Vitamin B3 is a water-soluble vitamin whose derivatives such as NADH, NAD, NAD+, and NADP play essential roles in energy metabolism in the living cell and DNA repair. The designation Vitamin B3 also includes the amide form, nicotinamide or niacinamide. Severe lack of Vitamin B3 causes the deficiency disease pellagra, whereas a mild deficiency slows down the metabolism decreasing cold tolerance. The liver can synthesize Vitamin B3 from the essential amino acid tryptophan (see below), but the synthesis is extremely slow and requires vitamin B6; 60 mg of tryptophan are required to make one milligram of Vitamin B3. Bacteria in the gut may also perform the conversion but are inefficient.