The active form of Vitamin A from these sources is known as retinoic acid. All of these substances are known as retinoids. In plants, Vitamin A is derived from precursors known as carotenoids.
There are several carotenoids, which are compounds that give fruits and vegetables a yellow or orange color. Beta-carotene is the most common Vitamin A precursor derived from plants. The body can convert carotenoids into retinoids and Vitamin A.
Vitamin A has several roles in the body. It regulates the mucosal linings of the nose, mouth, throat, and reproductive organs. It is also involved in the creation of proteins under the direction of DNA. These proteins are used in a variety of processes, including the production of both red and white blood cells.
Vitamin A helps to regulate the formation of proteins from stem cells that are then used to create red blood cells. It also helps oxygen attach to hemoglobin, which transports the oxygen throughout the body.
Similarly, Vitamin A is necessary for the creation of proteins that manufacture white blood cells. White blood cells are the first line of defense used by the immune system.
Vitamin A also plays an important role in maintaining vision. The pigment rhodopsin, the retinal form of Vitamin A, is essential for vision at night.
Adults need about 3000 I.U. of Vitamin A per day. The best food sources of Vitamin A are cod liver oil, eggs, and whole milk.
Vitamin A can also be derived from beta-carotene, which is found in orange and yellow fruits and vegetables, such as carrots, squash, and cantaloupe, as well as some green vegetables, such as spinach, kale, and collard greens.
Vitamin A is stored in the liver, so deficiency symptoms take some time to develop. Since Vitamin A is fat-soluble, people on low fat diets and people who have conditions that impair fat absorption may be at some risk for Vitamin A deficiency. Adequate protein intake is also necessary for the body to use Vitamin A efficiently.
In some cases, Vitamin A supplements may be of value, but they must be taken with caution. As a fat-soluble nutrient, Vitamin A accumulates in fat cells, and excess amounts are not easily excreted. Taking too much preformed Vitamin A in supplement form can result in a long list of toxic effects, including liver damage, digestive system disorders, skin problems, and bone decalcification.
On the other hand, supplements of beta-carotene can usually be taken in large doses without harm and converted to Vitamin A as needed. If too much beta-carotene is consumed, it is stored in fat cells as well, but the stored beta-carotene is not yet converted to Vitamin A. The only notable side effect from too much beta-carotene is that it may turn skin yellow. This occurs only after enough beta-carotene has been converted into Vitamin A.
Vitamin A may interact with certain medications and nutrients, and the use of Vitamin A supplements by pregnant women is controversial. Cholesterol lowering medications known as HMB-CoA reductase inhibitors and certain oral contraceptives may raise blood levels of Vitamin A.
Conversely, cholesterol-lowering medications in the class known as bile acid sequestrants may decrease Vitamin A absorption. The antibacterial medication neomycin may decrease the absorption of Vitamin A as well. Medications used to treat acne and certain kinds of skin cancer are similar in chemical composition to Vitamin A, so people taking these medications should not take large doses of Vitamin A at the same time.
Very large doses of Vitamin A can also inhibit absorption of Vitamins D and K. Pregnant women or women who may become pregnant should consult a physician before using Vitamin A supplements.