A minimally invasive tracer protocol is effective for assessing the response of leucine kinetics and oxidation to vaccination in chronically energy-deficient adult males and children

Evidence for the Contribution of Gut Microbiota to Age-Related Anabolic Resistance

Globally, people 65 years of age and older are the fastest growing segment of the population. Physiological manifestations of the aging process include undesirable changes in body composition, declines in cardiorespiratory fitness, and reductions in skeletal muscle size and function (i.e., sarcopenia) that are independently associated with mortality. Decrements in muscle protein synthetic responses to anabolic stimuli (i.e., anabolic resistance), such as protein feeding or physical activity, are highly characteristic of the aging skeletal muscle phenotype and play a fundamental role in the development of sarcopenia. A more definitive understanding of the mechanisms underlying this age-associated reduction in anabolic responsiveness will help to guide promyogenic and function promoting therapies. Recent studies have provided evidence in support of a bidirectional gut-muscle axis with implications for aging muscle health. This review will examine how age-related changes in gut microbiota composition may impact anabolic response to protein feeding through adverse changes in protein digestion and amino acid absorption, circulating amino acid availability, anabolic hormone production and responsiveness, and intramuscular anabolic signaling. We conclude by reviewing literature describing lifestyle habits suspected to contribute to age-related changes in the microbiome with the goal of identifying evidence-informed strategies to preserve microbial homeostasis, anabolic sensitivity, and skeletal muscle with advancing age.

Intestinal microbial contribution to metabolic leucine input in adult men

New estimates of the indispensable amino acid requirements of adult humans are much higher than previously thought and questions the adequacy of cereal-based diets of low protein quality. However, dietary amino acid requirements may be supplemented by contributions from the intestinal microbiota. This study measured the contribution of intestinal microbes to leucine input in healthy adult men. Fourteen adult men were studied during each of 2 11-d periods (before and after intestinal antimicrobial treatment), in which leucine was supplied at 1.25 times the estimated average requirement (EAR) (d 1-7) and at 2.5 times the EAR (d 8-11) providing an l-amino acid diet. We estimated fasting- and fed-state leucine oxidation on d 7 and d 11 using a (13)C-leucine tracer infusion. The microbial contribution to body leucine input was calculated from the relationship of leucine oxidation to leucine intake and the reduction in leucine oxidation after antimicrobial treatment. Antimicrobial treatment did not affect the slope of the relationship of leucine oxidation to leucine intake. Mean and fed-state leucine oxidation declined by approximately 13 and 20%, respectively (both P < 0.05) after antimicrobial treatment with the 1.25 EAR diet, but not with the 2.5 EAR diet. The contribution of the intestinal microbiota to body leucine input was estimated to be between 19 and 22% at the 1.25 EAR diet. The contribution of the intestinal microbiota to body amino acid homeostasis may be significant at maintenance intakes, but its long-term nutritional importance remains to be determined.

Intestinal parasites increase the dietary lysine requirement in chronically undernourished Indian men

BACKGROUND

We previously used the 24-h indicator amino acid balance method to show that the lysine requirement in undernourished Indian men from low socioeconomic and unsanitary environments is approximately 50% higher than the mean requirement of 30 mg lysine. kg(-1). d(-1) in well-nourished men.

OBJECTIVE

It is possible that this higher lysine requirement in persons with chronic undernutrition is due to environmental influences, including the presence of intestinal parasites. We assessed this possibility by using 24-h indicator amino acid balance (with leucine) at both the "normal" requirement for lysine intake and the higher requirement, before and after successful treatment to eradicate intestinal parasites in affected, undernourished men.

DESIGN

Fourteen chronically undernourished men were studied before and after treatment for intestinal parasites, during each of two 7-d (6-d dietary adaptation plus 1-d tracer experiment) diet periods supplying either 30 (n = 7) or 45 (n = 7) mg lysine. kg(-1). d(-1) from an L-amino acid diet. Twenty-four-hour indicator amino acid balance was estimated on day 6 by [(13)C]leucine tracer infusion.

RESULTS

Before the parasite treatment, subjects were in neutral 24-h leucine balance at both lysine intakes. After the eradication of intestinal parasites, there was a significant (P < 0.001) improvement in 24-h leucine balances, which were positive at both lysine intakes.

CONCLUSIONS

On the basis of the 24-h indicator amino acid balance approach, it appears that intestinal infestation with parasites increases the requirement for lysine and that this may be one factor responsible for the higher lysine requirement observed in persons with chronic undernutrition.

Protein and amino acid requirements and the composition of complementary foods

In this paper, factorial models of the dietary requirements for protein, nitrogen and individual indispensable amino acids are developed from published information on the relationship between age and protein deposition and between protein (amino acid) intake and nitrogen balance. The results are used to develop recommendations on the protein-energy ratio and the amino acid pattern of the diet. As part of the development of the models, factors affecting dietary protein digestibility, bioavailability and efficiency of utilization are discussed. Over the age range of 6-24 mo the models predict a fall in the weight-specific protein and amino acid requirement that results almost entirely from the changes in the growth rate of the children. It is also concluded that the requirement for the maintenance of body protein equilibrium (so-called maintenance) changes little with age. This contrasts markedly with the relationship between age and energy requirements. The amino acid modeling implies that the optimum pattern of individual essential amino acids also changes only marginally across the age range considered in the report. The calculations of the dietary requirement for whole protein imply that achieving a minimum protein-energy ratio of 6.3% is desirable. The amount of protein needed from complementary foods for breast-fed children is discussed.

Leucine requirement and splanchnic uptake of leucine in chronically undernourished adult Indian subjects

BACKGROUND

We showed previously by the 24-h direct amino acid balance (DAAB) method that the leucine requirement of well-nourished Western and South Asian subjects is approximately 40 mg x kg(-1) x d(-1).

OBJECTIVE

It is not known whether this estimated leucine requirement is applicable in chronic undernutrition; therefore, we assessed the leucine requirement in Indian men with chronic, but stable, undernutrition.

DESIGN

We studied 26 chronically undernourished men during 2 randomly assigned 7-d diet periods consisting of an L-amino acid diet (n = 20) and supplying either 14 and 30 (n = 10) or 22 and 40 (n = 10) mg leucine x kg(-1)x d(-1) or consisting of the subjects' habitual cereal-and-lentil-based diets (n = 6). The 24-h DAAB was estimated on day 6 by using a [13C]leucine tracer infusion. The splanchnic uptake of leucine was determined at an intake of 40 mg x kg(-1) x d(-1) by administering [2H3]leucine orally.

RESULTS

By using mixed-models linear regression of leucine balance against leucine intake, we estimated a zero leucine balance at a leucine intake of 39.6 mg x kg(-1) x d(-1). The splanchnic first-pass uptake of [2H3]leucine was 22.7% and 11.5% of the intake in the fasted and fed phases, respectively. The subjects were in neutral leucine balance with their habitual cereal-and-lentil-based diets.

CONCLUSION

On the basis of the 24-h DAAB approach, a mean leucine requirement of 40 mg x kg(-1) x d(-1) is proposed for healthy and for chronically undernourished Indian adults.