⏱ 3 min read
Vitamin A is essential for vision, immunity, skin, and embryonic development, and it reaches the body by two very different routes. One is preformed retinol (and its esters) found in animal-source foods such as liver, egg yolk, and dairy: it is vitamin A that is ready to use. The other is provitamin A, mainly beta-carotene, which is abundant in orange vegetables —carrot, sweet potato, pumpkin— and in leafy greens, and which the body must convert.
Conversion takes place in the intestinal wall, where the enzyme beta-carotene oxygenase (BCMO1) splits the beta-carotene molecule into two of retinal, which is then reduced to retinol. In theory one molecule of beta-carotene yields two of vitamin A; in practice the yield is much lower and varies greatly from person to person.
Official estimates assume that roughly 12 units of beta-carotene from food are needed per unit of retinol, an efficiency much lower than the theoretical ratio. The reason is multifactorial: beta-carotene is trapped in the plant matrix (cooking and chopping release it), it needs fat in the meal to be absorbed, and its conversion is downregulated when vitamin A stores are sufficient. In addition, a notable fraction of the population carries variants of the BCMO1 gene that further reduce their conversion capacity, making them poor converters.
That regulated conversion is precisely what makes beta-carotene a safe source: the body makes only the retinol it needs and stores the rest as a harmless pigment (an excess merely produces a reversible orange coloration of the skin). Preformed retinol has no such brake: at high doses it is toxic to the liver and, very importantly, teratogenic —it can harm the fetus—, which is why it is strictly limited in pregnancy. This difference is the best illustration of why, sometimes, providing the precursor is more prudent than providing the active vitamin.
Educational information, not medical advice.