⏱ 3 min read
The word vitamin evokes a molecule that we must ingest ready-made, but several of the most important vitamins reach the body —or are made in it— from a provitamin: an inactive form that an enzyme, sunlight, or a metabolic pathway transforms into the functional version. Understanding these precursors explains everyday phenomena: why carrots do not provide as much vitamin A as they seem to, why vitamin D is actually a hormone that we make with the sun, or why a protein-rich diet protects against pellagra.
Conversion is never total or automatic. It depends on specific enzymes (with their genetic variability), on the person's nutritional status, and sometimes on external factors such as ultraviolet radiation. That is why the intake of a provitamin is not simply equivalent to the active vitamin: it is a potential source whose real yield varies from person to person.
The case of vitamin A is illustrative: the beta-carotene in plants is a safe precursor —the body converts only what it needs, avoiding the toxicity of preformed retinol—, but its conversion rate is low and highly variable between individuals, to the point that some people are poor converters because of their genetics. Vitamin D takes the idea to the extreme: its skin provitamin is activated only by light, so at high latitudes or with little sun exposure, production collapses despite having plenty of the precursor.
Niacin breaks the mold: besides being ingested as such, the body can make it —and with it NAD, the central coenzyme of energy metabolism— from tryptophan, the same amino acid that gives rise to serotonin. The efficiency is modest (about 60 mg of tryptophan per mg of niacin), but enough for a diet with adequate protein to prevent pellagra. It is a perfect example of how a single precursor feeds very different pathways according to the body's demand.
Educational information, not medical advice.