Vitamin A
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 Retinol (depicted) and retinal play a biological role in vision, but most of the effects of vitamin A are exerted by retinoic acid, which binds to nuclear receptors and regulates gene transcription. | |
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| AHFS/Drugs.com | Monograph |
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| Routes of administration | By mouth, intramuscular[1] |
| Drug class | Vitamin |
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| Formula | C20H30O |
| Molar mass | 286.46 g·mol−1 |
| 3D model (JSmol) | |
| Melting point | 62–64 °C (144–147 °F) |
| Boiling point | 137–138 °C (279–280 °F) (10−6 mm Hg) |
Vitamin A (also known as retinol, vitamin A1)[3][4] is a fat-soluble vitamin, a group of organic chemicals including retinal and several carotenoids. The human body does not make these chemicals, and must take them from food. It stores them in fat (lipids). Because it can be stored by the body it is not needed every day.[4] Egg yolks and dairy (for example, milk and cheese) are rich in vitamin A.[3][4]
The source of vitamin A in diet affects the dose of vitamin A that is needed, as they have different bioavailabilities.[5]The form that vitamin A is in when it is taken into the body is part of the reason of what causes this. The efficiency of the conversion can also depend on genetics and fat consumption.
Leafy yellow, green, and red vegetables, and yellow fruits, such as mango, papaya, and apricots, are the main sources of beta-carotene.[4]
Many parts of the body need vitamin A. For example, vitamin A helps sight and is good for the immune system. It is also important for a growing embryo.
Effects
Vitamin A is essential in humans. It is used for metabolic functioning of the retina and helps people see in low light,[4] and the functioning of the immune system, development of an embryo (reproduction), as well as the growth of bone. It also helps in keeping epithelial tissue, such as the nose, healthy.[3][4]
Diet should provide all of the vitamin A that is needed in humans.[4] The total vitamin A content of a food is usually expressed as micrograms (µg) of retinol equivalents (RE). The amount of vitamin A adults aged 19 to 64 need is 700 µg a day for men and 600 µg a day for women.[4]
Vitamin A Deficiency
Too little vitamin A is dangerous to health.[6] People who do not get enough of the vitamin may lose the ability to see in poor light and suffer from a weakened immune system. They may also have problems with memory, because vitamin A is important for the brain. People with malnutrition often have too little vitamin A. This is common in poor countries.
Hypervitaminosis A
Too much vitamin A is also dangerous to health.[6] It is a condition known as hypervitaminosis. Vitamin A is not soluble in water, and the human body can not get rid of the excess vitamin A easily by urination like some other vitamins.[7] Someone might get too much vitamin A from taking too many vitamin supplements or from eating too much A-rich food like liver.
Vitamin A does not need to be eaten every day. Because it can be stored in fat, anything that is not used by the body is stored for later.[4] Eating too much vitamin A can lead to problems later in life, like easily fractured bones. This is a risk in women when combined with their risk of postmenopausal bone breakdown (osteoporosis).
What applies to humans applies to all mammals at least. Some large predators, such as the big cats, seem not to eat the liver of their prey. On the other hand, polar bears apparently do, and they store vitamin A in their liver cells.[8] However, they have long, sometimes very long, periods between kills. In contrast, African big cats kill about every three or four days, and they do seem to avoid eating the liver.
Types
Retinol (pre-formed) in animal meat is more bioactive than plant-based retinol.Plant-based retinol is in the beta-retinol (β-retinol) form.β-carotene (beta carotene), a provitamin found in foods like carrots, can be converted to vitamin A in the body.[9] In this (plant-based) form, it is found as beta-carotene and is twice as bioactive as alpha-carotene.[5] Supplements containing beta-carotene are as useful to the body as pre-formed retinol (from animal meats) and requires the same dose..
Dietary alpha-carotene, or beta-cryptoxanthin is half as bioactive as beta-carotene, which is found in plants.
Dose
The dose depends on the type of retinol as well as other factors, such as gender. Men need 700mcg (micrograms, sometimes seen as µg), or 600mcg for women.[4] Supplements are as useful (bioactive) to the body as the natural form found in meat, called pre-formed retinol. This form of retinol is more useful (bioactive) to the body than beta-retinol, for example the plant-based vitamin A found in a healthy, balanced diet. Vitamin A is dosed in mcg (micrograms) of retinol equivalents (RE or sometimes RAE).[4] The retinol found in plants or other food sources is different to meat-based retinol. Beta-cryptoxanthin is found in non-meat based dietary food.
The RE (dose) of the forms is based on their bioavailability. 1 mcg (microgram) RE equals 1 IU (international unit) divided by how potent the form of vitamin A is:[5]
- Pre-formed retinol (from animals): 1mcg = 1 IU / 3.33
- Beta-carotene (from supplements): 1mcg = 1 IU / 3.33
- Beta-carotene (from plants): 1mcg = 1 IU / 20
- Beta-cryptoxanthin (from food): 1mcg = 1 IU / 40
- Alpha-carotene (from food): 1mcg = 1 IU / 40
Vitamin A Media
Typical location of Bitot's spots
Carrots are a rich source of β-carotene.
Vitamin a supplementation coverage rate children ages 6-59 months
Retinoids: Tretinoin is all-trans-retinoic acid; initial tradename: Retin-A. Isotretinoin is 13-cis-retinoic acid; initial tradename: Accutane. Etretinate and Acitretin, its non-esterified metabolite, are used orally to treat severe psoriasis.
Vitamin A biosynthesis from β-carotene
Chemical structure of β-ionone.
Frederick Gowland Hopkins, 1929 Nobel Prize for Physiology or Medicine
George Wald, 1967 Nobel Prize for Physiology or Medicine
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Related pages
- Vitamin B and folic acid (B9)
- Fentanyl, a drug that is lipid-soluble
References
- ↑ Vitamin AThe American Society of Health-System Pharmacists. Retrieved 8 December 2016.
- ↑ Aquasol A - vitamin a palmitate injection, solution. DailyMed (14 August 2020). Retrieved 18 January 2024.
- ↑ 3.0 3.1 3.2 MeSH Browser. meshb.nlm.nih.gov. Retrieved 2026-04-01.
- ↑ 4.00 4.01 4.02 4.03 4.04 4.05 4.06 4.07 4.08 4.09 4.10 Vitamins and minerals - Vitamin A (in en). nhs.uk (2017-10-23). Retrieved 2026-04-01.
- ↑ 5.0 5.1 5.2 EBOOK-cutting-through-labeling-confusion.indd.
- ↑ 6.0 6.1 Dietary Supplement Fact Sheet: Vitamin A and Carotenoids (2006-04-23)Ods.od.nih.gov. Retrieved 2010-08-20.
- ↑ Anderson J, Young L. Fat-Soluble VitaminsColorado State University, Cooperative Extension. Retrieved 20 May 2007.
- ↑ Senoo H, Imai K, Mezaki Y, Miura M, Morii M, Fujiwara M, Blomhoff R. 2012. Accumulation of vitamin A in their liver cell of arctic top predators. Anatomical Record 295 (10): 1660–8 [1]
- ↑ Chichili, Gurunadh Reddy. β-Carotene Conversion into Vitamin A in Human Retinal Pigment Epithelial Cells (in en). Investigative Ophthalmology & Visual Science 46 (10) (2005-10-01). p. 3562–3569. doi:10.1167/iovs.05-0089.