Vitamin A: Benefits, Dosage, and Safety

What vitamin a is

Vitamin A is the name of a group of fat-soluble retinoids, primarily retinol and retinyl esters. The diet supplies it in two forms: preformed vitamin A (retinol and retinyl esters) from animal foods such as dairy, eggs, fish, and organ meats, and provitamin A carotenoids (beta-carotene, alpha-carotene, and beta-cryptoxanthin) from plant foods, which the body converts to retinol in the intestine. Retinol is oxidized to retinal and retinoic acid, the two main active metabolites, and most body stores are held in the liver as retinyl esters.

What it does

• Supports normal vision as an essential component of rhodopsin, the light-sensitive protein in the retina, and maintains the conjunctival membranes and cornea
• Involved in immune function
• Supports cellular communication (cell signaling)
• Supports cell growth and differentiation, including normal formation and maintenance of the heart, lungs, eyes, and other organs
• Involved in growth and development
• Involved in male and female reproduction

How much vitamin a per day

These are population reference values from the NIH and the Dietary Reference Intakes. They are targets for healthy people, not personal prescriptions. The last column is the Tolerable Upper Intake Level, the most you should get from all sources combined.

Deficiency and who is at risk

The most common clinical sign is xerophthalmia. The earliest symptom is night blindness (impaired vision in low light) from low rhodopsin levels. Deficiency also damages the cornea and can lead to permanent blindness, and it is one of the top causes of preventable childhood blindness. Chronic deficiency is associated with abnormal lung development, respiratory disease such as pneumonia, increased risk of anemia, and greater severity and mortality risk of infections, particularly measles and infection-associated diarrhea. A serum or plasma retinol of 20 mcg/dL (0.70 micromoles/L) or less reflects moderate deficiency, and 10 mcg/dL (0.35 micromoles/L) or less indicates severe deficiency. Frank deficiency is rare in the United States.

Who may need more

• Premature infants (under medical supervision)
• People with fat malabsorption disorders such as cystic fibrosis, Crohn disease, or celiac or pancreatic disease
• People in settings where dietary vitamin A is limited
• People taking orlistat, who are advised by the manufacturers to take a multivitamin containing vitamin A and beta-carotene

Food sources

Highest in preformed vitamin A: beef liver (6,582 mcg RAE per 3 ounces), fish, eggs, and dairy products. Rich provitamin A plant sources: baked sweet potato (1,403 mcg RAE per whole), boiled spinach (573 mcg RAE per half cup), pumpkin pie, raw carrots (459 mcg RAE per half cup), cantaloupe, sweet red peppers, and mangos. Milk and margarine are often fortified, and some ready-to-eat cereals are fortified with vitamin A.

Forms and absorption

Supplements commonly contain retinyl acetate or retinyl palmitate (preformed vitamin A), provitamin A beta-carotene, or a combination. Absorption of preformed vitamin A esters from supplements is about 70% to 90%, while beta-carotene absorption ranges from about 8.7% to 65%. Conversion equivalence: 1 mcg RAE = 1 mcg retinol = 2 mcg supplemental beta-carotene = 12 mcg dietary beta-carotene = 24 mcg dietary alpha-carotene or beta-cryptoxanthin. Stand-alone supplements often provide 3,000 mcg RAE (333% of the DV); multivitamins commonly provide 750 to 1,050 mcg RAE. The UL applies only to the preformed (retinol/ester) portion, so the beta-carotene share of a product does not count toward it.

Safety, excess and interactions

Too much

Because vitamin A is fat-soluble, excess is stored in the liver and can accumulate. Acute toxicity (hypervitaminosis A) can occur within days to weeks after one or a few very high doses, typically more than 100 times the RDA, causing severe headache, blurred vision, nausea, dizziness, aching muscles, and coordination problems; in severe cases raised cerebrospinal fluid pressure, drowsiness, coma, and death. Chronic hypervitaminosis A (regular high doses) can cause dry skin, painful muscles and joints, fatigue, depression, and abnormal liver test results. Intakes of preformed vitamin A above the UL, and some oral retinoid medications, can cause congenital birth defects (malformations of the eye, skull, lungs, and heart). Beta-carotene is not known to be teratogenic; long-term excess can cause carotenodermia (harmless yellow-orange skin) that reverses when intake stops.

Interactions to know

• Orlistat (Alli, Xenical): this weight-loss drug can reduce absorption of vitamin A, other fat-soluble vitamins, and beta-carotene, lowering plasma levels; manufacturers recommend taking a multivitamin containing vitamin A and beta-carotene
• Synthetic retinoids such as acitretin (Soriatane) and bexarotene (Targretin): can increase the risk of hypervitaminosis A when combined with vitamin A supplements
• Retinoid medications used as topical or oral therapies (for example isotretinoin for severe acne and etretinate for severe psoriasis): combined with excess preformed vitamin A, can contribute to teratogenic effects and birth defects

How status is measured

Status is usually assessed with serum or plasma retinol, though this is not a sensitive marker because levels stay normal until liver stores are nearly depleted and can fall during infection. A serum/plasma retinol of 20 mcg/dL (0.70 micromoles/L) or less reflects moderate deficiency and 10 mcg/dL (0.35 micromoles/L) or less severe deficiency. Liver vitamin A concentration is the best indicator of adequacy; research settings estimate it indirectly using isotope-dilution or dose-response methods.

Regulated as a dietary supplement ingredient under DSHEA, not FDA-approved to treat, cure, or prevent disease. The FDA Daily Value (DV) for vitamin A is 900 mcg RAE for adults and children age 4 years and older. Vitamin A is a required nutrient on the Nutrition Facts label; food labels must list it only when vitamin A has been added.

What the evidence supports

We grade the strength of evidence behind each common reason people take vitamin a, mirroring how the NIH describes it.