Niacin is widely distributed in plant and animal foods. Good sources are bakers' yeast and meats (including liver), cereals, legumes and seeds but significant amounts are also found in many other foods such as milk, green leafy vegetables, and fish, as well as coffee and tea. Niacin occurs in plants mostly as nicotinic acid and in animal tissues mostly as nicotinamide.
In plants niacin is found mostly in a bound form (sometimes referred to as niacytin or niacinogen) affecting its availability in the diet. In maize, niacin is present in covalently bound complexes with small peptides and carbohydrates and is therefore unavailable when eaten. The bioavailability of the bound form of niacin can be improved substantially by hydrolysis with a mild alkali. It has long been a tradition in Central America to soak maize in lime-water prior to the preparation of tortillas. This practice effectively liberates the bound niacin and appears to be responsible for effective protection against pellagra in that part of the world. In other plant foods, the bound niacin is heat-labile and the niacin can be released by heating. Thus, the roasting of coffee beans increases the available nicotinic acid content in coffee from 20 to 500 mg/kg coffee beans (Combs, 1992). This practice might have contributed to the prevention of pellagra in South and Central America along with the regular consumption of beans that are good sources of bioavailable nicotinic acid.
