Vitamin B3, also called niacin, is a member of the B complex of vitamins. Like its relatives in this complex, niacin is also necessary to free energy from food, particularly the conversion of proteins, fats and carbohydrates into energy the body can use. The body uses niacin to synthesize glycogen, a reserve form of energy. Niacin is also important for the normal structure of the mucous membranes and skin, and for healthy functioning of the nervous system.