Generally, there are two categories of vitamin A, depending on whether the food source is an animal or a plant.
Vitamin A from an animal food source is preformed vitamin A. It is absorbed in the form of retinol, your body’s most usable (active) form of vitamin A. This highly absorbing retinol can also be converted into retinal and retinoic acid in your body, other active forms of vitamin A.
Sources include liver, whole milk, eggs, and some fortified food products such as fortified low fat milk and breakfast cereal.
Vitamin A source from a dark colorful fruit or vegetable is provitamin carotenoid, which can be made into retinol in your body. The common provitamin A carotenoids found in plant foods are beta-carotene, alpha-carotene, and beta-cryptoxanthin.
Beta-carotene is made into retinol most efficiently, while alpha-carotene and beta-cryptoxanthin are only half as efficient. Major dietary contributors of provitamin A carotenoids are carrots, cantaloupes, sweet potatoes, and spinach.
Laboratory studies have shown that some provitamin A carotenoids function as antioxidants, however, this role is inconclusive for humans. Antioxidants protect cells from oxygen metabolism’s by product of free radicals, which are potentially damaging and contributors to chronic disease development.
Vegetarians not consuming eggs and dairy foods need to include a minimum of five servings of orange and yellow fruits and dark leafy green vegetables in their daily diet to meet their vitamin A needs.