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Niacin (vitamin B3) is peculiar because it does not depend on a single source: it arrives from the diet as nicotinic acid and niacinamide, but the body can also make it from tryptophan, the essential amino acid. All of these routes lead to the same key molecule: NAD (nicotinamide adenine dinucleotide), the coenzyme that takes part in hundreds of energy-yielding reactions and that also fuels DNA repair enzymes and longevity-regulating enzymes such as sirtuins and PARPs.
The pathway from tryptophan —the kynurenine pathway— is long and inefficient: it is estimated that roughly 60 mg of tryptophan per mg of niacin produced are needed, and it also requires vitamins B2 and B6 as cofactors. Despite that inefficiency, a diet with enough protein provides enough tryptophan to cover a good part of niacin needs, which explains why pellagra appeared mainly in populations that depended on corn, which is poor in tryptophan and in available niacin.
Pellagra —the three classic Ds: dermatitis, diarrhea, and dementia— was a plague in regions where corn was the staple food. The key was not only the lack of niacin, but the double deficiency: corn has little bioavailable niacin and little tryptophan to make it from. The traditional Mesoamerican solution, nixtamalization (treating corn with lime), released the niacin and prevented the disease, a brilliant example of culinary wisdom ahead of biochemistry.
Current interest has shifted from preventing deficiency to trying to raise NAD, which declines with age. Two precursors have concentrated the research: nicotinamide riboside (NR) and nicotinamide mononucleotide (NMN), which enter NAD synthesis through a salvage pathway that is shorter and more efficient than tryptophan. Human studies confirm that they raise NAD levels; what is still not solidly established is what that increase translates into clinically in healthy people. It is a promising but still immature field.
The two dietary forms are not interchangeable in every respect. Nicotinic acid at high doses changes blood lipids (raises HDL cholesterol, lowers triglycerides) and produces the characteristic flushing from vasodilation; niacinamide has no such effect on lipids and does not cause flushing. Both work as a vitamin and as an NAD precursor, but their pharmacological effects at high doses differ, which is worth keeping in mind when choosing one or the other.
Educational information, not medical advice.