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Provitamins are the precursors to vitamins. They are dietary substances that can be converted via normal metabolic processes into active vitamins. Depending on the provitamin and vitamin involved, the conversion process takes place in various parts of the body with differing levels of efficiency.
Vitamins themselves are organic nutrients essential to human life in small amounts. Eleven of the 13 vitamins are supplied by food, and two require additional conditions. Vitamin D, for example, forms within the body upon the skin’s exposure to ultraviolet light. Many vitamins also have provitamins, which help provide humans with necessary vitamins through additional dietary channels.
Vitamin A can be directly obtained through ingesting foods that contain retinol, the preformed vitamin. These foods include animal and fish products, especially livers, and several dairy products, including whole milk, butter, and cheese. Many people receive their daily vitamin A from an important additional source: provitamin A, also known as cartenoids. Cartenoids can be found in many fruits and vegetables, especially those that are red, orange, yellow or green.
Not all fruits and vegetables that contain these cartenoid pigments also contain provitamin A, but many of them do, including carrots, spinach, turnip greens and palm oil. Beta-carotene, the provitamin A most important in the diets of most humans, can be found in these vegetables as well as dark yellow squashes, pumpkin, sweet potatoes, tomatoes, broccoli and beet greens. Additional, lesser sources of beta-carotene include most fruits, summer squash, zucchini squash, beans, cabbage, corn, peas and many kinds of nuts.
Beta-carotene is converted to vitamin A in the human body with a yield of about 40 percent to 50 percent and provides as much as half of most people’s necessary vitamin A. A small amount of fat is needed to stimulate the secretion of digestive juices that induce the absorption of beta-carotene — and dietary vitamin A — into the body. Individuals with intestinal disorders that change or lessen their absorption of dietary fat are less able to absorb vitamin A and beta-carotene.
Another important provitamin is 7-dehydrocholesterol, also known as provitamin D. As stated above, the formation of vitamin D requires ultraviolet light, which acts on provitamin D to create vitamin D. Individuals who live above latitude 45 degrees north or below latitude 45 degrees south also must ingest dietary vitamin D, which can be found in egg yolk, butter, oily fish and enriched margarine, in order to meet their health needs.