Leucine is known regulator of muscle protein synthesis or MPS . It does this by initiating mRNA translation through the mTOR , p70S6K-4E-BP1 signaling pathways . A high concentration of leucine in the diet is thought to underline favorable affects on fat loss and body composition, via protein sparing and a stable glucose environment during energy-restricted periods .
Increasing the leucine concentration of a protein serving (to ~40% ) can increase resting rates of MPS. For these reasons recent investigations have assessed the effects of adding leucine to meals and protein supplements. [4-8]
It's interesting to note that adding leucine can restore the anabolic response of meals and training in older adults (60-year-olds) to levels similar to that observed in 20 year-olds . Importantly as i said, this improved muscle "rejuvenation" response occurs after leucine-rich meals but also after exercise . Therefore, leucine content of a meal may well determine the meals' anabolic response.
However, several research groups have failed to see additional benefit from adding leucine to a protein rich food or supplement - either by adding leucine to a moderate dose of high quality protein (10-30grams) at rest  and post exercise [8, 11]. In fact, a low dose of whey protein (6gm) with added leucine or all other essential amino acids (to the equivalent of 25g of whey protein isolate) was not as effective as 25g of whey protein isolate alone at sustaining increased rates of MPS post-exercise.
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Although these findings suggest that added leucine is unnecessary for the stimulation of MPS when sufficient EAAs are provided from a quality protein source, the potential role of leucine in stimulating muscle anabolism in populations susceptible to protein loss and proteolytic conditions such as calorie deprivation (athletes making weight) and sustained endurance-type exercise, warrants further research.
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1. Norton LE, Layman DK. Leucine regulates translation initiation of protein synthesis in skeletal muscle after exercise. J Nutr. 2006;136(2):533S-537S.
2. Crozier SJ, Kimball SR, Emmert SW, Anthony JC, Jefferson LS. Oral leucine administration stimulates protein synthesis in rat skeletal muscle J Nutr. 2005 Mar;135(3):376-82.
3. Layman DK, Walker DA. Potential importance of leucine in treatment of obesity and the metabolic syndrome. J Nutr. 2006;136(1 Suppl):319S-23S.
4. 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;84(3):623-32.
5. Koopman R, Verdijk LB, Beelen M, Gorselink M, Kruseman AN, Wagenmakers AJ, Kuipers H, van Loon LJ. Co-ingestion of leucine with protein does not further augment post-exercise muscle protein synthesis rates in elderly men. Br J Nutr. 2008, 99(3):571-80.
6. Coburn JW, Housh DJ, Housh TJ, Malek MH, Beck TW, Cramer JT, Johnson GO, Donlin PE. Effects of leucine and whey protein supplementation during eight weeks of unilateral resistance training. J Strength Cond Res. 2006, 20(2):284-91.
7. Glynn EL, Fry CS, Drummond MJ, et al. Excess leucine intake enhances muscle anabolic signaling but not net protein anabolism in young men and women. J Nutr 2010;140:1970-6.
8. Tipton KD, Elliott TA, Ferrando AA, Aarsland AA, Wolfe RR. Stimulation of muscle anabolism by resistance exercise and ingestion of leucine plus protein. Appl Physiol Nutr Metab. 2009 Apr;34(2):151-61.
9. Katsanos CS, Kobayashi H, Sheffield-Moore M, Aarsland A, Wolfe RR. A high proportion of leucine is required for optimal stimulation of the rate of muscle protein synthesis by essential amino acids in the elderly. Am J Physiol Endocrinol Metab. 2006, 291(2):E381-7.
10. Pasiakos SM, McClung HL, McClung JP, Margolis LM, Andersen NE, Cloutier GJ, Pikosky MA, Rood JC, Fielding RA, Young AJ.Leucine-enriched essential amino acid supplementation during moderate steady state exercise enhances postexercise muscle protein synthesis. Am J Clin Nutr. 2011 Sep;94(3):809-18.
11. Churchward-Venne TA, Burd NA, Mitchell CJ, West DW, Philp A, Marcotte GR, Baker SK, Baar K, Phillips SM. Supplementation of a suboptimal protein dose with leucine or essential amino acids: effects on myofibrillar protein synthesis at rest and following resistance exercise in men. J Physiol. 2012 Jun 1;590(Pt 11):2751-65