Leucine plays a key-role in protein synthesis during moderate steady state exercises

Leucine is one of the essential amino acids (EAA) and belongs to the group of BCAAs (branched chained amino acids), the only ones (together with Isoleucine and Valine) that are not degraded in the liver. BCAAs are found mainly in the skeletal muscle and offer an important contribution to the muscle building. Studies suggest that leucine offers the greatest contribution to the energy production at muscular level, slowing the degradation of the tissue by stimulating muscle protein synthesis. In fact, through the insulin-pathway signaling, leucine seems able to trigger muscle anabolic process.

Nevertheless, EFSA‘s Panel on Dietetic Products, Nutrition and Allergies (NDA) published a scientific opinion regarding BCAAs, providing a negative opinion related to the proposed claims, such as ‘Growth or maintenance of muscle mass’, ‘Faster recovery from muscle fatigue after exercise’ and ‘Reduction in perceived exertion during exercise’, noting that there was no proof of benefit over that of all aminoacids as protein building blocks. Some countries, however, are still allowing claims to be made on leucine and BCAAs.

A recent study was performed in the US to evaluate how supplementation with essential amino acids containing two different amounts of leucine can influence post-exercise muscle protein synthesis. Eight adults drank two amino acid based beverage containing respectively 1.87 g and 3.5 g of leucine during their physical exercises. Muscle protein synthesis was determined by using two stable carbon and hydrogen isotopes as marker. Results showed that muscle protein synthesis was 33 % greater (P < 0.05, then statistically significant) after drinking the leucine-enriched beverage, than after consumption of non-enriched one. This trial suggests that ‘increasing the concentration of leucine in an EAA supplement consumed during steady state exercise elicits a greater MPS response during recovery’. It should be noted however that EFSA has indicated that the markers used are not adequate, as positive effects on muscle mass should be proved via actual measurements of mass, not through markers of degradation or synthesis.

The study supports higher ratios of leucine in BCAA supplements, or increasing the leucine content of supplements and other foods for sportsmen.

Given EFSA’s negative opinion, claims on the effect of BCAAs on muscle protein synthesis (which EFSA said is not a benfit per se) rest on shaky ground. It’s just possible, and only for now, to claim BCAAs effects allowed at national level (e.g. for Italy: ‘trophism and muscle recovery’); this must be checked country by country.

Armando Antonelli – Sport Nutrition Team

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