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Niacin (nicotinic acid or nicotinamide) is essential in the form of the coenzymes nicotinamide adenine dinucleotide (NAD) and NAD phosphate (NADP) in which the nicotinamide moiety acts as an electron acceptor or hydrogen donor in many biological redox reactions. NAD functions as an electron carrier for intracellular respiration, as well as a codehydrogenase with enzymes involved in the oxidation of fuel molecules. NADP functions as a hydrogen donor in reductive biosyntheses such as in fatty acid and steroid syntheses and, like NAD, as a codehydrogenase. Nicotinic acid and nicotinamide are rapidly absorbed from the stomach or the intestine. Nicotinamide, the major form in the bloodstream, arises from enzymatic hydrolysis of NADP in the intestinal mucosa and liver. It is transported to tissues that synthesize their own NAD as needed. Niacin and NAD are biosynthesized from dietary tryptophan via the kynurenine pathway and quinolinic acid. Excess niacin is excreted in the urine primarily as N1-methylnicotinamide and N1-methyl-2-pyridone-5-carboxamide. Deficiencies: Pellagra, the classic niacin deficiency disease, is characterized by bilateral dermatitis in sun exposed areas, glossitis, diarrhea, and dementia. Often associated with a largely cereal diet such as maize or sorghum, the disease is now rarely seen in industrialized countries but still appears in India, China, and Africa. Pellagra is often associated with other micronutrient deficiencies and may also develop in cases of disturbed tryptophan metabolism (carcinoid syndrome, Hartnup's). Clinical uses: Nicotinic acid (but not nicotinamide) given as a drug in doses of 1.5-4 g/day improves the blood cholesterol profile. Nicotinamide acts as a tumor-specific radiosensitizer, possibly due to its effect on vasorelaxation and increased tumor oxygenation. Diet recommendations: The Recommended Dietary Allowances (RDAs) are expressed in niacin equivalents (NE) in which 1 NE = 1 mg niacin or 60 mg tryptophan. The RDA ranges from 13-19 NE/day for adults or 6.6 NE per 4.186 MJ (1000 kcal). There is an additional allowance of 2 NE/day for pregnancy and 5 NE/day for lactation. The RDA for infants to 1 year ranges from 5-6 NE/day and 9-13 NE/day for children ages 9-13 years. Food sources: Niacin is widely distributed in plant and animal foods, mainly as the pyridine nucleotides NAD and NADP. Good sources are yeast, meats including liver, cereals, legumes, seeds, milk, green leafy vegetables, and fish. Toxicity: Large doses of nicotinic acid given to lower cholesterol may produce flushing of the skin, hyperuricemia, and hepatic abnormalities. These effects are reversed if the drug is reduced in amount or discontinued. Recent research: NAD is the substrate for three classes of enzymes that transfer ADP-ribose units to proteins involved in DNA processing, cell differentiation, and cellular calcium mobilization. Nicotinamide is under investigation for helping prevent and control diabetes. For further information: Swendseid, M.E., & Jacob, R.A. (1994) Niacin. In: Modern Nutrition in Health and Disease (Shils, M.E., Olson, J.A. & Shike, M., eds.), 8th ed., pp. 376-382. Lea & Febiger, Philadelphia, PA.
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