Vitamin A refers to the preformed retinoids and provitamin A carotenoids that can be converted to vitamin A activity. Retinoids is a collective term for the biologically active forms of vitamin A, and they exist in 3 forms: retinol, retinal, and retinoic acid.
Vitamin A is categorized as a fat-soluble vitamin. Fat-soluble vitamins are absorbed along with dietary fat. Thus, adequate absorption of fat-soluble vitamins depends on the efficient use of bile and pancreatic lipase in the small intestine to digest dietary fat. Under optimal conditions, about 40 ro 90% if the fat-soluble vitamins are absorbed when they’re consumed in recommended amounts.
Vitamin A in food
Retinoids (preformed vitamin A) are found in liver, fish, fish oils, fortified milk, and eggs. Fat-free, low-fat, and reduced-fat milk products can be fortified with vitamin A. The provitamin A carotenoids are found mainly in dark green and yellow-orange vegetables and fruits, such as:
- Spinach and other greens
- Winter squash
- Sweet potatoes
- Bell Peppers
Beta-carotene accounts for some of the orange color in carrots and other carotenoid-rich foods. In dark green vegetables, this yellow-orange coloring is masked by the dark green pigment cholophyll, although these vegetables do contain provitamin A.
Dietary vitamin A activity is currently expressed in Retinol activity Equivalents (RAE). One RAE is equal to 1µg of retinol, 12 µg of beta-carotene, and 24 µg of the other 2 provitamin A carotenoids (alpha-carotene and beta cryptoxanthin).
Vitamin A Needs
The RDA for vitamin A is 900 µg Retinol Activity Equivalents (RAE) per day for adult men and 70 µg RAE per day for adult women. At this intake, adequate body stores of vitamin a are maintained in healthy adults.
The Daily Value used on food packages and supplements is 5000 IU, or about 1000 µg.
Vitamin A functions:
- Growth and Development. Retinoids play an important role in embryonic development. Retinoic acid is necessary for the production, structure, and normal function of epithelial cells in the lungs, the trachea, the skin, the GI tract, and many other systems.
- Cell Differentiation. Vitamin A is especially important in maintaining normal differentiation of cells that make up the structural components of the eye, such as the cornea and the retina.
- Vision. Vitamin a (as retinal) is needed in the retina of the eye to turn visual light into nerve signals to the brain.
- Immune Function. Vitamin A helps maintain the ephithelium, a barrier that protects the body against the entry of disease pathogens.
- Use of Vitamin A Analogs in Dermatology. Several synthetic compounds with chemical makeup similar to that of vitamin A (analogs) have been used in topical and oral medications to treat cane and psoriasis. Retinoid-based medications also have been used to lessen the damage from excess sun and UV-light.
Vitamin a Toxicity
The signs and symptoms of toxicity from excessive vitamin A (called hypervitaminosis A), appear with long-term supplement use at 5 to 10 times the RDA for retinoids.
Correspondingly, the Upper limit Level is set at 3000 µg/day of retinol to prevent harmful effects. No Upper Limit is set for carotenoids because vitamin A toxicity results only from excess intakes of retinoids.