The Beta Carotene Paradox
Epidemiological studies have consistently shown that increased intake of fruit and vegetables which are rich in carotenoids, is associated with decreased risk of lung cancer.
Since lung cancer is caused by smoking, which is a pro-oxidant, and since smokers have decreased levels of beta carotene and other anti-oxidants, supplementing the diet with beta carotene in smokers to prevent the development of lung cancer was a reasonable strategy.
However, a well designed, large clinical study conducted in Finland found just the opposite. Studies have shown that beta carotene, at high concentrations, has a pro-oxidant effect when the oxygen pressure is also high. Therefore in the lungs, where the oxygen tension is high, administering large doses of beta-carotene may lead to oxidative DNA damage and a higher incidence of cancer.
Extracted from an article by Omer Kucuk in Oncology Issues November/December 2000