Hypervitaminosis is the result of excessive intake of a vitamin, and this especially applies to fat soluble vitamins A, D, E, and K. Vitamin A stores in the human liver, and it is not necessary for a person to consume it each day.
Vitamin A occurs in two forms. One form is retinol or preformed vitamin A, which is almost ready for the body to use after its absorption from the gastrointestinal tract. Humans obtain retinol or preformed vitamin A from animal sources such as liver, eggs, milk, some fortified food products, margarine, cheese, and certain other foods. The other form of the vitamin is available as beta carotene, which one can consume from colorful fruits and vegetables. The preformed vitamin A from animal sources is the one that may lead to toxicity as it stores in the liver.
The liver stores serve an important role because when vitamin A intake or levels of it decline in the other tissues, mobilization of the vitamin liver stores occurs so that it becomes available for the rest of the body.
Beta carotene may not pose any risk for toxicity because it undergoes conversion to preformed vitamin A in the small intestine. Excessive or unnecessary intake of preformed vitamin A supplementation, on the other hand, can lead to excessive stores of the vitamin in the liver.
Prevention of cancer
Vitamin A is an antioxidant, and these compounds help to slow or prevent cell damage. This discovery has led many to believe that supplementation with vitamin A, and other nutrients, will prevent cancer or enable one to live longer than they would without the supplement. Though several clinical studies have taken place to examine these contentions, there is no definitive evidence that vitamin A prevents cancer or prolongs life.
In fact, vitamin A supplementation may predispose a person to the development of cancer, and this has been observed in clinical trials with smokers. Hence, people who smoke should not take vitamin A supplements as there is an even greater risk of lung cancer in these patients.
Pregnant women require vitamin supplementation so that the mother and fetus will have an adequate supply of various nutrients. However, it is important that she take the proper prenatal vitamin which her physician recommends. Specifically, she should never take a vitamin supplement during pregnancy which contains retinol or preformed vitamin A as it can lead to birth defects in the child. Prenatal supplements with beta carotene are a reasonable choice for these patients.
Very high doses of vitamin A may cause irregular menstrual periods, liver damage, osteoporosis, hair loss, nausea and vomiting, diarrhea, loss of appetite, fatigue, headaches, dizziness, blurred vision, poor muscle coordination, itching and scaling of the skin, and bone pain. It may also cause problems with the central nervous system, reduction of bone mineral density, and predisposition to bone fracture.
Although vitamin D has a role in the preservation of bone, hypervitaminosis A appears to interfere with that, and this may be the reason for the fractures.
Excessive intake of vitamin A can also pose a problem with skin creams and other topical preparations which contain vitamin A. These therapeutic creams and ointments are synthetic retinoids and are similar in their chemical structure to vitamin A. Babies and children are especially sensitive to the vitamin A in skin creams, and doses much less than what an adult takes may pose a problem for them.
In children and teens, synthetic retinoid skin therapies may cause delay in growth. One must also keep in mind that synthetic retinoid skin creams to treat acne and psoriasis may become toxic in high doses. Sexually active women of childbearing age who take topical synthetic retinoids for skin conditions should obtain birth control as the use of these agents during pregnancy can harm the fetus and cause birth defects.
Elderly people who take vitamin A supplements should use the beta carotene preparation and consume foods fortified with vitamin A. The preformed vitamin A supplement is not advisable for them because their blood levels of retinol increase with age, and they may have more risk of toxicity from it than younger individuals who take vitamin A supplementation.
The sale and distribution of vitamin supplements in the United States does not require approval from the Food and Drug Administration, and one will not really know how safe these agents are. Moreover, the United States Institute of Medicine does not recommend vitamin A supplementation for the general public. For those who take supplements, the Institute of Medicine recommends that a person not exceed 10,000 international units of retinol per day.