Long-term effects of leucine supplementation on body composition

PURPOSE OF REVIEW

Leucine does not only serve as a substrate for protein synthesis but is also recognized as a potent signal nutrient that regulates protein metabolism. Accordingly, leucine supplementation has been suggested to develop muscle mass or prevent protein loss in several conditions characterized by muscle protein wasting. In the present review, we reported the recent results related to the effect of dietary leucine or leucine-rich amino acid mixture and proteins on whole body composition.

RECENT FINDINGS

Although recent studies corroborate that increasing plasma leucine concentration generally induces an increase in muscle protein synthesis, long-term dietary leucine supplementation has been poorly investigated. Chronic free leucine supplementation alone did not improve lean body or muscle mass during resistance training or in elderly, whereas it was able to limit the weight loss induced by malnutrition. Contradictory data were also reported concerning the effect of leucine supplementation for weight management in obese patients. Leucine-rich amino acid mixture or proteins appeared more efficient than leucine alone to improve muscle mass and performance, suggesting the efficacy of leucine depends nevertheless on the presence of other amino acids.

SUMMARY

Until now, there is no evidence that chronic leucine supplementation is efficient in promoting muscle mass or preventing protein loss during catabolic states. Further studies are required to determine the duration and nutritional conditions of long-term leucine supplementation and to establish whether such nutritional interventions can help to prevent or treat muscle loss in various pathological or physiological conditions.

Effects of inflammation and/or inactivity on the need for dietary protein

PURPOSE OF REVIEW

Protein requirement in healthy young and old individuals is traditionally defined as the lowest protein intake sufficient to achieve neutral body protein balance. This concept, however, cannot be applied to those conditions characterized by unavoidable protein catabolism despite optimal nutrition, such as inactivity and diseases associated with systemic inflammation.

RECENT FINDINGS

The ability of dietary proteins to promote protein anabolism is reduced by inactivity and inflammatory mediators, whereas physical exercise ameliorates the efficiency in using dietary proteins. Consequently, the protein intake level associated with the lowest rate of catabolism in inactivity and/or inflammation is greater than the minimum protein intake required to achieve neutral protein balance in healthy, physically active individuals. A protein intake of 1.2 g.kg.day is currently recommended for inactive healthy individuals, whereas guidelines recommend up to 1.5 g.kg.day in patients with severe systemic inflammation, such as those affected by critical illness or cancer. High protein intake accelerates progression of renal insufficiency but does not affect renal function in healthy individuals.

SUMMARY

In inflammation and/or inactivity a relatively high protein intake may be required to promote synthesis of specific proteins, prevent depletion of selected amino acids (e.g., glutamine or arginine), modulate immune functions, counteract insulin resistance and redox unbalance. Thus, an optimal protein/amino acid intake may be greater than that required to decrease whole body protein wasting.