When protein synthesis equals protein breakdown, there is no net gain of muscle protein. This is important in maintaining muscle mass


HMB aids in increasing muscle protein (mass) by two mechanisms. HMB decreases protein breakdown caused by hard workouts at a molecular level by disrupting proteolysis through the ubiquitin-proteasome proteolytic pathway, the proteolytic enzyme that breaks muscle protein down.


HMB is also a metabolite of the BCAA leucine which has been shown to stimulate protein synthesis through the mTOR pathway. Potentially, HMB could play this role.


HMB increases protein synthesis



HMB stimulates protein synthesis in muscle by multiple mechanisms, including the mTOR/p70S6k pathway, which stimulate translation initiation of protein synthesis. (Many of these effects are shared with leucine; however, HMB is more potent than leucine in promoting muscle gain).


HMB decreases protein breakdown



HMB decreases protein breakdown at a molecular level caused by hard workouts. HMB results in decreasing the key regulating components of the ubiquitin-proteosome proteolytic pathway which results in LESS muscle loss.


Therefore, the increase in synthesis and decrease in breakdown result in twice the gain of muscle over exercise alone.

https://www.hmb.org/research/how_works.cfm