Selenium is a mineral with a long and interesting history. Discovered in 1817, it was considered a poison during much of the 19th century. In the 20th century, selenium found a use in many industries, from ceramics to rubber to agriculture. No longer considered toxic, selenium is the active ingredient in many therapeutic shampoos. But the greatest potential for selenium is as a supplement to reduce the risk of prostate cancer.
Meet the mineral
Selenium in the earth's crust finds its way into plants, then works its way up the food chain. In general, the best sources of selenium are whole grains, tomatoes and other vegetables, seafood, nuts (particularly Brazil nuts), garlic, and onions; meat and poultry also provide significant amounts.
Selenium is essential for human health. The Recommended Dietary Allowance for adult men is 55 micrograms. Daily amounts as high as 400 micrograms appear entirely safe; beyond that, supplements can cause hair loss, nausea, or diarrhea.
Selenium and the heart
Scientists don't fully understand how selenium functions in the body, but one important role is its antioxidant activity. As a result, doctors have wondered if it might help fight atherosclerosis. Indeed, a 1991 study of Finnish men linked low selenium levels to atherosclerosis of the carotid artery. But in 1995, the Harvard Physicians' Health Study cast considerable doubt on the hypothesis that selenium protects the heart. In fact, a 2006 study reported that taking selenium supplements had no effect on cardiovascular disease.
Researchers are now concentrating on selenium and cancer. The first glimmers came from the observation that cancer is less common in parts of the world that have high levels of selenium in the soil. Experiments in test tubes and laboratory animals followed, but a 1996 publication really put selenium on the map.
The Nutritional Prevention of Cancer Trial
To learn if selenium supplements might reduce the risk of recurrent skin cancer, a team of scientists working at seven American health centers gave either 200 micrograms of selenium or a placebo to 1,312 volunteers with an average age of 63. When the results were analyzed in 1996, the researchers were disappointed to learn that there was no difference in the occurrence of skin cancer in the two groups, but they were startled to find that selenium was linked to a significant reduction in deaths from lung, colon, esophageal, and prostate cancer; protection appeared strongest for prostate cancer, with 63% fewer deaths in the men who took selenium. These results were greeted with great interest but also with caution; most doctors stressed the need for further research.
The Nutritional Prevention of Cancer Trial has issued two subsequent reports. The initial hopes that selenium might reduce the incidence of lung and colorectal cancer did not hold up. But the men who received selenium continued to enjoy a 49% lower risk of prostate cancer through a follow-up period that averaged 7.6 years.
The Harvard studies
Every study, however well done, requires confirmation. Indeed, a large trial of selenium for prostate cancer prevention is already under way.
Just two years after the 1996 report, Harvard's Health Professionals Follow-up Study weighed in with a study of 33,737 men between the ages of 40 and 75. The researchers did not administer selenium supplements, nor did they measure blood levels of the mineral. Instead, they asked the volunteers to submit toenail clippings at the start of the study. The clippings were analyzed for selenium concentration.
When the scientists tracked the men for six years, they found that the men with the highest selenium levels at the start of the study had a 65% lower incidence of advanced prostate cancer than the men with the lowest levels. The Harvard team calculated that a daily consumption of 159 micrograms of selenium would prove protective.
Tomorrow and today
It's a familiar refrain: The only way to sort out an unsettled area of research is to perform additional studies. In the case of selenium, a definitive randomized clinical trial is already in progress. Sponsored by the National Cancer Institute, the Selenium and Vitamin E Cancer Prevention Trial (SELECT) will evaluate the effects of selenium and vitamin E, singly or in combination, in 32,400 men.
SELECT will provide much-needed answers about selenium and another controversial supplement, vitamin E. But the results won't be announced for years. In the meantime, what's a man to do?
The choice is yours. It's far too early to recommend a supplement for all men, but it's certainly important to recommend a healthful diet that will provide good amounts of the mineral. And if you are attracted to supplements, selenium is a reasonable choice. The best dose and form of the mineral are not known, but since the 1996 trial used 200 micrograms a day, that dose seems appropriate.
April 2007 update