Glutamine is an amino acid synthesized by the body from another amino acid, called glutamic acid or glutamate. Glutamine is referred to as a conditionally essential amino acid because under certain circumstances the body is unable to produce enough glutamine to meet its needs, so it becomes "essential" during these times to obtain glutamine from the diet. The most abundant amino acid in the blood and muscle tissue, glutamine participates in many important physiological functions and is especially important in maintaining the health of the gastrointestinal tract and the immune system. In recent years, glutamine has become increasingly popular among athletes, as it is believed that glutamine helps prevent infections following athletic events and speeds post-exercise recovery.

Glutamine is best known for its ability to serve as a source of fuel for the cells that line the gastrointestinal tract. More specifically, glutamine is the preferred fuel source for cells lining the small intestine. By nourishing these cells, glutamine helps maintain the health and integrity of the gastrointestinal tract. A healthy gastrointestinal tract is vital to preserving overall well-being, as the lining of the gastrointestinal tract serves as a first line of defense against disease-causing microorganisms and also minimizes the absorption of potentially allergenic molecules. Glutamine also serves as a source of fuel for muscle and immune cells.

In addition, glutamine plays a role in maintaining the body's proper acid-base balance. Glutamine is synthesized from glutamate and ammonia. Ammonia is a toxic waste compound with a high pH value, meaning that it is basic (as opposed to acidic). When ammonia levels are elevated, the body clears ammonia from the blood by synthesizing glutamine. If the blood is too acidic (pH too low), the body can break down glutamine into glutamate and ammonia to increae the pH of the blood.

Glutamine also serves as precursor to the antioxidant glutathione, participates in glycogen synthesis (the storage form of carbohydrate), and provides nitrogen compounds for the manufacture of nucleotides which are used to make DNA and RNA.