Reduce the amount of other amino acids, but add more leucine. The result: a supplement, sports drink or shake that stimulates exercise-induced muscle growth better than similar protein products. Researchers at Tufts University and the American army published the results of an experiment they did in the American Journal of Clinical Nutrition.
Leucine is an interesting amino acid. Muscle cells can 'see' leucine, as it were, and depending on the number of 'leucine prompts' they've had, they decide how hard they want to make their anabolic machinery work. The more leucine a muscle cell contains, the more energy the cell invests in building up muscle proteins.
The researchers got the soldiers to cycle for an hour at 50-60 percent of their VO2max, a level at which they were unable to hold a conversation. During that hour the subjects drank a total of 500 ml of a sports drink containing 10 g amino acids. On one occasion the soldiers got a standard mix [EAA]; on the other they got a mix containing extra leucine and less of the other amino acids [L-EAA]. The EAA mix contained 1.9 g leucine; the L-EAA mix contained 3.5 g leucine.
When the researchers examined muscle fibre samples, they noticed that the leucine-rich mix resulted in 33 percent more muscle protein synthesis than the standard mix. That's a statistically significant difference. This was partly because the extra leucine reduced the breakdown of muscle tissue. And these effects were statistically significant too.
The researchers have not yet worked out how extra leucine boosts muscle growth. The theory is that leucine activates anabolic signal molecules like IRS-1, Akt and mTOR, but the researchers found no evidence of this. Other more recent studies have shown evidence however. So leucine works, but probably in a different way to how we think it does.
"Consumption of a 10-g dose of EAA enriched with leucine during moderate endurance-type exercise stimulated increased MPS when compared with an isonitrogenous EAA supplement with an amino acid profile consistent with high quality proteins", the researchers write. "Increasing leucine availability during steady state exercise promotes skeletal muscle protein anabolism and spares endogenous protein."
Am J Clin Nutr. 2011 Sep;94(3):809-18.