Vitamin B3. Polarised light micrograph of vitamin B3 crystals. Vitamin B3 exists in two main forms: niacin and niacinamide. It is one of the B vitamins complex, and takes part in many metabolic reactions. Good sources of vitamin B3 are liver meat, groundnuts and sunflower seeds. Deficiency in the diet causes the disease pellagra. Pellagra symptoms include a swollen tongue, diarrhoea, mental confusion and depression. Magnification: x80 when printed 10cm wide.