Glutamine plays significant roles in various parts of the body. The immune system for example uses more glutamine for its functions than it does glucose, or sugar. Every part of the immune system uses glutamine in order to build killer cells, repair damage, and increase cytokine function. In gut physiology, glutamine promotes enterocyte proliferation, regulates tight junction proteins, suppresses pro-inflammatory signaling pathways, and protects cells against apoptosis and cellular stresses during normal and pathologic conditions. Thus healing the gut lining and allowing for the intestines to function at their highest levels.
Athletes have been taking glutamine for decades now as a way to significantly increase healing and reduce the chances that they will get exercise induced immune suppression, which is where you work out so hard you lower your immune system and get sick. This is a key nutrient for any athlete that works out hard and is under any stress from daily life, or is around any illness.