The total amount of muscle mass is the sum of muscle protein synthesis (MPS), and muscle protein breakdown (MPB). If MPS is greater than MPB, muscle mass increases. A protein kinase called the mechanistic target of rapamycin (mTOR) has been widely recognized as a key regulator of muscle growth. Elevations in energy status, growth factors, amino acids and specific nutrients, including β-hydroxy-β-methylbutyrate (HMB) and phosphatidic acid (PA), can increase MPS through an mTOR-dependent mechanism.
Resistance exercise mechanically induces MPS increases through mTOR signaling, resulting in a hypertrophic response. Exercise has been shown to increase intracellular levels of the phospholipid PA, and PA activates mTOR signaling.
Resistance exercise and protein ingestion stimulate MPS and are synergistic when protein consumption occurs before or after resistance exercise. Protein consists of individual amino acids. When analyzing the effect of different amino acids on MPS, essential amino acids (EAAs) outperform non-essential amino acids, and the branched chain amino acid (BCAA) leucine seems to play a key role. Therefore, an acute protein dose should contain up to 3,000 mg of leucine, in addition to a balanced array of the EAAs to optimally active MPS.
HMB is a naturally occurring metabolite of the essential amino acid leucine. Similar to leucine, HMB increases mTOR signaling, with leucine’s effect on MPS being more pronounced than HMB’s. HMB induces acute muscle anabolism via a distinct, and/or additional mechanism to leucine. This ingredient works optimally when protein intake is suboptimal or during times of increased MPB (e.g., during calorie restriction, times of insufficient recovery or periods of overtraining).
To read this article in its entirety, check out the January 2020 digital magazine, Muscle quest: Developing products to promote lean mass.
Ralf Jäger, co-founder and partner of Increnovo LLC, is a certified sports nutritionist (CISSN),fellow of the International Society of Sports Nutrition (FISSN) and a member of the editorial board of the leading peer-reviewed Journal of International Society of Sports Nutrition (JISSN).