vitamin a

Vitamin A is the name for a group of fat-soluble retinoids — primarily retinol and retinyl esters — that the body can also make from provitamin A carotenoids such as beta-carotene found in plant foods. According to the NIH Office of Dietary Supplements, it is involved in immune function, cell growth and differentiation, growth and development, reproduction, and vision, where it forms part of rhodopsin, a light-sensitive protein in the retina. Most of the body's vitamin A is stored in the liver.

The best-established role for vitamin A is correcting deficiency, which is rare in the United States but more common in some lower-income countries. For cancer, the NIH describes the relationship between vitamin A levels or supplementation and cancer risk as "unclear," with observational studies showing mixed results; notably, two large trials (CARET and ATBC) found that high-dose beta-carotene supplements — combined with retinyl palmitate in CARET and with vitamin E in ATBC — increased lung cancer risk in smokers. For age-related macular degeneration (AMD), the AREDS trial found a benefit from a multi-nutrient formula, but the follow-up AREDS2 study showed beta-carotene was not a required ingredient and it was later replaced with lutein and zeaxanthin, so the evidence does not support vitamin A on its own. For measles, WHO recommends high-dose vitamin A for children in deficient settings; a Cochrane review found that two doses were associated with lower mortality in younger children, but it is used as a treatment in deficient populations rather than as a way to prevent infection, and benefits have not been consistently shown across all groups.

The Recommended Dietary Allowance is 900 mcg RAE/day for adult men and 700 mcg RAE/day for adult women (needs are higher during pregnancy and lactation). The Tolerable Upper Intake Level for preformed vitamin A (retinol/retinyl esters) is 3,000 mcg/day for adults; this upper limit does not apply to beta-carotene. Most people in the US meet their needs through food and do not need a supplement, and because vitamin A is fat-soluble and stored in the liver, excess can accumulate over time. Talk to a clinician before supplementing — especially if you are or could become pregnant, smoke, or take other medications.

Because vitamin A is stored in the body, too much preformed vitamin A can be harmful. The NIH warns that exceeding the upper limit can cause congenital birth defects, so people who are or might be pregnant or who are lactating are advised not to take more than 3,000 mcg RAE (10,000 IU) per day of vitamin A supplements. Smokers and people exposed to asbestos should be especially cautious: high-dose beta-carotene and vitamin A supplements were linked to increased lung cancer and death risk in clinical trials of these groups. Beta-carotene is not known to be teratogenic, but the NIH still advises against beta-carotene supplements for the general population except to prevent deficiency. This is general information, not medical advice.

Acute toxicity (hypervitaminosis A) from one or a few very high doses can cause severe headache, blurred vision, nausea, dizziness, aching muscles, and coordination problems; in severe cases, rising spinal-fluid pressure can lead to drowsiness, coma, and even death. Chronic intake of high doses can cause dry skin, painful muscles and joints, fatigue, depression, and abnormal liver test results. Long-term high intake of beta-carotene can turn the skin yellow-orange (carotenodermia), a harmless and reversible condition.

The weight-loss drug orlistat (Alli, Xenical) can reduce absorption of vitamin A, beta-carotene, and other fat-soluble vitamins. Prescription retinoids derived from vitamin A — such as acitretin (Soriatane) for psoriasis and bexarotene (Targretin) — may raise the risk of hypervitaminosis A when combined with vitamin A supplements. The NIH advises anyone regularly taking these or other medications to discuss their vitamin A status with a health care provider.