Vitamin A

What is vitamin A?

Vitamin A arrives on a shelf in two quite different shapes. One is the ready-made kind, which labels write as retinol, retinyl palmitate or retinyl acetate, and which is also what comes from liver, dairy and eggs, and which has gone into fortified oil, rice, flour and sugar for decades. The other is beta-carotene, one of a family of fat-soluble plant pigments that colour carrots, sweet potatoes and tomatoes, and which the body turns into vitamin A for itself. Inside us the vitamin moves between three forms, retinol, retinal and retinoic acid, the last of which does most of the actual work, and its reputation rests on what researchers saw happen to eyesight and to illness where people went short.

Why do people take vitamin A?

What does vitamin A do in the body?

Vitamin A dissolves in fat rather than water, so it travels with the fats in a meal. Beta-carotene has to be converted into vitamin A inside the body first, whereas retinyl palmitate and retinyl acetate are already vitamin A and need only unwrapping. Once in, it shifts between three forms, retinol, retinal and retinoic acid, the last of which does most of the work.

The single most useful thing to know is where it goes next. The liver is the main store, so the body banks what it has no use for today rather than passing it out, which is why researchers judging a population’s vitamin A look at liver reserves, and why one very large dose lifts the blood reading for under two months. In the cells, reviews describe the vitamin as a regulator of immune cells and the carotenoids as part of the defence against peroxides.

What do people say about taking vitamin A?

These are anecdotes people have shared, not evidence.

What forms of vitamin A are there?

What else is worth knowing about vitamin A?

• Labels give the amount per serving, and a serving may be more than one capsule. Amounts appear in micrograms on some labels and in international units on others, and the two forms are counted separately: the ready-made kind in micrograms or milligrams of retinol equivalents, beta-carotene in milligrams of beta-carotene. It is worth checking which form and which unit you are reading before comparing two tubs.
• Too much vitamin A in pregnancy can cause birth defects. Reviews of the question describe an excess in the mother affecting how the baby’s brain and spinal cord form, and one names the first 60 days after conception as the window in which that can happen; an excess is also linked with raised blood calcium. Those reviews note that this is mainly a wealthier-country problem, because the excess comes from multivitamins and supplements rather than from food, and that routine vitamin A supplementation in pregnancy is not advised except where deficiency is a public health problem. Too little carries its own risks, among them night blindness and anaemia. The trials that did give it in pregnancy, 19 across more than 310,000 women, were run where deficiency is common; they found no difference to deaths of mothers or babies around birth, less night blindness and less anaemia, and a review of 23 trials noted a possible harm through increased HIV transmission in some populations. Pregnancy is named in the line that closes this section for this reason.
• Vitamin A is stored in the liver rather than passed out, so the amount matters more than it does with a water-soluble vitamin: one review notes that populations eating plenty of the ready-made kind in liver, dairy, eggs, fortified foods and one-a-day multivitamins are watched for exactly that reason. A meta-regression of 53 randomised trials with a low risk of bias, covering 241,883 adults, found the dose of vitamin A positively associated with deaths from any cause, although above the recommended allowance of 800 micrograms the difference did not reach significance and below it there were too few trials to say. On bones, two trials that gave adults large amounts for years, 998 adults on 7.5 mg of retinol equivalents a day and 2,322 adults on 25,000 IU a day for as long as 16 years, found no rise in broken bones.
• Beta-carotene carries a finding of its own. A pooled analysis of eight trials in 167,141 people found more lung cancer on beta-carotene than on a dummy tablet, more so still in smokers and in people who had worked with asbestos, and a Finnish trial of 29,133 male smokers found the same whatever their cigarettes. A separate review of 31 trials in 216,734 people found no difference in deaths from any cause overall but a higher rate of death from lung cancer, and the meta-regression above put beta-carotene’s line at 9.6 mg a day. A metabolomic study inside that Finnish trial also saw changes consistent with the liver enzymes that handle medicines being switched up, and called for interactions between supplements and medication to be looked into, particularly in smokers.

If you take medication, are pregnant, or have a health condition, check with a pharmacist or GP first.