What is Vitamin A?

Vitamin A is a fat soluble vitamin that is required for a number of bodily functions. This vitamin is needed to maintain good vision and a healthy immune system, as well as being essential for growth and development.

Vitamin A describes a group of compounds that include retinol, retinoic acid, retinal, and a number of provitamin A carotenoids such as beta-carotene. All types of vitamin A contain a beta-ionone ring with an isoprenoid chain attached which is referred to as the retinyl group. Both structures are required for vitamin activity.

Functions of Vitamin A

In the eyes, retinal is combined with a protein called opsin to give rhodopsin, an essential light absorbing molecule needed for color vision and seeing in dim light.

Another form of Vitamin A, retinoic acid, is a key hormone-like growth factor for epithelial cells and other cell types in the body.

Sources of Vitamin A

Vitamin A is found in both plant and animal food sources. The form of vitamin A absorbed when animal sources are consumed is retinyl palmitate, which gets converted into an alcohol called retinol. Retinol acts as a storage form of vitamin A, which can be converted to and from retinal, the active aldehyde form of the molecule. One of the richest animal sources of Vitamin A is liver.

Plant sources of Vitamin A include orange and yellow fruits and vegetables such as carrots, pumpkin, sweet potato, apricots and mangoes. The orange/yellow pigment occurs due to the presence of provitamin A carotenoids such as beta-carotene. These compounds need to be converted into vitamin A or retinol in the body before they can be used.

The daily amount of vitamin A required by adults is 0.7 mg for men and 0.6 mg for women. Any vitamin A that is not required immediately is stored for future use in bodily functions.

Sources

1. http://www.nhs.uk/Conditions/vitamins-minerals/Pages/Vitamin-A.aspx
2. http://www2.moh.gov.my/images/gallery/rni/11_chat.pdf
3. http://ocw.jhsph.edu/courses/InternationalNutrition/PDFs/Lecture3.pdf
4. http://www.who.int/publications/cra/chapters/volume1/0211-0256.pdf
5. http://www.unicef.org/publications/files/Vitamin_A_Supplementation.pdf