Vitamin B3 (Niacin)

What Is Vitamin B3 (Niacin)?

Niacin is a water-soluble B vitamin that encompasses two closely related compounds: nicotinic acid and nicotinamide (niacinamide). Both are converted in the body to the coenzymes nicotinamide adenine dinucleotide (NAD) and its phosphorylated form (NADP), which are essential to hundreds of enzymatic reactions. The body can also synthesize a small amount of niacin from the essential amino acid tryptophan (approximately 60 mg tryptophan yields 1 mg niacin equivalent, NE).

The RDA is 16 mg NE/day for men and 14 mg NE/day for women. The tolerable upper intake level (UL) for supplemental nicotinic acid is 35 mg/day due to flushing.

Key Functions

• Energy metabolism: NAD and NADP participate in over 400 redox reactions, including glycolysis, the TCA cycle, and fatty acid synthesis.
• DNA repair: PARP (poly-ADP-ribose polymerase), which uses NAD as a substrate, is critical for DNA strand-break repair.
• Cell signaling: NAD functions as a substrate for sirtuins, deacetylase enzymes linked to metabolic regulation and aging research.

Deficiency, Toxicity, and Food Sources

Niacin deficiency causes pellagra, characterized by the "four Ds": dermatitis (sun-sensitive skin rash), diarrhea, dementia, and, if untreated, death. High-dose nicotinic acid (1-3 g/day) is used clinically to raise HDL cholesterol and lower triglycerides but requires medical supervision.

Food sources high in niacin include chicken breast, tuna, salmon, beef, peanuts, and fortified cereals. Corn-based diets are at risk for pellagra unless the corn is nixtamalized (processed with alkali), which releases bound niacin.