Vitamin A supplements: retinol, beta-carotene and the pregnancy caution

MedlinePlus describes two dietary routes. Preformed vitamin A, or retinol, is found in liver, fish, eggs and dairy. Provitamin A carotenoids, of which beta-carotene is the best known, give carrots, sweet potatoes, mangoes and dark leafy greens their colour and are turned into the active vitamin in the gut and liver only as the body needs it.

That regulation is why eating plenty of orange vegetables is not a route to vitamin A poisoning, whereas repeated large servings of liver or high-strength retinol capsules can be.

The NHS lists three jobs: helping the immune system work, supporting vision in dim light, and keeping skin and the linings of the airways and gut healthy. MedlinePlus adds the maintenance of teeth and soft tissue and a role in pregnancy and breastfeeding.

Deficiency leads to difficulty seeing in low light and, if severe, dry and damaged corneas (xerophthalmia). It is mostly a problem in countries with food shortages and in people whose digestion cannot absorb fat, rather than in a typical UK diet.

This is the most important practical message. The NHS pregnancy guidance says do not take cod liver oil or supplements containing vitamin A, and avoid foods very high in it such as liver. MedlinePlus states that large doses in pregnant women can cause birth defects.

The concern is specific to preformed retinol at high intakes during early pregnancy, when organs and the nervous system are forming. Beta-carotene from vegetables is not the target of the warning.

Multivitamin products vary, so check the label for vitamin A or retinol before starting one if you are pregnant or could become pregnant. Products marketed for pregnancy are normally formulated with this in mind, but the NHS advice is to choose a product with care and ask a pharmacist or midwife if unsure.

• Do not eat liver or liver pate while pregnant.

• Do not use cod liver oil or other fish liver oil capsules.

• Check multivitamin labels for retinol or vitamin A content.

• Mention any retinoid medicine you use, as some acne and skin medicines have their own strict pregnancy rules set by the prescriber.

Outside pregnancy, the NHS warns that taking more than 1.5mg a day on average over many years may make bones weaker and more likely to break. It singles out older people and postmenopausal women, who are already at greater risk of osteoporosis.

The practical advice is arithmetic. If you eat liver once a week, the NHS says you should not also take a vitamin A supplement. People in the higher-risk groups can reduce exposure by having liver no more than occasionally and checking the labels on fish liver oil and multivitamins.

NCCIH notes that high-dose supplemental vitamin A may increase lung cancer risk in smokers, which is part of why the carotenoid component used in some eye formulas has changed over time. People taking prescription retinoid tablets should not add vitamin A supplements without asking the prescriber, as the effects can add together.

A blood retinol level can be measured but is rarely needed. It may be requested when someone has night blindness, fat malabsorption after bowel or bariatric surgery, or suspected toxicity. Headache, nausea, skin peeling or visual changes in someone taking large amounts should prompt medical advice and a stop to the supplement.