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How is selenium used in cancer treatment? – Part 1

06/02/10

Selenium is found in many plant and animal foods. It is an essential micronutrient, part of several antioxidant enzymes (Burk, 2002), and also involved in endocrine and immune system functions.

No specific deficiency condition has been described in humans. On the other hand, excess selenium can be toxic, affecting liver, skin, nails and hair. Recommended intake and upper tolerable level are 40-55 and 300 micrograms/day, respectively (Alexander, 2007).

A role of selenium for cancer prevention was first suggested some forty years ago (Shamberger & Frost, 1969). Since then, other interesting studies confirmed this benefit of selenium.

Blood selenium levels in men with prostate cancer were found to be lower than average (Pourmand, 2008).

In a Chinese 2-year trial (Yu, 1991), selenium supplements were shown to reduce the incidence of liver cancer. Another Chinese trial (Blot, 1993) observed that supplementation with combined beta-carotene, vitamin E, and selenium for 5 years had 13 to 21 percent reductions in gastric cancer incidence, gastric cancer mortality, and total cancer mortality in a poorly nourished population.

A study in a French population (Hercberg, 2004) showed a 31 percent reduction in overall cancer risk in men but not in women by use of vitamin C, vitamin E, beta-carotene, selenium, and zinc for 8 years.

The Nutritional Prevention of Cancer (NPC) trial (Duffield-Lillico, 2002, 2003) tested the efficacy of selenium supplementation (200 mcg) for cancer prevention in 1,312 men and women with a recent history of skin cancer. Over 8 years, selenium intake was associated with a 25% lower risk of all cancers and a 50% lower risk of prostate cancer. Lung and bowel cancer were also reduced, however in a non-significant manner.

In almost 14,000 US adults from the Third National Health and Nutrition Examination Survey, followed up for 12 years, it was found that higher blood selenium levels were associated with a decrease in all-cause mortality (by 17%), cancer mortality (by 31%), and cardiovascular mortality (by 6%) (Bleys, 2008).

Finally, a very recent combined analysis of the results of all relevant existing studies came to the conclusion that selenium supplementation was associated with a significant reduction in cancer incidence in men (by 23%) but not in women, and in cancer mortality (by 22%) (Bardia, 2008).

The mechanisms that may explain the preventive effects of selenium supplementation are largely unclear. Amongst all the diverse mechanism that have been proposed some important ones are (a) protective role of selenium-containing proteins and enzymes, (b) induction of apoptosis (cell death), (c) immune system effects, (d) detoxification of antagonistic metals, (e) inactivation of nuclear transcription factor, (f) regulation of lipoxygenases, (g) effect on advanced cancer condition, (h) reduction of oxidative stress, (i) induction of Phase II enzymes, (j) androgen receptor down regulation, (k) inhibition of DNA adduct formation, and (l) cell cycle arrest (Naithani, 2008).

Additional trials are needed to define the benefits and risks of different types and doses of selenium supplements, which in the future may be implemented for cancer and other chronic disease prevention.