Vitamin A

Vitamin A

Key Facts In Our Multivitamin
Form absorbed Retinol Form Retinyl Acetate
Active forms Retinal Amount 660 µg (RE)
Retinoic acid % NRV 82%
Nutrient Reference Value 800 µg (RE)
Stored in Liver

 

Vitamin A

Vitamin A is actually the name for a group of fat soluble compounds that are all related to retinol. They are obtained from the diet in products of animal origin as pre-formed vitamin A, which includes retinol and retinyl esters such as retinyl acetate and retinyl palmitate. These vitamin A compounds, together with their metabolites and synthetic derivatives, are called retinoids. There’s also pro-vitamin A carotenoids, like beta-carotene. (1-3)

Vitamin A is important for:

✔️ supporting the normal function of the immune system

✔️ helping to maintain normal vision

✔️ helping to maintain normal skin.

     

    Vision

    Vision

    Vitamin A is critical for vision. It is an essential component of rhodopsin, a protein that absorbs light in the eye’s retinal receptors, and because it supports the normal differentiation and functioning of the conjunctival membranes and cornea. (2-4)

    Cell Growth

    Cell Growth

    Vitamin A also supports cell growth and differentiation, playing a crucial role in the normal formation and maintenance of the heart, lungs, kidneys, and other organs. (2)

    Immune

    Immune

    Not long after its discovery, vitamin A was called “the anti-infective vitamin”. However, more recent studies have shown that vitamin A does even more by enhancing recovery from infection. (5,7)

    Skin

    Skin

    Vitamin A is required for the integrity of epithelial cells throughout the body. These are cells that come from surfaces of your body, including skin, blood vessels and organs. (2)

     

     

    Foods that provide vitamin A (2,6)

    Dairy foods are rich in vitamin A.

    Good sources of vitamin A (retinol) include:

    • cheese
    • eggs
    • oily fish
    • fortified low-fat spreads
    • milk and yoghurt
    • liver and liver products such as liver pâté.

     

    How much vitamin A do I need? (6,9)

    The total vitamin A content of a food is usually expressed as micrograms (µg) of retinol equivalents (RE). Vitamin A is fat soluble, so any that your body doesn’t need immediately is stored for future use. (1)

    The amount of vitamin A that adults aged over 18 need is:

    • 700 µg a day for men
    • 600 µg a day for women

     

    How much is too much?

    Having more than an average of 1,500 µg a day of vitamin A over many years may affect your bones, making them more likely to fracture when you're older.

    Bear in mind that other supplements, such as fish liver oil, are also high in vitamin A. So if you take supplements containing vitamin A, make sure your daily intake from food and supplements doesn’t exceed 1,500 µg.

    If you're pregnant, you should avoid taking supplements that contain vitamin A – speak to your GP or midwife.

     

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    Interactions with other nutrients

    Getting too much vitamin A may decrease vitamin C tissue storage. Also, vitamin A can work against vitamin K. Deficiency in zinc may affect the absorption of vitamin A from the gut. Not getting enough vitamin A may result in lower iron absorption. (8)

     

    Absorption and bioavailability

    Vitamin A is fairly efficiently absorbed. Approximately 80% of dietary pre-formed vitamin A is absorbed, but this may be reduced if fat is not present in the diet. Retinyl esters are hydrolysed to retinol in the gut. The released retinol is then absorbed in the upper part of the small intestine (to be technical, it'll enter the body as a component of nascent chylomicrons). (2,8)

     

    Vitamin A chemical structures

     

    Chemistry

    Once absorbed, retinol is re-esterified to retinyl esters, mainly retinyl palmitate, to be stored in the liver. However, vitamin A must be metabolised to compounds called retinal and retinoic acid, which are the active forms of vitamin A, in order to support the vitamin’s important biological functions. Retinal is the major compound involved in vision. Retinoic acid is the the compound required for other processes, where it acts as a nuclear receptor-ligand. (2,3)

     

    Vitamin A to Z

     

    References

    1. Johnson, E. J.; Russell, R. M. Beta-Carotene. In: Encyclopedia of Dietary Supplements, 2nd ed.; Coates, P. M.; Betz, J. M.; Blackman, M. R., Eds. Informa Healthcare: London and New York, 2010; pp 115-120.
    2. Ross, C.A. Vitamin A. In: Encyclopedia of Dietary Supplements, 2nd ed.; Coates, P. M.; Betz, J. M.; Blackman, M. R., Eds. Informa Healthcare: London and New York, 2010; pp 778-791.
    3. Ross, A. Vitamin A and Carotenoids. In: Modern Nutrition in Health and Disease, 10th ed.; Shils, M.; Shike, M.; Ross, A.; Caballero, B.; Cousins, R., Eds. Lippincott Williams & Wilkins: Baltimore, 2006; pp 351-375.
    4. Solomons, N.W. Vitamin A. In: Present Knowledge in Nutrition, 9th ed.; Bowman, B.; Russell, R., Eds. International Life Sciences Institute: Washington, 2006; pp 157-183.
    5. Stephenson, C. B. Vitamin A, infection, and immune function. Rev. Nutr. 2001, 21, 167-192. (PubMed)
    6. https://www.nhs.uk/conditions/vitamins-and-minerals/vitamin-a/
    7. Ruhl, R. Effects of dietary retinoids and carotenoids on immune development. Nutr. Soc. 2007, 66, 458-469. (PubMed)
    8. Expert Group on Vitamins and Minerals; Safe Upper Levels for Vitamins and Minerals; Food Standards Agency: London, 2003.
    9. COMA; Dietary Reference Values for Food Energy and Nutrients for the United Kingdom; Report of the Panel on Dietary Reference Values; Committee on Medical Aspects of Food and Nutrition Policy, HMSO: London, 1991.
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