Water Soluble Vitamins
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.
Meat is an excellent source of niacin. Legumes, milk, eggs, and yeast are good sources. Grains are commonly enriched with niacin. Milk and eggs are actually low in niacin, but high in tryptophan, a precursor to niacin. The measure of niacin equivalents takes into account the tryptophan that may be converted to niacin. Corn is high in tryptophan and niacin, but also contains the amino acid leucine which inhibits the synthesis of NAD in red blood cells.
Niacin is used for the treatment of hyperlipidemia. Niacin decreases very low-density lipoprotein (VLDL) and increases high density lipoprotein (HDL) leading to a regression in artherosclerotic plaques. The recommended dose is gradually increased to 1.5-2 grams/day if the modified-release form of niacin is taken and 3 grams/day for the regular crystalline niacin form. Side effects of niacin therapy have included flushing, pruritus, urticaria, nausea, vomiting, diarrhea, bloating, and constipation. Research is being conducted to examine the potential effect of niacin therapy for the prevention of non-insulin dependent diabetes.