Vitamin B3, molecular model. The chemical formula for vitamin B3, also called niacin or nicotinic acid, is C6.H5.N.O2. Atoms are represented as spheres and are colour-coded: carbon (blue), hydrogen (gold), nitrogen (dark blue) and oxygen (red). Vitamin B3 is an essential nutrient that humans are unable to produce and so need to obtain from their diet. It is most concentrated in tuna, salmon, halibut, chicken, venison and asparagus. Vitamin B3 is needed to break down carbohydrates, fats, and proteins in the body. Deficiency can lead to the disease pellegra, with symptoms including cracked and inflamed skin, a poor digestive system, diarrhoea and depression.