Vitamin A
Description: A fat soluble vitamin which can be found in fish oil, liver, egg and dairy products. Concentrations of vitamin A may be very high in some animals tissues such as the liver. Preformed vitamin A in foods may be in the form of retinyl exters but plant sources contain provitamin A carotenoids.
Plants do not contain vitamin A already formed but they may contain carotenes such as Beta Carotene. (Beta carotene is a provitamin, a compound that requires an activation step in order to become biologically active, for vitamin A.) Concentrations of carotenoids in plants varies depending upon growing harvesting conditions. The vitamin A activity of beta carotene is greater than that of other carotenoids.
Actions: Vitamin A is necessary for normal vision, bone and muscle growth, reproduction, and maintaining healthy skin.
Cautions: Large amounts of vitamin A may result in significant toxicity.
Note: Cats require preformed vitamin A because they lack an enzyme (dioxygenase) necessary to cleave Beta Carotene and thus can not create the biologically active form. Cats, unlike dogs, cannot meet their needs for vitamin A by ingesting carotenoids.