Glutamine is a conditionally essential amino acid abundant in the human body, serving as a key nitrogen donor in nucleotide and amino sugar synthesis. It supports immune cell proliferation, intestinal barrier integrity, and acid-base balance via renal ammoniagenesis. In foods, it occurs naturally in proteins from meat, dairy, and grains, contributing to umami flavor. Chemically, its gamma-amide side chain enables hydrogen bonding and solubility in aqueous media. It is used as a dietary supplement and in cell culture media. Under physiological conditions, it exists as a zwitterion and participates in the glutamate-glutamine cycle in the brain.