Acutely, consumption of protein leads to increases in protein synthesis. But not all proteins stimulate protein synthesis to the same extent. Leucine is one of the essential amino acids, and has potent effects on preventing protein degradation while stimulating protein synthesis. Few studies however have documented the extent of protein synthesis in response to consuming protein plus additional leucine. In this study, detailed measures of protein synthesis after performing a bout of resistance training were made in both young and older subjects. After one trial, subjects repeatedly consumed a carbohydrate beverage, and during another identical trial they consumed the carbohydrate with whey protein and additional free leucine. Results showed a significant increase in muscle protein synthesis with protein plus leucine in both young and old subjects. In addition, protein plus leucine also suppressed protein breakdown. The end result was a dramatic increase in muscle protein balance in the protein plus leucine trials. The study confirms the positive effects of protein and specifically the potent effects of leucine on protein metabolism in muscle. Leucine appears to be one of the most important amino acids in promoting anabolism.
Koopman R, Verdijk L, Manders RJ, Gijsen AP, Gorselink M, Pijpers E, Wagenmakers AJ, van Loon LJ. Co-ingestion of protein and leucine stimulates muscle protein synthesis rates to the same extent in young and elderly lean men. Am J Clin Nutr. 2006 Sep;84(3):623-32.