Vitamin B3, molecular model. Vitamin B3 (niacin) is an essential nutrient that humans are unable to produce and 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. Vitamin B3's chemical formula is C6H5NO2.Atoms are represented as spheres and are colour-coded: carbon (C, grey), hydrogen (H, turquoise), nitrogen (N, blue) and oxygen (O, red).