Niacin, also called nicotinic acid, is involved in oxidation-reduction reactions as coenzymes that convert protein and glycerol from fats to glucose, and oxidize glucose to release energy. Niacin is involved in the synthetic pathway of adenosine tri-phosphate (ATP) and in ADP-ribose transfer reactions. The two niacin coenzymes involved in these processes are nicotinamide-adenine dinucleotide (NAD) and nicotinamide-adenine dinucleotide phosphate (NADP). NAD is a substrate for the enzyme poly (ADP-ribose) polymerase (PARP) which is involved in DNA repair.
Absorption and excretion
Niacin is absorbed into the intestine by diffusion. Approximately 15-30% of niacin is bound to protein and taken up by the tissues. Niacin metabolites are excreted in the urine.
Tryptophan is a precurser to niacin. The amount of tryptophan converted to niacin is dependent on diet and hormonal factors. If there is a deficiency in both tryptophan and niacin, tryptophan will be used for protein synthesis rather than niacin production.
Niacin deficiency leads to a disorder called pellagra. Pellagra is characterized by the 3 "D"s: dermatitis, diarrhea, and dementia. Skin exposed to sunlight develops dark, scaly dermatitis. There is a characteristic "necklace" lesion on the neck. Niacin deficiency can lead to inflammation of the mucous membranes of the tongue, esophagus, urethra, prostate, and vagina. Intestinal inflammation leads to diarrhea. Long-term niacin deficiency leads to central nervous system dysfunction manifested as confusion, apathy, disorientation and neuritis. Individuals may be predisposed to niacin deficiency if they are also consuming low levels of vitamin B6, copper, and riboflavin.
Niacin deficiency can also lead to a disease called Hartnup's syndrome. This is due to impaired synthesis of niacin from tryptophan.
Recommended intakes of niacin are expressed in milligrams of niacin equivalents. The 1989 recommendations are: 2-4 mg for infants, 6-8 mg for children, 12-16 mg for adolescents, 16 mg for men, 14 mg for women, 18 mg for pregnant women, and 17 mg for lactating women.