Beta-carotene to vitamin A conversion (BCO1)

BCO1 is the enzyme that turns beta-carotene (the orange pigment in carrots and leafy greens) into active vitamin A. A common variant lowers its activity, so carriers convert plant beta-carotene into vitamin A less efficiently.

What BCO1 does

Vitamin A comes in two forms. Preformed vitamin A, retinol, is found in liver, dairy, eggs and fish. Provitamin A carotenoids, mainly beta-carotene, come from plants and have to be converted into retinal in the intestinal wall. BCO1 is the enzyme that performs that conversion.

This variant, R267S, is one of two common changes in the enzyme's coding sequence that reduce how efficiently it works.

How much conversion is affected

The measurement that exists comes from a study of female volunteers looking at both common variants together. Women carrying the variant forms of both R267S, read here, and A379V, which is not read here, converted beta-carotene into retinyl esters about 69% less efficiently than others, and the double-variant enzyme showed roughly 57% lower activity in laboratory assays.

Two caveats follow from that design. The large figure applies to the combination rather than to this variant alone, and the study measured a small number of women, so the size of the effect for a person carrying only this variant is not well pinned down. ClinVar classifies the variant as benign, and the site's evidence file records no genome-wide association study for it.

When this actually matters

For most people eating a mixed diet, not much: preformed vitamin A from dairy, eggs and fish bypasses the enzyme entirely, and vitamin A deficiency is rare where those foods are available.

It matters more for people relying on plant sources alone, particularly strict vegans, and in settings where vitamin A deficiency is endemic and beta-carotene-rich staples are the intended solution. Research on biofortified crops takes conversion genotypes seriously for exactly that reason.

What not to do with this result

Do not take high-dose preformed vitamin A supplements on the strength of a genotype. Retinol is fat-soluble and accumulates, excess is toxic to the liver and bone, and it is teratogenic in pregnancy. Beta-carotene from food does not carry that risk, because the conversion is regulated, which is part of why the reduced-conversion genotype matters so little for most people.

High-dose beta-carotene supplements have their own history: trials in smokers found increased lung cancer rates, so supplementation is not a neutral default either.

What each rs12934922 genotype means

rs12934922 has three possible genotypes: AA, AT and TT.

Efficient converter (BCO1) (rs12934922 AA). You have the efficient version of BCO1, so your body readily turns plant beta-carotene (from carrots, sweet potato, leafy greens) into active vitamin A. A normal varied diet covers your vitamin A well.

Somewhat reduced conversion (rs12934922 AT). You carry one copy of the lower-activity BCO1 variant, so you convert plant beta-carotene into vitamin A a bit less efficiently. Eating a range of vitamin-A sources (including some from animal foods or fortified items, if you eat them) helps cover the gap.

Reduced converter (BCO1) (rs12934922 TT). You have two copies of the lower-activity BCO1 variant, so you turn plant beta-carotene into active vitamin A noticeably less efficiently. If you rely mostly on plant sources, getting some preformed vitamin A (dairy, eggs, fish, or fortified foods) or simply more carotenoid-rich produce helps ensure enough.

Common questions

Should I take vitamin A supplements? Not on the basis of this result. Preformed vitamin A accumulates and is toxic in excess, and it is teratogenic in pregnancy. If you eat dairy, eggs or fish you are getting retinol directly, and the conversion step this variant affects is bypassed.

I am vegan. Does this change anything? It is a mild reason to make sure carotenoid-rich foods are eaten regularly and with some fat, which improves absorption, and to raise vitamin A status with a clinician if there is any reason for concern. It is not a reason for high-dose supplementation.

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