Vitamin B3 is a water-soluble B vitamin occurring as niacin (nicotinic acid) and nicotinamide, widely distributed in foods — meat, fish, poultry, wholemeal grains, legumes, and nuts are good sources, and the body can also make it from the amino acid tryptophan. It is essential to energy metabolism: its coenzyme forms, NAD and NADP, carry electrons in hundreds of reactions that release energy from food. Severe deficiency causes pellagra — the classic four Ds of dermatitis, diarrhoea, dementia, and death — once common where maize-dominated diets lacked both niacin and tryptophan. It is destroyed little by cooking but leaches into cooking water. Fortification of flour has largely eliminated deficiency in wealthy countries.