Leucine is an essential, branched-chain amino acid and thus belongs to the group of BCAA. Compared with other amino acids, leucine receives particular attention in sports science because it is credited with a role in triggering muscle protein synthesis, the process by which the body builds new muscle proteins.
In practice, protein sources differ in their leucine content. Whey protein contains a comparatively high share of leucine compared with many other protein sources, which is why it is often mentioned in this context in sports science. Other protein sources such as eggs, meat or pulses also supply leucine, although in varying amounts per serving.
Detailed information on the role of leucine in building muscle is given in the entry Muscle protein synthesis, and the entry Whey protein explains more about whey as a leucine-rich protein source.
