Vitamin A

Vitamin A exists in two forms that behave quite differently. Preformed vitamin A, or retinol, comes from animal foods — liver, oily fish, egg yolk, dairy — and is ready to use immediately.

Provitamin A carotenoids, principally beta-carotene, come from plants and give carrots, sweet potato, peppers and leafy greens their colour. The body converts these to retinol as needed.

That conversion is regulated, which is the crucial practical difference. The body will only convert as much beta-carotene as it needs, so beta-carotene from food does not build up. Preformed retinol has no such brake, accumulates in the liver, and is the reason vitamin A has a meaningful upper limit at all. Two products both labelled vitamin A can therefore carry entirely different risk profiles.

Retinal, a form of vitamin A, combines with a protein in the retina to form the pigment that detects light. This is the most direct and best-known role, and why poor vitamin A status affects vision in dim light first.

Retinoic acid, another form, acts on nuclear receptors that regulate gene expression, influencing cell differentiation across many tissues. This is why vitamin A matters to skin, to the linings of the gut and airways, and to immune cell development.

Because it is fat-soluble, absorption requires dietary fat, and the liver stores substantial reserves — enough to buffer intake for months, which cuts both ways.

Vitamin A's essentiality is beyond dispute, and correcting genuinely low status produces clear improvement in vision and immune function. Low status is uncommon in the UK but a major issue globally.

For supplementation in well-fed people, there is little evidence of benefit and reasonable evidence of downside at high intakes.

One finding deserves specific mention: large trials of high-dose beta-carotene supplements in smokers reported worse outcomes in the supplemented groups rather than better. This was unexpected, has been replicated, and is a clear example of a nutrient behaving differently as an isolated high-dose supplement than as part of food. Anyone who smokes should avoid high-dose beta-carotene supplements.

Beta-carotene from food is safe, with the only notable effect being a harmless orange tint to the skin at very high intakes, which resolves on reducing it.

Preformed retinol is a different matter. Because it accumulates in the liver, sustained high intake causes headache, nausea, dry skin, hair loss, bone pain and liver problems. This is one of the few vitamins where genuine excess is realistic from supplements.

The pregnancy caution is firm rather than precautionary: high retinol intake can harm a developing baby. Avoid retinol supplements, cod liver oil and liver if pregnant or trying to conceive. Anyone who smokes should avoid high-dose beta-carotene supplements.