What it's actually good for
L-glutamine is the most abundant free amino acid in the human body, serving as a primary fuel source for enterocytes (gut lining cells) and immune cells. The strongest clinical evidence comes from critical care and surgical settings, where glutamine supplementation has been shown in multiple RCTs and meta-analyses to reduce infectious complications and support gut barrier function. For otherwise healthy individuals, the picture is more nuanced: glutamine does help maintain intestinal tight junctions and may benefit people with increased intestinal permeability, but evidence for treating general digestive complaints in healthy adults is limited. In the athletic context, some trials suggest glutamine reduces the incidence of upper respiratory tract infections following intense training, when plasma glutamine levels can drop significantly. However, for muscle recovery and performance per se, systematic reviews find the evidence inconsistent and the effects modest at best in well-nourished athletes. Glutamine earns a B because the gut barrier and immune evidence is mechanistically sound and clinically supported in stressed populations, but the benefits for healthy, well-fed individuals are less certain and the athletic performance claims are overstated by the supplement industry.