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The IL15RA gene is linked to lean body mass and muscle size (hypertrophy). One version is associated with muscle size and the other is linked to muscle strength in response to resistance-type training.
Source: 1000 Genome Project. Global averages for both sexes
The ‘interleukins’ are a large group of proteins that are involved in the regulation of the immune response and other inflammatory reactions. One of these, interleukin-15 (IL-15), is also a growth factor that is expressed in muscle and has been demonstrated to have anabolic effects, with increased levels being linked to muscle growth in various studies on rats, and mouse and human muscle cell lines. The discovery of these anabolic properties has inspired dozens of scientists all over the world to investigate whether IL-15 may be an effective treatment for muscle-wasting disorders and muscle loss caused by ageing.
In addition to its effects on muscle growth, IL-15 is also believed to play a role in controlling lean body mass, with reduced levels of IL-15 in humans being associated with propensity towards obesity. The action of IL-15 is part of a complex network, and is modulated by interactions with a number of other molecules. One of these molecules, interleukin-15 receptor alpha, or IL-15RA, is believed to regulate the bioavailability of the IL-15 protein. A genetic variation in the gene that produces IL-15RA has been linked to muscle growth and strength.
The studies described below enable us to begin to build a picture of how IL-15 and its receptor are playing a role in the growth of muscle and control of body fat in humans. There are still many unanswered questions around the action of IL-15 in different tissues types and how it is regulated. It may not be involved in normal muscle growth but it is clear that it is at least part of a systemic response to stress that inhibits the breakdown of muscle and promotes fat loss.