Niacin, or vitamin B3, exists in 2 forms- nicotinic acid (niacin) and nicotin0amide (niacinamide). Both forms are used to synthesize the niacin coenzymes: nicotinamide adenine dinucleotide (NAD) and nicotinamide adenine dinucleotide phosphate (NADP﹢).
NIACIN IN FOODS
Niacin can be obtained from foods as the vitamin itself (preformed niacin) or synthesized in the body from the essential amino acid tryptophan.
Sources are poultry, meat, fish, enriched bread and bread products. Coffee and tea also contribute a little preformed niacin to the diet. Other good sources are mushrooms, wheat bran, and peanuts. Protein-rich foods are also good sources of niacin because they provide tryptophan. Unlike some other eater-soluble vitamins, niacin is very heat stable and little is lost cooking.
FUNCTIONS OF NIACIN
Like the coenzyme forms of riboflavin, the coenzyme forms of niacin, NAD﹢ and NADP﹢, are active participants in oxidation-reduction reactions. The niacin coenzymes function in at least 200 reactions in cellular metabolic pathways, especially those that produce ATP. NAD﹢ is required mainly for the catabolism of carbohydrates, proteins, and fats. NAD﹢ acts as a an electron and hydrogen acceptor in glycolysis and the critic acid cycle.
Under anaerobic conditions, NAD﹢ is regenerated when pyruvate is converted to lactate. Under aerobic conditions, NADH + H﹢donates electrons and hydrogens to acceptor molecules in the electron transport chain, thereby contributing to ATP synthesis. Alcohol metabolism also requires niacin coenzymes.
NIACIN NEEDS AND UPPER LEVEL
The niacin RDA for adult men is 16 mg/day; for adult women, it is 14 mg/day/ The RDA for niacin is expressed as niacin equivalents (NE) to account for preformed niacin in foods and niacin synthesized from tryptophan. Typical intakes of niacin in the U. S. exceed the RDA; in fact, tryptophan is the major source of niacin.
The Daily Value for niacin on food and supplement labels is 20 mg. The Upper Level for niacin, 35 mg/day, applies only to niacin supplements and fortified foods.
ABSORPTION, TRANSPORT, STORAGE OF NIACIN
Nicotinic acid and nicotinamide are readily absorbed from the atomach and the small intestine by active transport and passive diffusion, so that generally almost all the niacin that is consumed is absorbed. However, the bioavailability of niacin is low in some grains, especially corn. This because the niacin is tightly pund to protein; thus, less than 30% can be absorbed.
Niacin can be released from the protein and its bioavailability improved by soaking corn in a solution of calcium hydroxide dissolved in water (water with lime). This practice is common among native peoples in Latin America, where corm is a staple food; it protects them from niacin deficiency. After being absorbed, niacin is transported via the portal vein to the liver, where it is stored or delivered to the body’s cells. Niacin is converted to its coenzyme forms in all tissues. Any excess niacin is excreted in the urine.
NIACIN DEFICIENCY
Pellagra-the deficiency disease of the B-vitamin niacin – is the only dietary deficiency disease ever to reach epidemic proportions in the U. S. In the early 1900s, pellagra affected thousands in southeastern states before scientists discovered its link with niacin-poor diets.
Pellagra has long been associated with corn-based diets. Although there is no evidence of pellagra among the native populations of North, Central, amd South America, where corn (maze) has been the spatle food in the diet for thousands of years, pellagra outbreaks followed the introduction of corn into Europe and Africa. As mentioned previously, the main reason for this was that the indigebous people of Latin America treated corn with alkali (from lime water or wood ashes), which released the niacin that is tightly bound to protein.
During the early 1900s, pellagra was rampant in the southeastern U.S., where corn was the staple food of poor people. More than 10.000 Americans died of pellagra in 1915. From 1918 until the end of World War II in 1945, approximately 200.000 Americans suffered from this disease. Many people has such severe dementia that they were forced to live out their lives in mental institutions.
One reason that pellagra remained a problem for so long was the false belief that pellagra was an infectious disease. Finally, in 1937, researchers discovered that nicotinic acid dramatically cures a similar disease in dogs, called black tongue. Soon after, the enrichment of grain products with niacin in the U.S. virtually eliminated pellagra, although isolated cases still occur due to severe malabsorption, chronic alcoholism, or Hartnup’s disease ( a genetic disorder in which the tryptophan to niacin pathway is blocked). Today, pellagra still can be found in Africa, particularly when famine occurs, or in refugee camps when ratios are mostly maize.
PHARMACOLOGICAL USE OF NIACIN
Niacin, as nicotinic acid, is sometimes prescribed by physicians to lower LDL cholesterol levels and increase HDL cholesterol levels. When combined with diet, exercise, and other cholesterol-lowering medications, nicotinic acid can reduce the risk of heart attack. The dose required, 1 to 2 g daily, is more than 60 times the RDA. The most common side effect is flushing of the skin, but GI tract upset and liver damage also can occur.
