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November 2006

By Julius G. Goepp, MD

Selenium Shows Promise in Preventing Cancer

Selenium’s antioxidant properties have spurred great interest in its potential to prevent cancer. This is not surprising, given our increasing awareness that cancer is caused and promoted by the same cascade of events—oxidative stress and cytokine release—that produces the autoimmune and other inflammatory conditions just discussed.

To date, more than 100 animal studies have investigated selenium’s effects on the mechanisms responsible for initiating cancer. In the overwhelming majority of these studies, selenium reduced tumor incidence and tissue changes that lead to cancer.8,37 The high incidence of various cancers in selenium-deficient regions of the world strongly supports a cancer-preventive role for selenium in humans as well, and epidemiological studies have demonstrated reductions in the rates of, and mortality from, all cancers in populations receiving selenium supplementation.38

Selenium Prevents and Slows Prostate Cancer

Additional studies of selenium’s effects in helping to prevent specific cancers have yielded results that are even more positive.

One of the most dramatic of these was the unexpected outcome of a study designed to examine selenium’s impact on skin cancer.39 This study of 1,312 individuals who received 200 mcg of selenium daily or a placebo showed no effect against skin cancer, but demonstrated “striking” results in preventing prostate cancer, the most common cancer in American men.40,41 The overall risk of prostate cancer was almost 50% lower in the supplemented group than in the controls, though the result was significant only in men who had relatively low prostate-specific antigen (PSA) levels and low initial selenium levels.

This result prompted a re-analysis of data from the SU.VI.MAX study, with a specific look at prostate cancer.42 Within that group, 5,141 men took the selenium-containing supplement or placebo for eight years, and biochemical markers of prostate disease were measured at the beginning and end of the study. Overall, a slight reduction in prostate cancer risk was reported; however, among men who had normal PSA levels at the study’s outset, a significant risk reduction of nearly 50% was recorded.

A 2005 study focusing on selenium’s effects in preventing prostate cancer dramatically confirmed this protective effect.43 Forty-eight patients with early prostate cancer took selenium, vitamin E, both L-selenomethionine and vitamin E, or a placebo for three to six weeks before undergoing prostatectomy (removal of the prostate). Levels of cancer markers were measured and compared with 29 healthy control subjects. The startling result was a change in classification from cancerous to healthy in the serum markers of disease in the men who took supplements compared to those who did not.

In addition to its clear role in preventing prostate cancer, selenium may slow the progression of already established prostate cancer. In a six-week trial, 37 men with prostate cancer and increasing PSA levels were given either a placebo or an antioxidant supplement containing selenium, plant estrogens, and other antioxidants.44 In the supplemented group, male hormone levels (known to stimulate prostate cancer growth) were lower during treatment. In addition, free PSA levels rose during treatment with the placebo, but decreased during antioxidant supplementation.

These studies, along with recent findings that selenium is selectively concentrated in prostate tissue,45 strongly support a role for selenium supplementation in both preventing and slowing the progression of prostate cancer.

Selenium Protects Against Lung Cancer

Lung cancer is the most common cause of cancer death in the world.46 The previously mentioned skin cancer study also demonstrated that selenium supplementation reduced rates of lung and colorectal cancers.39 Lung cancer is associated with antioxidative stress and low levels of antioxidants, including selenium.47 A 1993 study demonstrated a 50% reduction in lung cancer occurrence in people with the highest dietary selenium intake compared to those with the lowest intake.48 This finding is supported by a more recent study of more than 27,000 male smokers who were followed for nearly 15 years. In this study, researchers found that lung cancer risk was significantly lower in patients who had the highest intake of antioxidant vitamins and selenium.49

Selenium supplementation was highly effective in preventing lung cancer in a region of China where very low natural selenium concentrations contribute to some of the world’s highest rates of lung cancer. Forty Chinese tin miners were randomly assigned to receive either 300 mcg of selenium or a placebo daily for one year. As expected, selenium blood levels rose dramatically in the supplemented miners, while serum levels of the antioxidant enzyme glutathione peroxidase increased by 156%. At the same time, levels of lipid peroxide (a measure of cell membrane damage that leads to cancer) were reduced by 75% in the supplemented group, and there was laboratory evidence of protection from DNA damage, another prerequisite for cancer formation.50

Moreover, results from the US Nutritional Prevention of Cancer Trial demonstrated a statistically significant reduction in lung cancer incidence with selenium supplementation, with 200 mcg per day cutting the incidence of cancer by nearly 50%.51 A later re-analysis with additional data showed the effect to be most significant in people with low baseline selenium levels, again suggesting that supplementation is preventive when initiated early.37

Selenium Guards Against Colon Cancer

Selenium supplementation has also shown effectiveness in preventing colorectal cancer, the third most common cause of cancer death in the US.52 Glutathione peroxidase, a selenium-containing antioxidant enzyme, is genetically defective in a significant percentage of patients with colon cancer.53 Selenium actually activates the DNA repair mechanisms that help cells protect themselves against colon and other cancers,54 while inducing programmed cell death (apoptosis) in cancerous tissue.55 In an animal model of cancer, selenium-containing broccoli in the diet of laboratory rats protected the animals against chemically induced mammary and colon cancers.56

After demonstrating that patients with colon cancer routinely have selenium deficiencies, and that levels of vital antioxidant enzymes could be increased with selenium supplementation, one research team concluded, “If prospective trials confirm that selenium supplementation reduces colon cancer incidence rates, it may be concluded that selenium supplementation should be recommended for patients at risk.”57

Such confirmation is now beyond doubt. In a 1996 study of 44 patients with colon cancer, half were randomly assigned to receive selenium supplements and half were given a placebo. All the subjects had their tumors surgically removed, and all had abnormally low selenium levels at baseline. The selenium-supplemented patients demonstrated significant increases in anti-cancer immune system cells compared to levels in control patients, suggesting that selenium supplementation boosts cell-mediated immunity.58

More dramatic, unexpected evidence of selenium’s cancer-preventive properties comes from a 2006 study. Seeking to determine the maximum tolerable dose of the anti-cancer drug irinotecan, researchers administered a massive dose of selenomethionine (containing 2200 mcg of selenium) to protect against the drug’s toxicity.59 Selenium supplementation was begun one week before the first dose of irinotecan was administered to colon cancer patients who had previously not responded to chemotherapy. This small study of highly drug-resistant patients produced unexpected responses (one patient out of six showed a partial response to treatment) and disease stabilization. No adverse effects of the high dose were reported. The scientists recommended further study of high-dose selenomethionine to determine the most protective serum concentrations of selenium.

Cardiovascular Diseases, Atherogenesis, and the Metabolic Syndrome

The exciting evidence for selenium’s effectiveness in preventing inflammatory diseases and cancer has led scientists to explore its protective effects against other conditions involving oxidative damage. Many of these conditions were previously considered to be “natural” consequences of aging. With the discovery that atherosclerosis, cardiovascular disease, hypertension, obesity, and type II diabetes—components of the so-called “metabolic syndrome—are all caused or exacerbated by oxidant stress,60 selenium may prove to have a valuable preventive and therapeutic role in these conditions as well.

Patients with acute myocardial infarction (heart attack) have been shown to have lower levels of selenium in their plasma, red blood cells, and urine compared to control subjects.61 Selenium concentrations are also low in some patients with endothelial dysfunction, the inflammatory-mediated precursor to atherosclerosis and hypertension. Supplementation with selenium has been shown to reverse specific chemical markers of endothelial dysfunction.62 In addition, selenium may help protect against congestive heart failure.63,64

Oxidative stress is both a cause and consequence of chronic renal failure. Patients who suffer form this condition experience exceptionally high levels of cardiovascular disease and death,65 as well as impaired immune function.66 Selenium levels are markedly reduced in these patients.67 Selenium deficiency is especially pronounced in patients on kidney dialysis, who lose considerable amounts of selenium during the dialysis process. This renders them even more vulnerable to further oxidative damage to their cardiovascular systems.65 Selenium supplementation in dialysis patients has been used successfully to reduce oxidative stress,68 and has actually been shown to improve immune function as well.66


A review of recent research findings suggests that scientists have only begun to tap selenium’s potential as a vital antioxidant mineral.

As scientists continue to discover the many ways in which oxidative stress is related to inflammation and its destructive consequences—from atherosclerosis to prostate, lung, colon, and other cancers—the disease-preventive powers of selenium are likely to receive even greater scrutiny. For now, all health-conscious adults would be well advised to incorporate this vital mineral nutrient in their daily supplement regimen as part of a comprehensive disease-prevention program.


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