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updated 6/25/2004 12:20:40 PM ET 2004-06-25T16:20:40

A recent study links the consumption of the essential mineral selenium with a lower risk of prostate cancer. There are now seven population studies in the past six years that examined the possible connection between selenium and prostate cancer. All but one of them have found selenium protective. Yet, since scientists are still uncertain how prostate cancer starts or can be prevented, it is too early to say that selenium definitely protects the prostate. There is much more to learn.

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The latest study, however, is especially important because it is the largest study to date. It tracked the health of the men participating for up to 13 years. The duration of the study is significant because prostate cancer is usually a slow growing cancer, influenced by diet and lifestyle over decades as it develops. In this recent study, men with the highest levels of selenium in their blood were about half as likely to develop advanced prostate cancer as the men with the lowest blood selenium. Similarly, two past studies show that those with the lowest blood selenium have a moderately increased risk of prostate cancer.

More studies in progress
Besides examining blood levels of selenium, studies are under way to test the effectiveness of selenium supplements in reducing prostate cancer. One major U.S. study in progress supplies participants with a supplement of either selenium, vitamin E, or both. The development of prostate cancer among these people will be compared to other participants who receive no supplements. Another study in France is testing an antioxidant vitamin supplement that includes selenium. An earlier U.S. study found that selenium supplements cut the risk of prostate cancer nearly in half, except for those whose blood levels of selenium were already high.

Because it boosts the body's antioxidant capacity, scientists believe that selenium can control cell damage that may lead to cancer. Selenium may even act in other ways to stop early cancer cells in their development. While study results so far are fairly positive about selenium's anti-cancer potential, many questions remain: How much selenium is enough for men? Can men get enough from food? Would supplements help every man or only some?

Should you take supplements?
The Recommended Dietary Allowance for selenium was set at 55 micrograms for adult men and women to maximize only one antioxidant enzyme system of which selenium is an essential part. The studies that give men extra selenium in a supplement form generally give 100 to 200 mcg. It is unclear whether smaller supplement amounts could help prevent prostate cancer. Optimal anti-cancer effects, however, may require more than the RDA.

In the United States, almost everyone receives more than the RDA for selenium from foods. It's easy to see why. There is 40 to 70 mcg of selenium in a three-ounce portion of fish, 23 - 30 mcg in a 3-ounce portion of poultry or meat, and 15 - 35 mcg in one cup of pasta, rice or two slices of bread. The entire RDA for selenium can also be consumed in 3 to 4 Brazil nuts.

If you want to take a selenium supplement anyway, first check to see how much extra selenium is in any vitamin supplements you use (they often contain about 20 mcg), as well as in fortified cereals, bars or other foods. The National Academy of Science warns that too much selenium can cause nerve damage, hair loss and digestive disturbances. The maximum amount from food and supplements considered safe is 400 mcg a day.

If supplements help prevent prostate cancer, it's still unknown who would benefit. Some studies suggest benefits might be limited to older men or men whose diets are low in antioxidants. In a few years, the work of researchers should make the answers clearer.

Nutrition Notes is provided by the American Institute for Cancer Research in Washington, D.C.

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