1-13C]phenylalanine to 13CO2 in response to graded phenylalanine intakes in the presence of excess tyrosine.

METHODS

Twelve subjects (6 men, 6 women), aged 73.8 ± 6.7 y (mean ± SD) and with a BMI (in kg/m2) of 26.4 ± 4.8 and 25.2 ± 4.4 for men and women, respectively, were randomized to phenylalanine intakes ranging from 7.20 to 40.0 mg .kg-1 .d-1 for a total of 66 studies. Study diets were isocaloric and isonitrogenous, providing protein and energy at 1.0 g .kg-1 .d-1 and 1.5 × resting energy expenditure (REE), respectively. Protein was provided as an amino acid mixture patterned after egg protein, with an excess of tyrosine and alanine to balance the nitrogen as phenylalanine intakes were varied. Two days prior to the study day, subjects were adapted to a milkshake diet providing protein at 1.0 g.kg-1 .d-1 and energy at 1.7 × REE. The mean phenylalanine requirement was determined using biphase linear regression analysis, which identified a breakpoint in the F13CO2 in response to graded phenylalanine intakes.

RESULTS

The mean and upper 95% CIs (approximating the recommended dietary allowance) of phenylalanine requirements were estimated to be 9.03 and 15.9 mg.kg-1 .d-1, respectively.

CONCLUSION

These results are similar to previously derived estimates of 9.1 and 13.6 mg.kg-1 .d-1 in young adult men and suggest that higher protein needs of the elderly to stimulate similar muscle protein synthesis rates as young adults are not driven by an increased requirement for phenylalanine. This trial was registered at clinicaltrials.gov as NCT02971059.

Tryptophan requirements in small, medium, and large breed adult dogs using the indicator amino acid oxidation technique1

Tryptophan (Trp) is an indispensable amino acid (AA) for dogs of all life stages; however, although Trp requirements for growing dogs are derived from 3 dose-response studies, there are no empirical data on Trp requirements for adult dogs at maintenance. The study objective was to determine Trp requirements of adult dogs of 3 different breeds using the indicator amino acid oxidation (IAAO) technique. Four spayed or neutered Miniature Dachshunds (5.28 ± 0.29 kg BW), 4 spayed Beagles (9.32 ± 0.41 kg BW), and 5 neutered Labrador Retrievers (30.51 ± 2.09 kg BW) were used. After a 14-d adaptation to a Trp-adequate basal diet (Trp = 0.482% dry matter), all dogs were fed a mildly Trp-deficient diet for 2 d (Trp = 0.092% dry matter) before being randomly allocated to receiving 1 of 7 concentrations of Trp supplementation (final Trp content in experimental diets was 0.092, 0.126, 0.148, 0.182, 0.216, 0.249, and 0.283% dry matter) and all dogs received all Trp treatments. After 2-d adaptation to the experimental diets, dogs underwent individual IAAO studies. Total feed was divided in 13 equal meals; at the sixth meal, dogs were fed a bolus of L-[1-13C]-Phenylalanine (Phe) (9.40 mg/kg BW), and thereafter, L-[1-13C]-Phe was supplied (2.4 mg/kg BW) with every meal. Total production of 13CO2 during isotopic steady state was determined by enrichment of 13CO2 in breath samples and total production of CO2 measured using indirect calorimetry. The maintenance requirement for Trp and the 95% confidence interval (CI) were determined using a 2-phase linear regression model. Mean Trp requirements were estimated at 0.154, 0.218, and 0.157% (dry-matter) for Dachshunds, Beagles, and Labradors, respectively. The upper 95% CI were 0.187, 0.269, and 0.204% (dry-matter) for Dachshunds, Beagles, and Labradors. In conclusion, estimated Trp requirements are higher for Beagles compared with Labradors or Dachshunds, and all estimated requirements are higher than those currently recommended by the NRC and AAFCO.

Using the indicator amino acid oxidation technique to study threonine requirements in horses receiving a predominantly forage diet

Threonine has been reported to be the second limiting amino acid in typical equine diets, but its actual requirement has not been determined in horses. To evaluate amino acid metabolism and requirements, the indicator amino acid oxidation (IAAO) method has been successfully used in other species. The objective of this research was to estimate threonine requirements in mature horses fed timothy hay and concentrate in 4:1 ratio using the IAAO method. Six Thoroughbred mares (579.9 ± 46.7 kg) received each of 6 levels of threonine intake, 41, 51, 61, 70, 80 and 89 mg/kg BW/day, in a randomly determined order. Each study period was 7-day long, and on day 6, blood samples were collected before and 90 min after feeding to measure amino acid concentrations using HPLC. On day 7, horses underwent IAAO procedures, which included a 2-hr primed, constant intravenous infusion of [¹³ C]sodium bicarbonate to measure total CO₂ production and a 4-hr primed, constant oral administration of [1-¹³ C]phenylalanine to estimate phenylalanine oxidation to CO₂ . Blood and breath samples were collected to measure blood [¹³ C]phenylalanine, using GC-MS analysis and breath ¹³ CO₂ enrichment, using an infrared isotope analyser. Increasing threonine intake levels did not affect plasma phenylalanine oxidation by the ANOVA test (p > 0.05) but resulted in a linear decrease in phenylalanine oxidation (p = 0.04) without a breakpoint by the orthogonal linear contrast. This study is the first attempt to evaluate threonine requirements in horses by the IAAO method; however, threonine requirements are still unknown in mature horses at this time.

Phenylalanine requirements of enterally fed term and preterm neonates

BACKGROUND

Phenylalanine, which is an essential aromatic amino acid, is either used for protein synthesis or irreversibly hydroxylated to tyrosine. The provision of optimal amounts of dietary phenylalanine is not only important for growth and development but might also influence catecholamine synthesis and release rates. The current recommended aromatic amino acid requirement for infants aged 0-6 mo is based on the amino acid content of human milk.

OBJECTIVE

We quantified the requirements for phenylalanine in the presence of excess tyrosine (166 or 177 mg/kg per day for term and preterm infants, respectively) for term and preterm neonates by using the indicator amino acid oxidation method with l-[1-(13)C]lysine 2HCl as an indicator. Hence, we determined the minimum obligatory phenylalanine requirement.

DESIGN

Fully enterally fed term and preterm infants received randomly graded amounts of phenylalanine (5-177 mg/kg per day) as part of an elemental formula. Data are expressed as means ± SDs.

RESULTS

Twenty term (birth weight: 3.19 ± 0.34 kg; gestational age: 38.9 ± 1 wk) and 16 preterm (birth weight: 1.75 ± 0.17 kg; gestational age: 32.5 ± 0.6 wk) Asian infants participated at a postnatal age of 17 ± 8 d. In total, 44 studies were performed. The minimum obligatory phenylalanine requirement was 58 mg/kg per day (95% CI: 38-78 mg/kg per day) and 80 mg/kg per day (95% CI: 40-119 mg/kg per day) for term and preterm infants, respectively.

CONCLUSION

The determined mean phenylalanine-requirement estimates are lower than the contents of term and preterm formulas currently on the market. This trial was registered at www.trialregister.nl as NTR1610.

Metabolome-wide association study of phenylalanine in plasma of common marmosets

Little systematic knowledge exists concerning the impacts of cumulative lifelong exposure, termed the exposome, on requirements for nutrients. Phenylalanine (Phe) is an essential dietary amino acid with an aromatic ring structure similar to endogenous metabolites, dietary compounds and environmental agents. Excess plasma Phe in genetic disease or nutritional deficiency of Phe has adverse health consequences. In principle, structurally similar chemicals interfering with Phe utilization could alter Phe requirement at an individual level. As a strategy to identify components of the exposome that could interfere with Phe utilization, we tested for metabolites correlating with Phe concentration in plasma of a non-human primate species, common marmosets (Callithrix jacchus). The results of tests for more than 5,000 chemical features detected by high-resolution metabolomics showed 17 positive correlations with Phe metabolites and other amino acids. Positive and negative correlations were also observed for 33 other chemicals, which included matches to endogenous metabolites and dietary, microbial and environmental chemicals in database searches. Chemical similarity analysis showed many of the matches had high structural similarity to Phe. Together, the results show that chemicals in marmoset plasma could impact Phe utilization. Such chemicals could contribute to early lifecycle developmental disorders when neurological development is vulnerable to Phe levels.

Whole-body phenylalanine kinetics and skeletal muscle protein signaling in horses with pituitary pars intermedia dysfunction

OBJECTIVE

To compare whole-body phenylalanine kinetics and the abundance of factors in signaling pathways associated with skeletal muscle protein synthesis and protein breakdown between horses with pituitary pars intermedia dysfunction (PPID) and age-matched control horses without PPID.

ANIMALS

12 aged horses (6 horses with PPID and 6 control horses; mean age, 25.0 and 25.7 years, respectively).

PROCEDURES

Plasma glucose, insulin, and amino acids concentrations were determined before and 90 minutes after feeding. Gluteal muscle biopsy samples were obtained from horses 90 minutes after feeding, and the abundance and activation of factors involved in signaling pathways of muscle protein synthesis and breakdown were determined. The next day, horses received a priming dose and 2 hours of a constant rate infusion of (13)C sodium bicarbonate followed by a priming dose and 4 hours of a constant rate infusion of 1-(13)C phenylalanine IV; whole-body protein synthesis was determined.

RESULTS

Plasma glucose and insulin concentrations were higher after feeding than they were before feeding for both groups of horses; however, no significant postprandial increase in plasma amino acids concentrations was detected for either group. Phenylalanine flux, oxidation, release from protein breakdown, and nonoxidative disposal were not significantly different between groups. No significant effect of PPID status was detected on the abundance or activation of positive or negative regulators of protein synthesis or positive regulators of protein breakdown.

CONCLUSIONS AND CLINICAL RELEVANCE

Results of this study suggested that whole-body phenylalanine kinetics and the postprandial activation of signaling pathways that regulate protein synthesis and breakdown in muscles were not affected by PPID status alone in aged horses.

Recent advances in determining protein and amino acid requirements in humans

During the past 25 years a significant amount of research has been conducted to determine amino acid requirements in humans. This is primarily due to advancements in the application of stable isotopes to examine amino acid requirements. The indicator amino acid oxidation (IAAO) method has emerged as a robust and minimally invasive technique to identify requirements. The IAAO method is based on the concept that when one indispensable dietary amino acid (IDAA) is deficient for protein synthesis, then the excess of all other IDAA, including the indicator amino acid, will be oxidized. With increasing intakes of the limiting amino acid, IAAO will decrease, reflecting increasing incorporation into protein. Once the requirement for the limiting amino acid is met there will be no further change in the indicator oxidation. The IAAO method has been systematically applied to determine most IDAA requirements in adults. The estimates are comparable to the values obtained using the more elaborate 24h-indicator amino acid oxidation and balance (24h-IAAO/IAAB) model. Due to its non-invasive nature the IAAO method has also been used to determine requirements for amino acids in neonates, children and in disease. The IAAO model has recently been applied to determine total protein requirements in humans. The IAAO method is rapid, reliable and has been used to determine amino acid requirements in different species, across the life cycle and in disease. The recent application of IAAO to determine protein requirements in humans is novel and has significant implications for dietary protein intake recommendations globally.

Amino acid supplementation does not alter whole-body phenylalanine kinetics in Arabian geldings

Stable isotope infusion methods have not been extensively used in horses to study protein metabolism. The objectives were to develop infusion and sampling methodologies for [1-(13)C] phenylalanine and apply these methods to determine whether the addition of supplemental amino acids to a control diet affected whole-body phenylalanine kinetics in mature horses. Arabian geldings were studied using a 6-h primed (9 μmol/kg), constant (6 μmol · kg(-1) · h(-1)) i.v. infusion of L-[1-(13)C] phenylalanine, with blood and breath sampled every 30 min, to measure whole-body phenylalanine kinetics in response to receiving the control diet (n = 12) or the control diet supplemented with equimolar amounts of glutamate (+Glu; 55 mg · kg(-1) · d(-1); n = 5), leucine (+Leu; 49 mg · kg(-1) · d(-1); n = 5), lysine (+Lys; 55 mg · kg(-1) · d(-1); n = 5), or phenylalanine (+Phe; 62 mg · kg(-1) · d(-1); n = 6). The plasma concentrations of the supplemented amino acid in horses receiving the +Leu, +Lys, and +Phe diets were 58, 53, and 36% greater, respectively, than for the control treatment (P < 0.05). Isotopic plateau was attained in blood [1-(13)C] phenylalanine and breath (13)CO(2) enrichments by 60 and 270 min, respectively. Phenylalanine flux (+20%) and oxidation (+110%) were greater (P < 0.05) in horses receiving the +Phe treatment than in those fed the control diet. There was no effect of treatment diet on nonoxidative phenylalanine disposal or phenylalanine release from protein breakdown. The developed methods are a valuable way to study protein metabolism and assess dietary amino acid adequacy in horses and will provide a useful tool for studying amino acid requirements in the future.

Protein requirement of healthy school-age children determined by the indicator amino acid oxidation method

BACKGROUND

The current Dietary Reference Intake (DRI) recommendations for protein requirements in children are based on a factorial estimate and have not been directly determined.

OBJECTIVE

The objective of the current study was to determine the protein requirement in healthy, school-age children by measuring the oxidation of L-[1-(13)C]-phenylalanine to (13)CO(2) [label tracer oxidation (F(13)CO(2))] in response to graded intakes of protein.

DESIGN

Seven healthy children (6-11 y old) each randomly received a minimum of 7 protein intakes (range: 0.1-2.56 g · kg(-1) · d(-1)) for a total of 56 studies. The diets provided energy at 1.7 times the resting energy expenditure and were made isocaloric by using carbohydrate. Protein was given as an amino acid mixture on the basis of the egg-protein pattern, except for phenylalanine and tyrosine intakes, which were maintained constant across intakes. The mean protein requirement was determined by applying a 2-phase linear regression crossover analysis on F(13)CO(2) data, which identified a breakpoint (requirement) at minimal F(13)CO(2) in response to graded amounts of protein intake.

RESULTS

Mean and population-safe (upper 95% CI) protein requirements were determined to be 1.3 and 1.55 g · kg(-1) · d(-1), respectively. These results are significantly higher than the mean and population-safe protein requirements currently recommended by the DRI 2005 for macronutrients (0.76 and 0.95 g · kg(-1) · d(-1), respectively).

CONCLUSION

To our knowledge, this study was the first to directly estimate protein requirements in children by using stable isotopes and indicated that current recommendations are severely underestimated.

Evidence that protein requirements have been significantly underestimated

PURPOSE OF REVIEW

This review discusses recent evidence that suggests a significant underestimation of protein requirements in adult humans.

RECENT FINDINGS

Traditionally, total protein requirements for humans have been determined using nitrogen balance. The recent Dietary Reference Intake recommendations for mean and population-safe intakes of 0.66 and 0.8 g/kg/day, respectively, of high-quality protein in adult humans are based on a meta-analysis of nitrogen balance studies using single linear regression analysis. We reanalyzed existing nitrogen balance studies using two-phase linear regression analysis and obtained mean and safe protein requirements of 0.91 and 0.99 g/kg/day, respectively. The two-phase linear regression analysis is considered more appropriate for biological analysis of dose-response curves. Considering the inherent problems associated with the nitrogen balance method, we developed an alternative method, the indicator amino acid oxidation technique, to determine protein requirements The mean and population-safe requirements in adult men were determined to be 0.93 and 1.2 g/kg/day and are 41 and 50%, respectively, higher than the current Dietary Reference Intakes recommendations.

SUMMARY

The indicator amino acid oxidation-based requirement values of 0.93 and 1.2 g protein/kg/day and the reanalysis of existing nitrogen balance studies are significantly higher than current recommendations. Therefore, there is an urgent need to reassess recommendations for protein intake in adult humans.

Indicator amino acid oxidation is not affected by period of adaptation to a wide range of lysine intake in healthy young men

The number of days of adaptation to a specific amino acid intake required prior to the determination of amino acid requirements using the indicator amino acid oxidation method (IAAO) is still in debate. In this study, our objective was to determine whether adaptation for 8 h, 3 d, and 7 d to a wide range of lysine intakes had any effect on the oxidation of the indicator amino acid, l-[1-(13)C]phenylalanine, to (13)CO(2) (F(13)CO(2)). Five healthy young men randomly received each of 4 levels of lysine (5, 20, 35, and 70 mg x kg(-1) x d(-1)) along with an amino acid mixture to achieve a protein intake of 1.0 g x kg(-1) x d(-1) and energy intake of 1.5x resting energy expenditure during 4 separate 7-d study periods. IAAO studies were conducted on d 1, 3, and 7. During each study day, oral consumption of l-[1-(13)C]phenylalanine was followed by collection of breath for F(13)CO(2) and plasma for measurement of phenylalanine enrichment. F(13)CO(2) was affected by lysine intake but did not differ among adaptation periods of 8 h, 3 d, or 7 d. Phenylalanine flux was not significantly affected by period of adaptation. These results suggest that the minimally invasive IAAO model, where participants are adapted prior to protein intake for 2 d followed by study day adaptation to the test amino acid intake for 8 h, may be sufficient to estimate individual amino acid requirements in healthy young men.

A change-point regression approach for efficacy evaluation of dietary supplements

In clinical trials for dietary supplements and functional foods, the study population tends to be a mixture of healthy subjects and those who are not so healthy but are not definitely diseased (called "borderline subjects"). For such heterogeneous populations, the t-test and ANCOVA method often fail to provide the desired treatment efficacy. We propose an alternative approach for the efficacy evaluation of dietary supplements and functional foods based on a change-point linear regression model. The model does not require the assumption of a constant treatment effect and provides clinically interpretable results. By employing the AIC-based profile likelihood method, inferences can be made easily using standard statistical software. The proposed method was applied to the Garcinia study data, and the merit of the method was demonstrated by comparing it with traditional methods.

An approach to defining the upper safe limits of amino acid intake

The existing data on the safe upper limits of amino acid intake in humans is essentially observational; how much do individuals ingest and what side effects do they have? There are numerous studies in humans comparing the effects of high doses of amino acids given as protein bound vs. as free amino acids. These studies have shown that protein-bound amino acids have much less effect on plasma levels of the test amino acid, because protein intake stimulates protein synthesis as another sink for the increased amino acid intake. In practice, the highest amino acid intakes occur with free amino acid supplements that may be ingested by athletes who believe that the amino acids will benefit them in training and/or performance. Previously, in a piglet study, we were able to define the point at which maximal phenylalanine oxidation occurred, above which plasma phenylalanine concentration and body balance rose exponentially. We regard this value of maximal disposal (oxidation) of an amino acid as one metabolic marker of the upper limit of intake. Recently, others have demonstrated a similar maximal oxidation rate for leucine in rats. Based on these experimental data and the paucity of published human data in controlled experiments, we think that a systematic approach needs to be undertaken to define the maximal oxidation rate for all dietary indispensable amino acids and other amino acids that may be ingested in excess by humans. We believe that this will provide a rational basis to begin to define the upper limits of tolerance for dietary amino acids. However, some amino acids, such as threonine and methionine, will be more difficult to study, because they have more than 1 route of disposal or very complex metabolic regulation, in which case defining their upper limits will be more multifaceted.

Individual amino acid requirements in humans: an update

PURPOSE OF REVIEW

To discuss recent amino acid requirement studies in adult humans and school-age children, primarily determined using the indicator amino acid oxidation technique.

RECENT FINDINGS

Using the minimally invasive indicator amino acid oxidation model, requirements for most indispensable amino acids have been defined in adult humans. The estimates are comparable to the values obtained using the more elaborate 24-h indicator amino acid oxidation and balance model. The less-invasive indicator amino acid oxidation model has also been successfully applied to define requirements in healthy school-age children and children with disease. A recent adaptation of the indicator amino acid oxidation method to determine protein requirements in adult humans resulted in mean and safe values of 0.93 and 1.2 g protein/kg/day, respectively. These estimates are 40-50% higher than current recommendations and suggest an urgent need to reassess recommendations for protein intake in humans.

SUMMARY

In summary, indicator amino acid oxidation is a robust technique, and has resulted in the definition of amino acid and protein requirements in adult humans and children. A wider application of the technique in other vulnerable populations across life stages and in other diseases is now possible.

Aromatic amino acid requirements in healthy human subjects

Dietary aromatic amino acids are needed to meet the requirements for phenylalanine and tyrosine for protein synthesis. The amounts needed for neurotransmitter synthesis and other biological processes are small and quantitatively negligible. The earlier nitrogen balance-based estimates were judged to be inadequate. Very recently, there have been 3 estimates published based on the indicator amino acid oxidation technique, which average 42 mg.kg(-1).d(-1). This average value was obtained by feeding subjects a tyrosine-free diet and hence is an estimate of the mean maximum phenylalanine requirement. The mean minimum phenylalanine requirement estimate in the presence of an excess of tyrosine is 9.1 mg.kg(-1).d(-1). Hence, tyrosine can spare 78% of the dietary phenylalanine need. Finally the optimal proportions of dietary phenylalanine and tyrosine have been shown to be 60:40, respectively.

Evidence that phenylalanine may not provide the full needs for aromatic amino acids in children

Phenylalanine is nutritionally classified as an indispensable amino acid and can be converted to tyrosine by phenylalanine hydroxylation. The initial goal of the present study was to determine the aromatic amino acid (phenylalanine plus tyrosine) requirements in healthy children fed a diet without tyrosine by using the indicator amino acid oxidation (IAAO) method using lysine as the indicator amino acid. Healthy school-age children (n = 5) were fed in random order a diet with eight graded intakes of phenylalanine without tyrosine. The requirement was determined by the rate of recovery of CO2 from L-[1-C]lysine oxidation (FCO2). Phenylalanine (total aromatic amino acid) requirement, in the absence of tyrosine, for children was determined to be 28 mg/kg/d, which was only 64% of the adult requirement, which is biologically absurd. A possible reason for the lower estimate of phenylalanine requirement could be lower phenylalanine hydroxylation rate in children, which is supported by the finding of lower urinary tyrosine/phenylalanine ratios in children compared with adults. In conclusion, this study indicates that phenylalanine may not provide the total needs for aromatic amino acids in children fed an amino acid-based diet without tyrosine.

Leucine is not a good choice as an indicator amino acid for determining amino acid requirements in men

Leucine tracer has been widely used for examining whole-body protein turnover in humans, but has not been evaluated as an indicator to be used in the indicator amino acid oxidation (IAAO) method. The goal of this study was to determine whether the L-[1-(13C)]leucine isotope is an acceptable indicator by comparing it with an established tracer, L-[1-(13C)]lysine. Healthy men (n = 7; 29.9 +/- 4.8 y old) were fed in random order a diet with 7 graded intakes of phenylalanine without tyrosine. In the first study (n = 5), subjects were administered an excess leucine intake of 65 mg/(kg.d), and in the second study (n = 5), they were given the mean requirement of 45 mg/(kg.d) to determine whether leucine intake affected the pattern of response. Previous IAAO studies using lysine and phenylalanine demonstrated a clear pattern in 13CO2 production, i.e., increasing test amino acid intake resulted in a linear decrease to plateau, with a readily discernable breakpoint indicating the requirement. This pattern of production of 13CO2, indicates clear partitioning of the indicator amino acid between oxidation and protein synthesis. This was not observed with leucine at an intake of 65 mg/(kg.d). Conversely, at the lower leucine intake of 45 mg/(kg.d), a breakpoint was seen and a total aromatic amino acid requirement estimate that did not differ from that obtain using lysine as the indicator was obtained. In conclusion, leucine may be used as the indicator in the IAAO technique only when the daily intake leucine is given at its mean requirement level and the potential metabolic effects of other variables are taken into consideration.