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Excerpts from High-dose beta-carotene may up cancer risk

Reuters Health [01/06/99]

A study in ferrets suggests an association between high-dose beta-carotene supplementation and precancerous changes in lung tissue.

The researchers also note that these lung changes were even more pronounced when the animals were exposed to tobacco smoke. Their report is published in the January 6th issue of the Journal of the National Cancer Institute.

``Our findings shed light on the potential harmful effects of high-dose beta-carotene supplementation, particularly as it relates to cigarette smoke,'' Dr. Xiang-Dong Wang and colleagues at Tufts University in Boston, Massachusetts, report.

Previous studies suggested that high levels of beta-carotene in the blood of people who eat large amounts of fruit and vegetables might reduce their risk of lung cancer. But other studies showed that beta-carotene supplements appeared to increase the risk of lung cancer in smokers and asbestos workers.

To determine why this might be so, the investigators examined the lung tissues of ferrets, whose metabolic processes closely resemble those of humans. For 6 months, the ferrets were fed either a normal diet, or a diet supplemented with either low- or high-doses (equivalent to 30 mg per day in a human adult) of beta-carotene. Some of the ferrets were also exposed to high levels of tobacco smoke.

According to the authors, all of the ferrets receiving high-dose supplements had precancerous lesions in lung tissue upon biopsy. This response seemed to be ``enhanced by exposure to tobacco smoke,'' they say.

The lungs of ferrets consuming high-dose beta-carotene also displayed especially low levels of retinoic acid -- a form of vitamin A thought to help protect against cancer. The authors speculate that ``cigarette smoke, beta-carotene supplementation, or both may interfere with the metabolism of tissue beta-carotene and retinoic acid,'' leaving lung cells more vulnerable to carcinogenesis -- cellular processes that lead to cancer.

In an editorial, Dr. Reuben Lotan of The University of Texas M.D. Anderson Cancer Center in Houston, says that he believes the study ``provides a possible explanation'' for links between beta-carotene supplementation and human lung cancer.

SOURCE: Journal of the National Cancer Institute 1999;91:7-8, 60-66.

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