Skeletal muscle is the most abundant tissue in the human body and serves as the primary amino acid pool for the body when it needs it (e.g., under stress conditions like dieting, heavy training, injury, etc.).
Glutamine is the most abundant amino acid found in skeletal muscle and is depleted during and after heavy physical stress. Over several years there have been anecdotal reports on the benefits of glutamine supplementation for muscle growth, however scientific data supporting mechanisms of action have been conflicting.
Recent experimental evidence indicates that the muscle sparing and anabolic effects of glutamine supplementation may be due to its ability to inhibit myostatin, enabling greater muscle growth. In this study, published in Amino Acids, researchers from the University of Torino (in Italy) showed that when muscle cells were exposed to TNF (to induce catabolism) and supplemented with glutamine, it completely reversed the hyperactivity of myostatin (and halted catabolism).
Bonetto A, Penna F, Minero VG, Reffo P, Costamagna D, Bonelli G, Baccino FM, Costelli P. Glutamine prevents myostatin hyperexpression and protein hypercatabolism induced in C2C12 myotubes by tumor necrosis factor-α. Amino Acids. 2011 Feb;40(2):585-94. Epub 2010 Jul 10.