An organic chemical compound (or related set of compounds) is called a vitamin when the organism cannot synthesize the compound in sufficient quantities, and it must be obtained through the diet; thus, the term vitamin is conditional upon the circumstances and the particular organism.
https://en.wikipedia.org/wiki/Vitamin
As a result, most of the fat-soluble vitamins (such as the vitamins A, D, E, and K) that contain amounts above the daily allowance are drug products. The daily dosage of a vitamin supplement for example cannot exceed 300% of the recommended daily allowance, and for vitamin A, this limit is even lower (200%).
https://en.wikipedia.org/wiki/Vitamin
For the most part, vitamins are obtained with food, but a few are obtained by other means. For example, microorganisms in the intestine — commonly known as gut flora — produce vitamin K and biotin, while one form of vitamin D is synthesized in the skin with the help of the natural ultraviolet wavelength of sunlight.
https://en.wikipedia.org/wiki/Vitamin
Anti-vitamins are chemical compounds that inhibit the absorption or actions of vitamins. For example, avidin is a protein in egg whites that inhibits the absorption of biotin. Pyrithiamine is similar to thiamine, vitamin B1, and inhibits the enzymes that use thiamine.
https://en.wikipedia.org/wiki/Vitamin
For example, ascorbic acid (one form of vitamin C) is a vitamin for humans, but not for most other animal organisms. Supplementation is important for the treatment of certain health problems, but there is little evidence of nutritional benefit when used by otherwise healthy people.
https://en.wikipedia.org/wiki/Vitamin
Two forms of vitamin A are available in the human diet: preformed vitamin A (retinol and its esterified form, retinyl ester) and provitamin A carotenoids [1-5]. Preformed vitamin A is found in foods from animal sources, including dairy products, fish, and meat (especially liver).
https://ods.od.nih.gov/factsheets/VitaminA-HealthProfessional/
The various forms of vitamin A are solubilized into micelles in the intestinal lumen and absorbed by duodenal mucosal cells [5]. Both retinyl esters and provitamin A carotenoids are converted to retinol, which is oxidized to retinal and then to retinoic acid [2].
https://ods.od.nih.gov/factsheets/VitaminA-HealthProfessional/
The Roles of Vitamin A. Vitamin A is much more than just a vitamin that helps you see at night. It is a fat-soluble vitamin that plays a role in immunity, the body's development and maintenance of major organs. Available in two sources, vitamin A is derived from animals and from plants. Vitamin A from animal sources is preformed vitamin A and is active immediately. From plant sources, it is provitamin A — including a group of carotenoids, such as beta-carotene. Beta-carotene is the most common form of provitamin A and in plant foods it must be converted in the body to the active form of vitamin A.
http://www.eatright.org/resource/food/vitamins-and-supplements/types-of-vitamins-and-nutrients/vitamin-a
Vitamin A is available in multivitamins and as a stand-alone supplement, often in the form of retinyl acetate or retinyl palmitate [2]. A portion of the vitamin A in some supplements is in the form of beta-carotene and the remainder is preformed vitamin A; others contain only preformed vitamin A or only beta-carotene. Supplement labels usually indicate the percentage of each form of the vitamin.
https://ods.od.nih.gov/factsheets/VitaminA-HealthProfessional/
Most of the body's vitamin A is stored in the liver in the form of retinyl esters. Retinol and carotenoid levels are typically measured in plasma, and plasma retinol levels are useful for assessing vitamin A inadequacy.
https://ods.od.nih.gov/factsheets/VitaminA-HealthProfessional/
what is vitamin a derived from
Vitamin A is derived from animals and from plants. Vitamin A is derived from animals and plants.