Vitamin B3 (Niacin) (B3)

The conversion is the whole story. The NIH counts 60 mg of dietary tryptophan as equivalent to 1 mg of niacin, which is why nutrition science measures B3 in niacin equivalents rather than as a single substance. A diet heavy in protein can quietly cover a good share of the requirement through that route, while a plasma draw, taken at one moment after one meal, says little about the steady internal supply.

That is also why intake targets read the way they do. The NIH sets the recommended dietary allowance at 16 mg of niacin equivalents per day for adult men and 14 mg for adult women, rising to 18 mg in pregnancy and 17 mg during lactation. Every one of those figures is a niacin-equivalent number, written to account for the body's own manufacturing. A blood test reports none of that.

When clinicians do need to know a person's niacin status, the NIH points them toward the kidneys rather than the bloodstream. The breakdown products of niacin metabolism, N1-methylnicotinamide and N1-methyl-2-pyridone-5-carboxamide, show up in urine, and their excretion tracks status more reliably than a plasma value. It is a roundabout-seeming answer, measuring the waste to infer the supply, but it sidesteps exactly the problem the in-house production creates.

A real deficiency is uncommon in industrialized populations, which is part of why few people ever need this tested. When it does occur, the NIH describes the classic endpoint as pellagra, remembered as the three Ds: dermatitis, diarrhea, and dementia. The skin sign is distinctive, a pigmented, sunburn-like rash on areas exposed to light, and the mouth can show it too, with a bright red tongue. The digestive and neurologic changes round out the picture.

Because the body can make niacin from tryptophan, a shortage usually means both routes have failed at once. The NIH ties most deficiency in developed countries to alcohol use disorder, which is the commonest cause there, alongside malabsorption, diets genuinely low in both niacin- and tryptophan-containing foods, and specific conditions such as carcinoid syndrome and Hartnup disease that disrupt tryptophan handling.

This is also where the contrast with vitamin B6 and vitamin B2 matters: B vitamins are often tested as a group, but they answer different questions and fail in different ways, and a normal-looking B3 number sits among them without meaning much on its own.

Most people who think about niacin at all are reacting to the flush, not to a deficiency. A large dose produces warmth, redness, and an itching or tingling spread across the face and upper body. The NIH describes this as prostaglandin-mediated, driven by GPR109A activation, and it is harmless but startling. It is also the basis for the tolerable upper intake level the NIH sets at 35 mg per day for adults. So the flush is a sign of dose, not of prior shortage.

For years, high-dose niacin was prescribed to nudge cholesterol, raising HDL and lowering LDL on paper. The outcomes did not follow. AIM-HIGH, which enrolled 3,414 people with cardiovascular disease, low HDL cholesterol, and high triglycerides, was stopped early after about three years because adding extended-release niacin to a statin produced no reduction in cardiovascular events. HPS2-THRIVE, far larger at 25,673 statin-treated patients, confirmed it: major vascular events occurred in 13.2% on niacin versus 13.7% on placebo, a rate ratio of 0.96 (95% CI 0.90 to 1.03), with no real benefit. The same trial found more harm, with serious adverse events in 55.6% versus 52.7%, including new-onset diabetes (hazard ratio 1.32). That evidence is why high-dose niacin for moving LDL cholesterol fell out of favor.

If your panel reported a niacin value, the most useful thing to understand is what it can and cannot say. It cannot confirm your diet is adequate, because the tryptophan route runs underneath it. It cannot rule deficiency in or out the way a urinary metabolite assessment can. What it can do is sit alongside the rest of a vitamins and nutrients panel as one data point among several, best read by someone who knows your symptoms and history. The vitamin panel guide walks through how these markers fit together.

A plasma value also prints in different units depending on the lab and country, micrograms per liter in some reports and nanomoles per liter in others, so the same status can look like two unrelated numbers; a short primer on SI versus conventional units clears up which is which. Even then, for a vitamin the body partly builds for itself, that converted number is about as honest as a blood test for B3 can be.