Selenium is a trace mineral found in meats, grains, seafood and some nuts. However, how much you get varies according to where you live, because different areas have different concentrations of selenium in the soil. Those who live in areas where selenium intake is low have higher rates of colorectal and other cancers.

"Your risk of colorectal cancer can vary by the amount of selenium you consume," said lead researcher Elizabeth T. Jacobs, from the Arizona Cancer Center. "People with higher blood selenium levels tended to have a decreased risk of a recurrence of colon cancer."

Selenium may protect against not only colon cancer but also prostate cancer and lung cancer, Jacobs added.

Jacobs and her colleagues collected data from three randomized trials of colon cancer patients: the Wheat Bran Fiber Trial, the Polyp Prevention Trial and the Polyp Prevention Study. Looking at the data from these trials, they were able to determine the effect of selenium in the development of new cancerous colon polyps in patients.

The researchers found that those with the highest selenium levels had a 34 percent lower risk of developing a new colon cancer, compared with those who had the lowest selenium levels. Their report appears in the Nov. 17 issue of the Journal of the National Cancer Institute (news - web sites).

Whether increasing your selenium intake will actually be beneficial is not clear. Jacobs said that increasing selenium consumption will help people who have low levels of the element. "It will benefit those whose intake of selenium is low," she said. "However, it may not benefit those who already have adequate selenium levels."

The risks of having too much selenium are not known but are being studied, Jacobs said. In addition, studies are under way to determine the optimum selenium levels needed to prevent colon cancer and prostate cancer.

Jacobs also said there are ongoing studies to see if selenium can help treat patients with prostate cancer.

"Selenium is a promising preventive agent for colorectal cancer, but we are hopefully going to confirm this," Jacobs said.

"This study is not definitive," said Dr. Scott M. Lippman, chairman of the department of clinical cancer prevention at the University of Texas M.D. Anderson Cancer Center, "because it is not from a large randomized control trial of selenium."

Based on this study, Lippman believes that such trials should be done to test the benefit of selenium in preventing colon cancer. Data from ongoing trials may provide an answer, he said: "I believe that over the next five to 10 years, we will have some very compelling data one way or the other regarding selenium's ability to suppress colorectal cancer."

Lippman, who co-wrote an accompanying journal editorial, said there are biological reasons that may make selenium effective in preventing colon cancer. "There is laboratory information that suggests that selenium might prevent colorectal cancer," he said.

One of these mechanisms is selenium's role in turning on genes that prevent cancer. Another is the mineral's effect in altering the metabolism of polyunsaturated fatty acids found in red meats, which are involved in the development of colorectal cancer, Lippman said.

"Not only are the epidemiological data very consistent and supportive, but there are actually fairly strong biologic plausibility for selenium's ability to protect against colorectal cancer," he said.

However, Lippman cautioned that the results of this study do not mean that people should start taking selenium supplements in the hope of preventing cancers. "The epidemiological and biological data strongly support going to the next step of a definitive randomized trial to find out if in fact it does work," he said.