Vitamin A deficiency is a well known cause of increased susceptibility to cancer in laboratory animals. It also allows cultures of normal cells growing in test tubes to become more easily transformed into cancer cells by carcinogenic chemicals or by radiation. Applying this knowledge to research in human cancer, physicians long ago measured the blood levels of vitamin A in people with cancer and found them to be lower than those in people of the same age and race, etc., who did not have cancer.
Although this finding has been confirmed in several human studies, critics have argued that this does not prove a cause-and-effect relationship. Cancer, they have suggested, may use up extra amounts of vitamin A and thus be the cause rather than the result of low vitamin A blood levels.
Settling this dispute once and for all, cancer researchers have at last clearly established that low vitamin A blood levels in otherwise healthy persons more than doubles their chances of developing cancer in the next five years. This is consistent with a Norwegian study in which it was found that men classified as having a low vitamin A intake were four times more prone to develop lung cancer than equally heavy smokers taking a normal amount of vitamin A. There are, of course, other factors at work in causing cancer.
The message is clear: Make sure that you take the proper amount of vitamin A every day. One daily multi-vitamin tablet containing 10,000 units (or three milligrams) of vitamin A should be enough. Only pregnant women need more than this. Excess vitamin A is to be avoided because it can cause joint pains, hair loss, yellowness and dryness of the skin, and liver disease