Leucine is an amino acid which has the single letter amino acid code L, and the 3 letter code Leu. It is a hydrophobic and non-polar amino acid. It is coded for by the nucleotide base sequences: CUU, CUA, CUC and CUG .
Leucine is one of the three Essential amino acids that makes up a group called the Branched chain amino acids ( or BCAAs). These BCAAs have ben described as hormone Secretagogues, as well as having roles to play in intracellular signalling - affecting gene transcription and translation. 
Leucine is the only amino acid with the unique ability to stimulate muscle protein synthesis through the interaction with mammalian target of rapamycin (mTOR).  This has led to a lot of research which may indicate that leucine rich diets may prevent muscle wastage when ageing or engaging in high intensity sport, and may even help in repair of muscle following injury. 
- ↑ https://www.dna20.com/codontablewheel.php
- ↑ Berg, J. M., Tymoczko, J. L., &amp; Stryer, L. (2002). Biochemistry (5th ed.). New York: W.H. Freeman.
- ↑ Sreekumaran, N. Short, Kevin R. (2005) The Journal of Nutrition, vol. 135, no. 6, p. 15475-15525.
- ↑ Gran, P. Cameron-Smith, D. (2011) BioMed Central, doi: 10.1186/1472-6793-11-10
- ↑ Pereira, Marcelo G. et al. (2014) Nutrients. 6 (10), 3981-4001
- ↑ Luiking, Yvette C. et al, (2014) Nutritional Journal, 13: 9, 10.1186/1475-2891-13-9