The vitamin A or retinoid family contains several forms, natural and synthetic, that have vitamin A activity. In addition, a number of vegetable foods contain compounds that the body can convert into vitamin A, such as the alpha, beta, and gamma carotenes. Absorption of retenol requires bile, pancreatic enzymes, and antioxidants. In the form of retinoic acid, it is absorbed directly.
A six month supply of vitamin A is stored in the liver as retinyl esters.
Vitamin A can be toxic in dosages over 15,000 RE, but much of this toxicity can be prevented by also taking vitamin E. Vitamin E and vitamin A work together.
Vitamin A can promote differentiation in epithelial cells and regression of premalignant lesions. It inhibits the development of cancerous tumors. A lack of vitamin A will decrease antibody formation, while vitamin A supplementation increases cytotoxic action of T cells, NK cells, and macrophages.