"The number of basal cell carcinomas and squamous cell carcinomas observed are greater than expected," writes researcher Anna J. Duffield-Lillico, PhD, an epidemiologist with Memorial Sloan-Kettering Cancer Center in New York City.
Skin cancer is the most common of all cancers. Every year more than one million Americans will develop it. Avoiding sun exposure is important in preventing it. There are three forms of skin cancer: basal cell, squamous cell, and melanoma. Some studies have shown that death rates from cancers such as nonmelanoma skin cancer, lung, and colorectal cancers are lower in people with higher selenium blood levels or intakes.
Researchers conducted the study of more than 1,000 men and women -- all who previously had basal and squamous cell skin cancers -- during a 10-year period. It was designed to see if selenium supplementation could prevent nonmelanoma types of skin cancer. During that time, each participant took 200 micrograms of selenium daily or a placebo. At the beginning of the study researchers asked the participants about sun exposure and sensitivity. A dermatologist examined them every six months.
At the study's end -- after adjusting for factors that might affect the rate of skin cancer -- the researchers showed that there was a 25% increase in squamous cell skin cancers and 17% increase in total nonmelanoma skin cancers in the participants that took selenium. These seem to be among those patients who had the highest levels of selenium in the blood before taking supplements -- which could just be a chance finding, Duffield-Lillico says.
She also included in the study research about selenium's effect on other forms of cancer. The incidence of and death rate for nonskin cancers were lower in the selenium group. However, apparently this did not translate into nonmelanoma skin cancer prevention, Duffield-Lillico says.
Her results don't match findings from other studies of selenium and skin cancer risk -- or findings that applying selenium to skin protects against ultraviolet B radiation. She writes that the results of the study may have been attributed to the fact that the patients in the treatment group may have been less cautious about sun exposure after the study began.
It's too soon to advise people against taking selenium for skin cancer prevention, she writes. "Nonmelanoma skin cancer is rarely fatal, but these negative effects of selenium supplementation appear greatest" among those who had high selenium levels in their blood before taking supplements, she explains.
Duffield-Lillico says she suspects that some commonly used drugs or exposure to environmental contaminants, such as pesticides or air pollution, could alter selenium's effects in skin cancer prevention.
SOURCE: Duffield-Lillico, A. Journal of the National Cancer Institute. Oct. 1, 2003; vol 95: pp 1477-1481.