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.