Niacin is one of the most stable of the B vitamins. It is resistant to the effects of heat, light, air, acid, and alkali. A white crystalline substance that is soluble in both water and alcohol, niacin and niacinamide are both readily absorbed from the small intestine. Small amounts may be stored in the liver, but most of the excess is excreted in the urine.

Another important fact about vitamin B3 is that it can be manufactured from the amino acid tryptophan, which is essential (needed in the diet). So niacin is not truly essential in the diet when enough protein, containing adequate tryptophan, and other nutrients are consumed. When niacin is not present in sufficient amounts, extra protein is needed. Also, when we are deficient in such nutrients as vitamins B1, B2, and B6, vitamin C, and iron, we cannot easily convert tryptophan to niacin. Many foods that are low in tryptophan are also low in niacin or, as in corn, the niacin is not readily available. Corn is low in tryptophan and its niacin is bound, so it must receive special treatment. Native Americans knew this and would soak corn in ash water before or after grinding to release the niacin. Even when they subsisted almost solely on corn, they did not experience the serious niacin deficiency disease called pellagra. In the time around the American Civil War, in the South poor white farm workers subsisted on "quick cornmeal," the poorly prepared white people's version, and pellagra was epidemic until the discovery that it was a dietary deficiency disease. Pellagra, the disease of the "three Ds"--diarrhea, dermatitis, and dementia--historically has been a problem of corn-eaters, whereas beriberi has been a disease most correlated with rice-eating cultures.

Sources: Only small to moderate amounts of vitamin B3 occur in foods as pure niacin; other niacin is converted from the amino acid tryptophan, as just discussed. The best sources of vitamin B3 are liver and other organ meats, poultry, fish, and peanuts, all of which have both niacin and tryptophan. Yeast, dried beans and peas, wheat germ, whole grains, avocados, dates, figs, and prunes are pretty good sources of niacin. Milk and eggs are good because of their levels of tryptophan. Though B3 is stable, the milling and processing of whole grains can remove up to 90 percent of the niacin. Thus, manufacturers will often "enrich" their products by adding niacin.

Functions: Niacin acts as part of two coenzymes, nicotinamide adenine dinucleotide (NAD) and nicotinamide adenine dinucleotide phosphate (NADP), that are involved in more than 50 different metabolic reactions in the human species. They play a key role in glycolysis (that is, extracting energy from carbohydrate and glucose), are important in fatty acid synthesis and in the deamination (nitrogen removal) of amino acids, are needed in the formation of red blood cells and steroids, and are helpful in the metabolism of some drugs and toxicants. Thus, niacin is a vital precursor for the coenzymes that supply energy to body cells.