L-Glutamine

Small-intestinal epithelium preferentially oxidizes glutamine for energy to maintain villus height and tight junctions. Enterocytes use almost all incoming glutamine on the spot; plasma concentration often falls after surgery, infection, or high-intensity training. Chicken, fish, and dairy are common food sources; intestinal mucosal energy supply relies on this amino acid.

Lymphocytes and macrophages consume large amounts of glutamine when proliferating, as a nitrogen donor and nucleic-acid precursor, and oxidize it for energy. Skeletal muscle is the body's main glutamine store, shuttling amino groups to viscera in a nitrogen shuttle. After high training volume, infection, or surgery this supply chain is most easily strained; immune-cell fuel shows the gap first.

L-Glutamine is a nitrogen source for muscle and immune cells; plasma levels fall after intense training, and supplementation trials often watch recovery and upper respiratory infection rates. Beyond gut mucosa, high-training athletes often match gaps on this muscle-recovery path, alongside immune nitrogen-shuttle uses; effects are clearer with continued use.