Vitamin A (a.k.a. Retinol) and Provitamin A (a.k.a. Carotene or Beta Carotene)
(the first class of vitamins discovered)
Vitamin A is actually not, in the strictest sense, a vitamin, because it can be synthesized in the body. The real vitamin is carotene, which the body uses to make vitamin A. Beta-carotene is made of two vitamin A molecules covalently bonded. Carotene is present in carrots, broccoli, squash, spinach, kale and sweet potatoes.
Beta-carotene is a member of the carotenoid family which includes beta-carotene, alpha-carotene, and beta-cryptoxanthin which are converted to vitamin A and are referred to as provitamin A carotenoids. The sole known role of carotenoids is to act as a source of vitamin A in the diet. Fruits and vegetables are the main source of carotenoids in the human diet.
The beta form of carotene is a powerful antioxidant. It is possible to obtain "vitamin" A, already synthesized in the bodies of animals, by consuming animal meat and animal products. However, I suggest you not put these kinds of toxins into your body and be happy with the carotene present in the aforementioned vegetables. Also present are related useful substances, including alpha-carotene, all together known as "mixed carotenoids." It is likely that these work together for the best result, just as with the mixed tocopherols of vitamin E.
An abundant supply of carotene has the following effects on the body: moist, supple, youthful skin and mucous membranes, moist, well-lubricated eyes and good night vision. However, if you are treating a specific condition, which responds to vitamin A, as we sometimes do in nutritional medicine, only vitamin A will work. You will not get the same effect with beta-carotene.