By Dr Ananya Mandal, MD
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.
Reviewed by Sally Robertson, BSc
Last Updated: Oct 9, 2014