Vitamin A (retinol) is a yellow, fat-soluble, generic term for a large number of related compounds obtained from some carotenoids by conversion in the liver, its storage organ. Retinol (an alcohol) and retinal (an aldehyde) are often referred to as preformed vitamin A. While carotenoids are widely distributed in such foods as green and yellow vegetables, retinol is not found in any vegetable sources, but is concentrated in egg yolks and the livers of many animals. Vitamin A, either from animal sources or synthesized in our own liver, is essential for growth and reproduction, maintaining healthy vision, and supporting protein synthesis and cell differentiation.
Vitamin A and its analogs have shown the ability to help maintain proper DNA function.1-3 One study, however, has shown that for certain lifestyles (e.g., smokers) supplemental intake of too much beta-carotene in the absence of other carotenoids and antioxidants (such as at least 400 IU/day of vitamin E) may be harmful.4
Beta-carotene is the most potent precursor to vitamin A, but its conversion to vitamin A in the body is limited by a feedback system. It is an important antioxidant in its own right and one that can only build up to toxic levels in rare circumstances. Beta-carotene helps support immune health by enhancing the function of the thymus gland.5,6