In the human body, vitamin A exists in three main forms: retinol, retinal, and retinoic acid. Vitamin A mainly appears in the form of retinol in the liver and blood; in the eye mainly in the form of retinal; when they regulate gene expression in cells and control the life cycle of cells, they often appear in the form of retinoic acid. (this is the final form and cannot be changed back to the other two).
The vitamin A we eat (in animal foods) tends to be retinyl esters (combinations of fatty acids and retinols such as retinyl palmitate, retinyl acetate), which are converted into retinol in the body. Plant foods are completely free of vitamin A, but contain another compound: carotenoids. Carotenoids are a large class of compounds with more than 600 members, such as alpha-carotene, beta-carotene, beta-cryptoxanthin, lycopene, canthaxanthin, lutein, zeaxanthin. We can effectively synthesize vitamin A (retinal) in the body mainly rely on the first three carotenoids, especially beta-carotene.
Three forms of vitamin A | |||
Food | Retinyl esters | Carotenoids | |
(Animal foods) | (Plant foods) | ||
Body | Retinol | Retinal | Retinoic acid |
Storage and transportation | Visual function | Gene expression | |
The Source of Vitamin A
Vitamin A exists only in animals, plants do not contain vitamin A, but contain pro-vitamin A (precursor) - carotene (C40H56)
Leafy green vegetables are rich in vitamin A, but readily available retinol (the active form of vitamin A) comes only from animals. The richest sources of retinol are: organ meats; fatty fish such as herring and salmon and fish oils; butter, milk and cheese; eggs. At the same time, plant foods also contain carotenoids, which are antioxidant forms of vitamin A that are converted into retinol in the body. Carotenoid-rich plant foods include: squash, carrots, and other orange vegetables; broccoli, spinach, turnip greens, and other dark leafy greens; sweet potatoes; and orange fruits like cantaloupe, papaya, and mango.