The significance of d-isomers in stable isotope studies in humans is dependent on the age of the subject and the amino acid tracer

Protein Requirements of Elderly Chinese Adults Are Higher than Current Recommendations

BACKGROUND

Due to a lack of research data on the protein requirements of the elderly in China, the estimated average requirement (EAR) and the recommended nutrient intake (RNI) of protein in the elderly remain the same as those in young and middle-aged people at 0.98 g/(kg·d).

OBJECTIVE

The objective of this study was to determine the protein requirements of healthy Chinese adults >65y old through use of the indicator amino acid oxidation (IAAO) method.

METHODS

Seven healthy adult men and 7 healthy adult women participated in the study, with protein intakes ranging from 0.3 to 1.8 g/(kg·d). The diets were isocaloric and provided energy at a 1.5 resting energy expenditure. Protein was given based on the lactalbumin. Phenylalanine and tyrosine were added to protein doses of 0.3-1.5 g/kg according to the highest dose of protein content [1.8 g/(kg·d)]. Phenylalanine and tyrosine concentrations were kept constant at each protein dose. The mean protein requirement was determined by applying a nonlinear mixed-effects model analysis to the F13CO2, which identified a breakpoint in F13CO2 in response to graded amounts of protein. This trial was registered with the Chinese clinical trial registry as ChiCTR-BOC-17010930.

RESULTS

Protein EAR and RNI for healthy elderly Chinese adults were determined to be 0.91 and 1.17 g/(kg·d), respectively, based on the indicator amino acid oxidation technique.

CONCLUSIONS

The estimates of protein requirements for Chinese adults >65 y in the present study are 3.4% and 19.4% higher than the current estimated requirements, 0.88 g/(kg·d) for EAR and 0.98 g/(kg·d) for RNI.

Lysine requirement of the enterally fed term infant in the first month of life

BACKGROUND

Infant nutrition has a major impact on child growth and functional development. Low and high intakes of protein or amino acids could have a detrimental effect.

OBJECTIVE

The objective of the study was to determine the lysine requirement of enterally fed term neonates by using the indicator amino acid oxidation (IAAO) method. L-[1-(13)C]phenylalanine was used as an indicator amino acid.

DESIGN

Twenty-one neonates were randomly assigned to lysine intakes that ranged from 15 to 240 mg · kg(-1) · d(-1). Breath, urine, and blood samples were collected at baseline and during the plateau. The mean lysine 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)) and phenylalanine oxidation rates calculated from the L-[1-(13)C]phenylalanine enrichment of urine and plasma.

RESULTS

The mean (±SD) phenylalanine flux calculated from urine and plasma L-[1-(13)C]phenylalanine enrichment data were 88.3 ± 6.9 and 84.5 ± 7.4 μmol · kg(-1) · h(-1), respectively. Graded intakes of lysine had no effect on phenylalanine fluxes. The mean lysine requirement determined by F(13)CO(2) was 130 mg · kg(-1) · d(-1) (upper and lower CIs: 183.7 and 76.3 mg · kg(-1) · d(-1), respectively). The mean requirement was identical to the requirement determined by using phenylalanine oxidation rates in urine and plasma.

CONCLUSIONS

The mean lysine requirement of enterally fed term neonates was determined by using F(13)CO(2) and phenylalanine oxidation rates calculated from the L-[1-(13)C]phenylalanine enrichment of urine and plasma. These methods yielded a similar result of 130 mg lysine · kg(-1) · d(-1). This study demonstrates that sampling of (13)CO(2) in expired air is sufficient to estimate the lysine requirement by using the IAAO method in infants. This trial was registered at www.trialregister.nl as NTR1610.

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.

Arginine can be synthesized from enteral proline in healthy adult humans

There is considerable controversy recently in identifying dietary precursors for arginine synthesis. We have previously shown in human neonates and piglets that proline is the sole dietary precursor for arginine synthesis. It is unclear in adult humans whether proline is a dietary precursor for arginine. We performed a multi-tracer stable isotope study in adults using (15)N(2)-ureido arginine and (15)N proline to elucidate synthesis of citrulline and arginine and determine whether proline is a precursor for arginine. Primed, intermittent infusions of the labeled amino acids were given enterally to 5 healthy men consuming a standardized milkshake diet. Blood was sampled during plateau enrichment between 1.5 and 3 h. Plasma enrichment occurred for both tracers, giving enteral turnover estimates of 93 μmol · kg(-1) · h(-1) for arginine and 154 μmol · kg(-1) · h(-1) for proline. Appearance of the label from proline in arginine and the intermediaries, ornithine and citrulline, was measured in all participants. The rate of synthesis of arginine from proline was 3.7 μmol · kg(-1) · h(-1), which is estimated to be ~40% of newly synthesized arginine. In this first study in adult humans using an enteral proline tracer, we have demonstrated synthesis of arginine from this dietary amino acid. Therefore, as in newborns, proline must now be considered a dietary precursor for arginine in healthy adults.

Arginine synthesis from enteral glutamine in healthy adults in the fed state

Recent studies have documented transfer of labeled nitrogen from [2-(15)N]glutamine to citrulline and arginine in fasting human adults. Conversely, in neonates and piglets we have shown no synthesis of arginine from [2-(15)N]glutamate, and others have shown in mice that glutamine is a nitrogen, but not a carbon donor, for arginine synthesis. Therefore, we performed a multitracer study to determine whether glutamine is a nitrogen and/or carbon donor for arginine in healthy adult men. Two glutamine tracers, 2-(15)N and 1-(13)C, were given enterally to five healthy men fed a standardized milkshake diet. There was no difference in plasma enrichments between the two glutamine tracers. 1-(13)C isotopomers of citrulline and arginine were synthesized from [1-(13)C]glutamine. Three isotopomers each of citrulline and arginine were synthesized from the [2-(15)N]glutamine tracer: 2-(15)N, 5-(15)N, and 2,5-(15)N(2). Significantly greater enrichment was found of both [5-(15)N]arginine (0.75%) and citrulline (3.98%) compared with [2-(15)N]arginine (0.44%) and [2-(15)N]citrulline (2.62%), indicating the amino NH(2) from glutamine is mostly transferred to arginine and citrulline by transamination. Similarly, the enrichment of the 1-(13)C isotopomers was significantly less than the 2-(15)N isotopomers, suggesting rapid formation of α-ketoglutarate and recycling of the nitrogen label. Our results show that the carbon for 50% of newly synthesized arginine comes from dietary glutamine but that glutamine acts primarily as a nitrogen donor for arginine synthesis. Hence, studies using [2-(15)N]glutamine will overestimate arginine synthesis rates.

Nutritional and medicinal aspects of D-amino acids

This paper reviews and interprets a method for determining the nutritional value of D-amino acids, D-peptides, and amino acid derivatives using a growth assay in mice fed a synthetic all-amino acid diet. A large number of experiments were carried out in which a molar equivalent of the test compound replaced a nutritionally essential amino acid such as L-lysine (L-Lys), L-methionine (L-Met), L-phenylalanine (L-Phe), and L-tryptophan (L-Trp) as well as the semi-essential amino acids L-cysteine (L-Cys) and L-tyrosine (L-Tyr). The results show wide-ranging variations in the biological utilization of test substances. The method is generally applicable to the determination of the biological utilization and safety of any amino acid derivative as a potential nutritional source of the corresponding L-amino acid. Because the organism is forced to use the D-amino acid or amino acid derivative as the sole source of the essential or semi-essential amino acid being replaced, and because a free amino acid diet allows better control of composition, the use of all-amino-acid diets for such determinations may be preferable to protein-based diets. Also covered are brief summaries of the widely scattered literature on dietary and pharmacological aspects of 27 individual D-amino acids, D-peptides, and isomeric amino acid derivatives and suggested research needs in each of these areas. The described results provide a valuable record and resource for further progress on the multifaceted aspects of D-amino acids in food and biological samples.

Arginine is synthesized from proline, not glutamate, in enterally fed human preterm neonates

In neonatal mammals, arginine is synthesized in the enterocyte, with either proline or glutamate as the dietary precursor. We have shown several times in piglets that proline is the only precursor to arginine, although in vitro evidence supports glutamate in this role. Because of this uncertainty, we performed a multitracer stable isotope study to determine whether proline, glutamate, or both are dietary precursors for arginine in enterally fed human neonates. Labeled arginine (M + 2), proline (M + 1), and glutamate (M + 3) were given enterally to 15 stable, growing preterm infants (GA at birth 30-35 wk) at 1-3 wk postnatal age. Enrichment in urine of the tracer amino acids and the M + 1 and M + 3 isotopomers of arginine were measured by LC-tandem mass spectrometry to determine the contribution of proline and glutamate to arginine synthesis. Plateau enrichments of arginine and proline tracers were measurable in urine. Urinary glutamate enrichment was not detected. Conversion of proline to arginine was detected. However, the M + 3 isotopomer of arginine, which would have been synthesized from glutamate, was not detected. We conclude that, in contrast to the current consensus in the literature based on in vitro studies, proline is the major contributor to arginine synthesis in human preterm infants.