Intravenously infused 13C-leucine is retained in fasting healthy adult men

Compartmental analysis: a new approach to estimate protein breakdown and meal response in health and critical illness

Purpose of review

This study aimed to discuss the use of the pulse stable isotope tracer approach to study changes in metabolism in healthy individuals and critically ill patients.

Recent findings and conclusion

We found that in the postabsorptive state and healthy condition, intracellular protein breakdown and net intracellular protein breakdown, when calculated using the pulse tracer approach, are about double what has previously been reported using the more traditional primed-constant and continuous stable isotope approaches (600 versus 300 grams of protein/day). In critically ill patients, protein breakdown is even higher and calculated to be approximately 900 grams of protein/day, using the pulse tracer approach. Based on these data, we hypothesize that reducing protein breakdown in the postabsorptive state is key when trying to improve the condition of critically ill patients. Moreover, we also used the pulse tracer approach during feeding to better estimate the intracellular metabolic response to feeding. Our first observation is that endogenous protein breakdown does not seem to be reduced during feeding. We also have shown that when consuming a meal with a certain amount of protein, the biological value of that protein meal can be calculated with the pulse tracer approach. In conclusion, using the pulse stable isotope tracer approach to study protein kinetics in the postabsorptive state and during feeding expands our understanding of how dietary proteins can affect human protein metabolism. The intracellular protein synthesis stimulatory effect of a meal is an important factor to consider when calculating the exact protein requirements and needs, particularly in critical illness.

Branched-chain amino acid requirements in healthy adult human subjects

There is now an expanding body of evidence to recommend, in the case of adult humans, the use of revised indispensable amino acid requirement values; these are approximately 2 to 3 times higher than the current international recommendations. The earlier methodologies for determining amino acid requirements, based on nitrogen balance, were criticized because of their design and the associated high energy intakes. The 1985 World Health Organization/Food & Agriculture Organization/United Nations University requirement for leucine has been demonstrated to be too low by short- and long-term (24-h) tracer-derived estimates of leucine oxidation and balance. The best values for leucine requirements come from 24-h direct amino acid oxidation (DAAO) and direct amino acid balance (DAAB) studies. Finally, we also collated all available data from studies on fed-state leucine oxidation with an adequate dietary adaptation period to assess the inflection on the leucine oxidation-leucine intake curve. The mean requirements for leucine, valine, and isoleucine are likely to be 40, 17-25, and 19 mg . kg(-1) . d(-1), respectively. This adds up to a total of approximately 84 mg . kg(-1) . d(-1), which is much lower than the lowest estimate of the total BCAA requirement of approximately 110 mg . kg(-1) . d(-1) made by the short-term indicator amino acid oxidation (IAAO) method, which determined the BCAA requirement from the pattern of oxidation of an indicator amino acid (phenylyalanine) at different levels of BCAA intake. An additional estimate of the leucine requirement was also made by a meta-analysis of all available 24-h DAAO/DAAB data from different studies. This resulted in a higher value for the leucine requirement than that obtained by the specific studies that utilized the 24-h DAAO/DAAB approach; however, even adding this value to the total BCAA requirement does not account for the difference in the total BCAA requirement estimates and the summed individual BCAA estimates.

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.