Although glutamine is produced in the human body, depletion can occur in multiple scenarios.  This is why it is recommended that you supplement with pure pharmaceutical grade L-Glutamine, especially if you train hard.  Science tells us that glutamine is anabolic, anti-catabolic, promotes protein synthesis, helps muscles repair and recover, supports hormone levels, supports a healthy immune system, and promotes a healthy gut.  Bodybuilders and athletes who do a lot of high intensity training can benefit greatly from replenishing with L-Glutamine.