Vitamin B3 crystals, polarised light micrograph. Vitamin B3 exists in two main forms: niacin and niacinamide. It is part of the vitamin B complex, and takes part in many metabolic reactions. Good sources of vitamin B3 are liver meat, groundnuts and sunflower seeds. A deficiency of this vitamin in the diet causes the disease pellagra. Magnification: x140 when printed at 10 centimetres wide.