Selenium supplements may protect against cancer for those with low levels of the mineral, but excessive doses could be harmful. Animal studies show benefits for cancer prevention, but human studies are inconclusive. Too much selenium can lead to health risks.
Selenium supplements for cancer may offer some protection for people who lack sufficient levels of the mineral in their bodies. Human studies show that people with low selenium levels face increased risks of developing cancer, but once selenium reaches normal levels, taking extra doses of selenium for cancer is probably not good. Animal studies show a significant benefit of selenium for cancer prevention, but no evidence exists to show similar benefits in humans. In fact, too much selenium could pose health risks.
More than 200 animal studies of selenium for cancer have been conducted since the 1970s. Two-thirds of these research projects showed that selenium could slow or inhibit tumor growth by 15 to 35 percent. Animals given more than 30 times the recommended daily amount of the mineral for humans showed significant protection against cancer. Only one of the experiments found no correlation.
Eight enzymes in the human body require selenium for antioxidant functions. The researchers believe that selenium levels could be altered by increased levels of vitamin E and vitamin C. Selenium occurs naturally in rocks and soil, allowing for uptake of the mineral by plants. The recommended daily amount needed for good health varies among health organizations, but between 30 and 50 micrograms per day is considered adequate.
Most of the research on selenium for cancer has consisted of observational studies. The scientists analyzed the study participants’ diets and selenium levels in their blood, recording which participants later developed cancer. Only a few of these experiments involved randomized, scientific trials using control groups assigned placebos. Randomized trials showed no correlation between the mineral and cancer prevention, specifically skin and prostate cancers. Researchers reviewing numerous previous studies reported in 2011 that too much selenium could increase the risk of diabetes.
One study added selenium to salt fed to Chinese men living in areas where soil selenium is scarce. They were also given high doses of carotene and vitamin E. The results showed fewer cases of esophageal and stomach cancer. The researchers were unable to determine whether the selenium supplements corrected a deficiency or what role the vitamins played in the findings.
The scientists concluded that too little or too much selenium could pose health risks. The toxic effects of excess selenium include an impaired immune system, fatigue, hair loss, and thinning nails. Some scientists advise not taking more than 200 micrograms of selenium per day to avoid adverse reactions.
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