Niacin, also known as vitamin B3, is an important nutrient. Niacin is a vitamin that acts as an antioxidant and plays a role in cell signaling and DNA repair. Deficiency is characterized by skin problems, dementia and diarrhea. In fact, every part of your body needs it to function properly. Niacin is one of the eight B vitamins, and it's also called vitamin B3. Niacin is water-soluble, so the body doesn't store it (see: Wikipedia).