The major dietary sources of niacin are meats, poultry, and fish, followed by dairy and grain products.[2] Preformed niacin exists in foods as nicotinamide, nicotinic acid, or the pyridine nucleotide coenzymes, NAD and


N 1


N 1

Nicotinic acid (Pyridine 3-carboxylic acid)

Nicotinamide (Pyridine 3-carboxamide)

Pyridine Carboxylic group

Pyridine Carboxamide group

NADP (Fig. 3). L-Tryptophan, the in vivo precursor of nicotinamide (Fig. 2), also contributes to the total niacin-equivalent (NE) content of foods and should be taken into account when calculating the vitamin intake. Eggs and milk, for instance, with their high tryptophan content, are a significant source of NE. Niacin intake is, therefore, generally expressed in NE. It is estimated that 60 g of the amino acid is converted to 1 g of the vitamin, with a variation of about 30% (standard deviation) among individuals. The efficiency of tryptophan conversion to nicotinic acid depends on nutritional history and hormonal factors.[3]

Quantitatively, tryptophan is primarily used for protein biosynthesis, even in conditions of niacin deficiency.