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Vitamin A describes a group of metabolites involved in cell and tissue growth, and several bodily functions, including vision, bone metabolism, and immune function. Vitamin A metabolites include preformed vitamin A — retinols, retinals, retinyl esters, and retinoic acids — and precursors (i.e., provitamin A molecules) like beta-carotene, which can be converted by the body into the other forms of vitamin A.
Vitamin A is most often used for
Last Updated:January 8, 2024
Vitamin A describes a group of fat-soluble metabolites that include preformed vitamin A molecules — retinols, retinals, retinyl esters, retinoic acids — and provitamin A molecules — carotenoids like beta-carotene — which are precursors that the body can convert into the other forms of vitamin A.[3][4][5] Vitamin A metabolites are involved in cell and tissue growth and in several bodily functions, including vision, bone metabolism, and immune function.[3][4]
Because they are fat soluble, vitamin A metabolites can be stored in the body (mainly in the liver) as a retinyl ester.[6][7][5][3] However, dietary intake is necessary because humans cannot synthesize vitamin A from scratch.[6][7][3][4] Humans obtain preformed vitamin A — retinols, retinals, retinyl esters, and retinoic acids — from animal products (e.g., fish, eggs, dairy, liver), and obtain provitamin A carotenoids (like beta-carotene) from plants (e.g., sweet potatoes, carrots, leafy greens); the latter can be converted to preformed vitamin A by the body.[1][5] In some countries, some foods — milk, margarine, cereals, etc. — are also fortified with vitamin A.[8][9][10][11]
Vitamin A is also taken as a dietary supplement. Such supplements typically contain preformed vitamin A (retinyl acetate or retinyl palmitate), provitamin A (beta-carotene), or a combination of both.[12] Vitamin A is also found in high levels in some fish oil supplements, such as cod liver oil, and in many multivitamins.[12]
Sufficient amounts of vitamin A can be obtained through a balanced diet, but vitamin A deficiency can occur due to insufficient intake.[1][3][4] Supplementation with vitamin A is used to treat vitamin A deficiency, which is most common in malnourished infants/children living in developing countries.[1][3][4] The evidence shows that supplementation with vitamin A can improve growth, vision, and survival in malnourished infants/children[13][14][15][16][17][18] and in premature babies.[19][20] However, the precise recommendations for treating vitamin A deficiency are unclear because results are inconsistent among studies.[14][15][16][17][18][19][20]
Observational studies show that higher dietary intake of vitamin A, which includes total vitamin A intake from all sources (foods, drinks, and supplements), is associated with a reduced risk of depression.[21] However, this association is derived from cross-sectional and cohort study designs in which vitamin A intake was estimated from historical diet recall using food frequency questionnaires. This makes it difficult to prove a causal link between vitamin A intake and depression.
Besides dietary vitamin A, all-trans retinoic acid (Tretinoin) and 13-cis-retinoic acid (Isotretinoin) are types of vitamin A used in prescription drugs that are effective in treating acne.[22][23][24]
High intakes of preformed vitamin A can cause serious side effects — symptoms include severe headache, blurred vision, nausea, dizziness, muscle aches, and problems with coordination.[1][25][26][12][27]
Excessive intakes of preformed vitamin A have been associated with an increased risk of bone problems — e.g., osteoporosis and hip fracture[28][29] — and some types of cancer, including ovarian cancer,[30] pancreatic cancer,[31] lung cancer,[32] and gastric cancer.[33] However, this evidence is derived from observational studies using cross-sectional and cohort study designs in which vitamin A intake data was estimated from historical diet recall using food frequency questionnaires. This makes it difficult to prove the causality between vitamin A intake and disease risk.
Excessive intake of preformed vitamin A during pregnancy can also cause birth defects, and excessive intake when breastfeeding can impair a child’s growth.[1][25][26][12] Therefore, high-dose supplementation with preformed vitamin A is not recommended when pregnant or lactating — consult your doctor if you are unsure.
In severe cases, excessive intake of preformed vitamin A can cause coma and death,[1][25][26][12][27] and supplementation with high doses of vitamin A has also been associated with increased mortality in well-nourished populations.[25]
There are also several drug interactions with Vitamin A, including, but not limited to, several antibiotics, some weight loss drugs (e.g., orlistat), and vitamin-A-containing drugs (e.g., isotretinoin). Always consult your doctor if you are taking over-the-counter or prescription medicines and planning to use a vitamin A supplement.
Because of the side effects and potential toxicity, there is an upper limit for total daily intake of preformed vitamin A (from food, beverages, and supplements combined):[1][2]
Ages | Upper limit of daily intake |
---|---|
Birth to 12 months | 600 micrograms (mcg or µg) |
Children 1–3 years | 600 mcg |
Children 4–8 years | 900 mcg |
Children 9–13 years | 1,700 mcg |
Teens 14–18 years | 2,800 mcg |
Adults 19 years and older | 3,000 mcg |
Unlike preformed vitamin A, high intake of beta-carotene (a provitamin A carotenoid) is not associated with serious side effects or health problems, and beta-carotene does not have an upper limit of intake. However, this does not mean that taking a higher-than-recommended dose of beta-carotene induces a greater effect.
When ingested, beta-carotene, a provitamin A carotenoid, is converted in the gastrointestinal system into a retinal form of vitamin A, which can be subsequently converted into retinol, retinoic acid, or retinyl ester forms of vitamin A.[5][3][6][7] Retinyl esters are the storage form of vitamin A, and are primarily stored in the liver.[5][3][6]
The metabolism of vitamin A is complex: there are many interconvertible forms, and each has a different role.[6][3][12][7] For example, different types of retinols, retinals, and retinyl esters are precursor molecules to the synthesis of rhodopsin, a pigment involved in vision;[3] retinols can act as cofactors in several enzymatic processes;[12] and retinoic acids directly regulate gene expression while also regulating vitamin A metabolism by, for example, modifying the intestinal absorption of beta-carotene.[3]
Recommended intake is similar between males and females, except during pregnancy and lactation when vitamin A requirements are elevated. The recommended dietary allowances (RDA) are:[1][2]
Ages | Recommended daily intake |
---|---|
Birth to 6 months | 400 micrograms (mcg, or µg) |
Infants 7–12 months | 500 mcg |
Children 1–3 years | 300 mcg |
Children 4–8 years | 400 mcg |
Children 9–13 years | 600 mcg |
Teen males 14–18 years | 900 mcg |
Teen females 14–18 years | 700 mcg |
Teen females, pregnant | 750 mcg |
Teen females, breastfeeding | 1,200 mcg |
Adult males | 900 mcg |
Adult females | 700 mcg |
Adult females, pregnant | 770 mcg |
Adult females, breastfeeding | 1,300 mcg |
Due to potential toxicity, the upper limit of total daily intake of preformed vitamin A (from food, beverages, and supplements combined) is as follows:[1][2]
Ages | Upper limit of daily intake |
---|---|
Birth to 12 months | 600 mcg |
Children 1–3 years | 600 mcg |
Children 4–8 years | 900 mcg |
Children 9–13 years | 1,700 mcg |
Teens 14–18 years | 2,800 mcg |
Adults 19 years and older | 3,000 mcg |
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Full page update
We updated this page and it's database with the newest meta-analyses.