Vitamin A
Vitamin A is not a single compound but a group of chemical compounds that are structurally similar. These compounds include retinol, retinaldehyde, retinoic acid, and provitamin A caretenoids which include beta-carotene, alpha-carotene, gamma-carotene and cryptoxanthin. Retinol and beta-carotene are some of the most common forms of vitamin A found in food and supplements, with the former being found in animals and the latter in plants.
Vitamin A plays a vital role in bone growth, reproduction and immune system health. It also helps the skin and mucous membranes repel bacteria and viruses more effectively. It is essential to healthy vision, and may slow declining retinal function in people with retinitis pigmentosa.
Some studies found that late-stage breast cancer patients have lower serum vitamin A levels, and that diets richer in micronutrients including vitamin A can improve immune functioning and prognosis in head and neck cancer patients, and reduce the risk of oral and pharyngeal cancers. However, vitamin A supplementation does not have a protective effect against non-small cell lung cancer or prolong survival for melanoma patients, and may actually elevate the risk of prostate cancer - provitamin A carotenoids should be used instead.
Vitamin A is fat-soluble and overuse may cause accumulation in the body. Supplementation with doses greater than the recommended Daily Value of 5,000 IU (Retinol) or in conjunction with certain medications or pre-existing conditions may result in adverse effects or toxicity.
Dosage
The vitamin A in fruits and vegetables is in the form of “provitamin A” -vitamin A precursors also known as carotenoids, which must be converted by the human body into usable retinoids. There are several distinct classes of carotenoids, and evidence suggests that carotenoids work together to provide their health benefits. Taken together in supplement form, these are commonly called “mixed carotenoids.” The body can make all the vitamin A it needs from these plant-derived carotenoids. They are water-soluble and do not accumulate in the body, so toxicity is rare. A suggested dosage of is 10,000 - 25,000 IU of mixed carotenoids daily.
Safety
Excessive, chronic intake of some forms of vitamin A can be toxic. Avoid taking supplemental vitamin A as retinol or retinoic acid, and instead use plant-derived vitamin A precursors such as beta-carotene (in addition to other mixed carotenoids). Also avoid concentrated animal sources such as cod liver oil (although some forms of cod liver oil are vitamin A reduced and are safe – check the label). Warning symptoms of overdose include hair loss, confusion, liver damage and bone loss.