Provitamin A is a nontoxic form of vitamin A, also known as the precursor of vitamin A. Vitamin A from animal sources (fish oils) is a retinoid, whereas provitamin A is from plant sources and is a carotenoid. Provitamin A is most commonly known as beta carotene, but alpha and gamma carotene are also sold as provitamin A.
Studies have shown that people with lots of beta-carotene rich fruits and vegetables in their diets got less cancer and heart disease. Beta carotene is an antioxidant. It also neutralizes free radical reactions in the body. Considerable research is directed at the relationship between both vitamin A and carotenes and the incidence of epithelial cancer, i.e. cancer of the lungs, gastrointestinal tract, genito-urinary tract and skin. Carotenes are plant pigments which protect the plant cell from being destroyed during the process of photosynthesis by acting as potent antioxidants. About 30 of the 400 or so carotenes so far characterised are able to be converted into vitamin A by the human body. Studies have demonstrated an inverse relationship between carotene intake and cancer incidence, i.e. the higher the intake of carotenes, the lower the incidence of cancer. Most of the research has focused on beta-carotene.
Beta-carotene has been shown to potentiate interferon's stimulatory action on the immune system. In addition, as carotenes are better antioxidants than vitamin A, they may be more advantageous in protecting the thymus gland, since the thymus gland is particularly susceptible to free radical and oxidative damage. A study on normal human volunteers demonstrated that oral beta-carotene (180 mg/day, approximately 300,000 IU) significantly increased the frequency of helper T cells by approximately 30 per cent after seven days and all T cells after 14 days. As helper T cells play a critical role in determining host immune status, this study indicates that oral beta-carotene may be effective in increasing the immunological competence of the host in conditions that are characterized by a selective lowering of the helper subset of T cells, such as AIDS. Oral beta-carotenes may also be useful for boosting anti-tumor immunity in cancer patients (in proper conjunction with other treatments).
A Finnish study reported 18 percent more cases of lung cancer among heavy smokers who took beta-carotene supplements. A similar result was found in a study performed by the National Cancer Institute. A twelve year study of 22,000 physicians found no evidence that beta-carotene supplements were protective against cancer and heart disease.
It is important to point out that none of these studies showed that beta-carotene caused cancer. They showed that beta carotene failed to prevent cancer among smokers. Some researchers believe that smoking may trigger the release of free radicals. Obviously, beta carotene cannot cure cancer which is already there. But, study after study has shown the protective effect of high levels of beta carotene in the blood. It is also possible that the protective effect may not entirely from beta-carotene itself. It could be the whole family of cartenoid pigments.
In summary, there is much evidence that vitamin A and carotenes significantly affect the immune function and can be used preventively as well as therapeutically to improve the status of the immune system. Particularly good sources of carotenes include all green leafy vegetables, coloured root vegetables like carrots, sweet potatoes and yams, apricots, cantaloupe melons, broccoli and squash.
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