Occurrence of Niacin
In natural sources the vitamin occurs as free nicotinic acid, bound nicotinic acid, or nicotinamide in coenzyme form. Ghosh and others determined the content of these forms in a variety of natural food products. About 40 per cent of the nicotinic acid of oil seeds and most of it in cereals is present in bound form. Yeast, crustacea, fish, animal tissues, and milk contain principally free and amide forms of nicotinic acid. The vitamin is rather widely distributed in both plant and animal tissues. Certain animal organs, including liver, kidney, and heart, as well as lean meat and some fish flesh are outstanding sources. Yeasts, peanuts, wheat germ, and dried legumes are in the same category. Fresh legumes and a few green vegetables are good sources, whereas milk, eggs, and most fruits are generally classed as poor sources.
The niacin content of 160 Indian foodstuffs, many of which are common constituents of the American diet, has been reported. According to the data of this report, dried yeast may contain from 20 to 62, rice polishings around 28, wheat germ 7, sheep liver 15, and pork muscle 2.8, all in milligrams per cent. Milk and eggs contain only 0.1 mg per cent. Elvehjem and co-workers also gave an extensive list of foods and their niacin content. Niacin, like other B vitamins, resides in wheat primarily in the bran and middling fraction. These workers list the niacin-content of patent flour 0.80, fancy first clear flour 1.68, second clear flour 5.55, clean shorts 5.44, and wheat feed and screenings 19.2 mg per cent.
Niacin Requirement
It should be noted that in the table of recommended dietary allowances, the niacin allowance is stated in milligram equivalents. It has been established that in man about 60 mg of food tryptophan are equivalent to 1 mg niacin (due to conversion), so that the required niacin intake varies with the tryptophan content of the diet. A diet providing 60 g of mixed protein contains around 600 mg of tryptophan or 10 niacin equivalents from the amino acid.
On the basis of other studies and allowing for a liberal safety factor, the recommended daily allowance (1963 revision) has been set at 19 mg niacin equivalents for a 70 kg adult and up to 22 for boys 15 to 18 years old (68 kg). In a controlled study in four women the mean urinary excretions of Nl-methylnicotinamide and its 6-pyrone were 5.8 and 7.3 mg per day, respectively. With a diet providing 8.7 mg nicotinamide and 770 mg of tryptophan from 60 g of protein daily, these excretions indicate the adequacy ofthe 21 niacin equivalents ingested. More recent studies upheld the original value of about 60 mg tryptophan equivalent to 1 mg niacin in humans.
