Vitamin A can exist naturally in active form or as pro-vitamin:
- from animal sources as preformed retinoids (retinol)
- from plant sources as provitamin carotenoids (provitamin A)
The most well known provitamin A carotenoid is beta-carotene.
Vitamin A content is reported as Retinal Equivalents (RE)
The currently conversion factors as defined by WHO/FAO are shown below:
1 mg retinol = 1 RE
1 mg beta-carotene = 0.167 mg RE
1 mg other provitamin A carotenoids = 0.084 mg RE.
Vitamin A: contributes to the maintenance of normal vision contributes to normal iron metabolism contributes to the maintenance of normal skin, normal mucous membranes contributes to the normal function of the immune system has a role in the process of cell specialization.
Retinoids, such as retinal and retinoic acid, are found in animal sources such as liver, kidney, eggs, and dairy produce.Carotenoids such as beta-carotene (which has the highest vitamin A activity) are found in plants such as dark or yellow vegetables, carrots, and tree nuts.