Vitamin B3

Niacin (B3) at supplemental doses above the flush threshold produces one of the most dramatic and immediately perceptible effects of any vitamin: the niacin flush. Within fifteen to thirty minutes, the skin turns red and hot, beginning in the face and spreading down the chest and arms. There is an intense, prickling warmth that can be uncomfortable or, for some, oddly pleasant. The flush is caused by prostaglandin-mediated vasodilation and is harmless, lasting twenty to sixty minutes. Beyond the flush, niacin supports mood and cognitive function through its role in NAD+ synthesis, but these benefits are gradual and subtle.

Vitamin B3 encompasses three principal vitamers with distinct pharmacological profiles: nicotinic acid (niacin), nicotinamide (niacinamide/NAM), and nicotinamide riboside (NR). All three serve as precursors to NAD+ (nicotinamide adenine dinucleotide) but via different biosynthetic routes.

Nicotinic acid is converted to NAD+ via the Preiss-Handler pathway: nicotinic acid → nicotinic acid mononucleotide → nicotinic acid adenine dinucleotide → NAD+. Niacin also activates the G-protein coupled receptor GPR109A (hydroxy-carboxylic acid receptor 2/HCA2), which mediates its lipid-modifying effects (reduced LDL, reduced triglycerides, increased HDL) through hepatic mechanisms, and also triggers the niacin flush — prostaglandin D2 and E2 release from Langerhans cells in the skin, causing vasodilation, warmth, and redness. Aspirin pretreatment blocks this flush by inhibiting cyclooxygenase.

Nicotinamide enters the NAD+ salvage pathway: NAM → NMN (via NAMPT, the rate-limiting enzyme) → NAD+ (via NMNAT enzymes). Nicotinamide does not activate GPR109A and causes no flush. However, at high doses, nicotinamide can inhibit sirtuins (by accumulating as a byproduct of sirtuin-catalyzed reactions, as nicotinamide is released when sirtuins cleave NAD+). This sirtuin inhibition may paradoxically reduce the beneficial effects of NAD+ on longevity pathways — an important consideration when choosing between niacin forms.

Nicotinamide riboside (NR) is phosphorylated by NR kinases to NMN, then to NAD+. NR bypasses the NAMPT bottleneck (the rate-limiting step of the salvage pathway) and efficiently raises NAD+ levels without flushing and without the sirtuin-inhibiting nicotinamide accumulation.

NAD+ functions as: electron carrier in oxidative phosphorylation and glycolysis; substrate for sirtuins (SIRT1-7, longevity-associated deacetylases/ADP-ribosyltransferases); substrate for PARPs (DNA repair enzymes consuming large amounts of NAD+); and substrate for CD38/CD157 (calcium signaling, immune function).

Pellagra — the niacin deficiency disease — was endemic in the American South and parts of Europe into the early 20th century, where corn (maize) was the dietary staple. Corn's niacin is in a bound, bioavailable form only after alkali processing (nixtamalization — the traditional Mexican/Mesoamerican process), which was not used in the American South and Europe. Consequently, populations dependent on non-nixtamalized corn developed severe pellagra.

Casimir Funk (who coined the term "vitamin") suspected a nutritional cause in the 1910s, and Joseph Goldberger of the US Public Health Service conducted definitive epidemiological studies from 1914–1929 demonstrating that pellagra was nutritional, not infectious. Goldberger's work — including volunteers deliberately inducing pellagra by eating pellagra-associated diets — was a landmark in nutritional epidemiology.

Conrad Elvehjem and colleagues at the University of Wisconsin identified nicotinic acid (niacin) as the anti-pellagra factor in 1937 by curing black tongue disease (canine pellagra analog) with liver extracts and pure nicotinic acid. This led to immediate enrichment programs: by the early 1940s, niacin fortification of flour was standard in the US and UK, and pellagra was essentially eliminated.

The lipid-modifying properties of high-dose niacin were discovered in 1955 by Rudolph Altschul, who found that large doses reduced serum cholesterol. This initiated decades of research and use of niacin as a cardiovascular drug — it remained the most effective HDL-raising agent until the development of modern lipid medications, though cardiovascular outcome trials (AIM-HIGH, HPS2-THRIVE) failed to show additional benefit over statins.

The NAD+ and longevity angle of vitamin B3 research emerged from David Sinclair's work on sirtuins in the 2000s and was dramatically expanded by Charles Brenner's discovery of NR as a novel NAD+ precursor in 2004 and subsequent clinical development of NR and NMN supplements.