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.
Determination of Niacin and Related Compounds
Dogs and chickens have been used for the biological assay of the vitamin. In these procedures total vitamin activity is determined. Microbiological methods are far more practical from the standpoint of time and expense. The present USP procedure employs Lactobacillus arabinosus as the test organism. The culture medium contains all essentials for growth except niacinactive compounds.
By graded addition of test material, the amounts added can be estimated by determining acid production and comparing this to acid produced upon addition of niacin. A microbiological method useful in the differential determination of nicotinic acid, bound nicotinic acid, and nicotinamide in natural materials was reported by Ghosh and others. Chemical methods are principally refinements of the original cyanogen bromide procedure adapted to niacin. After extraction of niacin-active substances, hydrolysis converts to niacin, which is reacted with CNBr and a reducing agent to yield a color.
The color density is compared to that produced from a known amount of niacin. Methods for a number of niacin metabolites can be found in the paper by Chang and
coworkers. A sensitive spectrophotometric method for DPN and TPN in the oxidized and reduced forms was described by Clock and McLean.
