L-glutamine is the most abundant free amino acid in plasma and muscle. Conditionally essential — needs exceed synthesis during major stress, illness, or heavy training.
Mechanism — Primary fuel for enterocytes (gut epithelial cells) and lymphocytes. Supports tight junction integrity. Modest role in glycogen replenishment and ammonia management post-exercise.
Use case — Gut barrier support (5-15 g/day), recovery from heavy training or illness, post-surgery. Best on empty stomach.
Caveats — Marketing in muscle-building context oversells the effect. Real value is in gut and immune support. Don't use in advanced liver disease without medical oversight.