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by MedMaven, Medically Reviewed by Dr. C.H. Weaver M.D. 01/2021

Some studies have suggested that people with higher selenium levels have a lower risk of certain types of cancer; a review conducted by the Cochrane Collaboration, however, found no clear evidence that selenium supplements provide a cancer benefit.

Selenium is an essential trace element that is necessary for the formation and function of at least 13 proteins. Studies performed in animals have shown that increasing the dietary intake of selenium can lead to a reduction in the incidence of some cancers. As a result, in recent years, many studies have been designed to explore the link between selenium levels and the development of cancer in humans. Researchers speculate that selenium may play a role in the repair and prevention of oxidative damage.1

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To summarize the available evidence, researchers evaluated 55 studies that involved a total of more than one million participants.The studies with the most reliable results found that selenium supplements did not reduce the risk of prostate cancer or non-melanoma skin cancer.

The researchers concluded “Currently, there is no convincing evidence that individuals, particularly those who are adequately nourished, will benefit from selenium supplementation with regard to their cancer risk.”

There is still much to be learned about selenium and the role nutrients may play in the prevention of cancer. People who are considering the use of dietary supplements are advised to discuss the risks and benefits with their physician.

Summary of Selected Selenium Studies

Gastric and Esophageal Cancer

In a study in China, researchers found a highly significant association between low levels of selenium in the blood and a high incidence of esophageal and gastric cancers. The study, published in the Journal of the National Cancer Institute, led researchers to estimate that over 25% of esophageal and gastric cancers in China were due to low levels of selenium.

In this study, researchers evaluated the relationship between selenium levels and the subsequent development of esophageal and gastric cancers. To do this, they selected subjects from the General Population Trial of Linxian, China, which was a large trial (29,584 subjects) designed to test four different combinations of nutritional supplements over a period of 5.25 years. One year prior to the start of the General Population Trial, all subjects had blood drawn and stored.

For the new study, researchers evaluated the results from the General Population Trial and selected a large group of subjects who had developed cancer as well as a large group of subjects with no cancer. Overall, researchers selected 1079 case subjects (590 with esophageal cancer, 489 with gastric cancer) and 1062 control subjects. They then measured the selenium levels from the blood that was drawn prior to the original study.

The researchers found a highly significant relationship between low levels of selenium and the subsequent development of esophageal and gastric cancers. The average selenium levels were approximately 4% higher in the control subjects than in the subjects with cancer. Individuals in the highest quartile of selenium developed esophageal and gastric cancers at half the rate as individuals in the lowest quartile.

Furthermore, one year after having blood drawn, approximately half of the 2,141 subjects in this study began taking a supplement that included selenium, beta-carotene, and vitamin E as part of the General Population Trial. The subjects who received this particular supplement had significantly lower rates of esophageal and gastric cancers (both incidence and mortality) than those who did not. However, despite the reduced risk resulting from the supplement, the researchers found that the relationship between selenium levels and cancer risk did not change regardless of whether or not subjects received the supplemental selenium. The researchers speculated that while the supplement could reduce risk, this particular dose might not be capable of immediately changing long-term low levels of selenium.

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The researchers concluded that selenium might play a role in the prevention of esophageal and gastric cancers. More research is necessary to determine whether the results of this trial have implications for the U.S. population. Typically the selenium levels in the U.S. population are higher than those of the Chinese population in this study. Furthermore, the major risk factors for esophageal and gastric cancers in the U.S. are smoking and alcohol consumption. Some of the other risk factors associated with these cancers, such as obesity, reflux disease, and Barrett’s metaplasia, are virtually absent in the Chinese population. Research will be ongoing to determine the role that selenium plays in the prevention of cancer. Individuals interested in the prevention of esophageal, gastric, and other cancers may wish to ensure that their diets include adequate amounts of selenium.3

Selenium May Reduce Risk of Developing Lung, Colorectal, and Prostate Cancer

Researchers from several centers in the United States conducted a study to determine whether the use of selenium supplements would result in a reduced risk of developing cancer or a recurrence (return) of cancer in 1312 persons who had a history of basal cell or squamous cell cancer of the skin. The researchers assigned the patients to receive either 200 micrograms of selenium per day or a placebo. The findings showed that the selenium did not have any impact on whether the patients developed the skin cancer again. 

However, the selenium was associated with fewer cancer-related deaths. Of the group receiving placebo, 57 persons died of cancer; of the group receiving selenium, 29 persons died of cancer. Of the cancers that were diagnosed, 119 were in the placebo group and 77 were in the selenium group. Cancers that were shown to be reduced in the selenium group included lung, colorectal, and prostate cancers. Because of these favorable results showing reductions in the incidence of lung, colorectal, and prostate cancers and the reduction in deaths from cancer, this study was stopped early.4

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Selenium Shows Promise in Reducing the Risk of Colorectal Cancer

The results of a study recently published in the Journal of the National Cancer Institute indicate that patients with higher levels of selenium in their blood are significantly less likely to develop new colon polyps, which may decrease their risk of colorectal cancer.

Previous studies have been inconclusive regarding the benefit of selenium with regard to colorectal cancer. Researchers in this trial combined the data collected from 3 previous randomized trials: 1) The Wheat Bran Fiber Trial, 2) The Polyp Prevention Trial, and 3) The Polyp Prevention Study. These previous trials had tested various nutritional interventions among patients who had recently had a colon polyp removed during colonoscopy. In the current trial, selenium concentrations in the blood were measured in 1,763 patients. Extensive analysis was performed to estimate the association between selenium blood concentrations and the risk of colorectal cancer.

Evaluation of the combined data found that individuals with the highest levels of selenium had a significantly lower chance of developing new colon polyps when compared to those patients with lower levels of selenium. This finding supports previous theories that higher levels of selenium may be related to a decreased risk of colorectal cancer. Patients are encouraged to speak to their physician regarding selenium supplementation and their risk of colorectal cancer.5

Prostate Cancer

According to results recently published in the Journal of Urology, men with low plasma levels of selenium are at increased risk for developing prostate cancer. Based on this evidence, researchers suggest that supplemental selenium may reduce the risk of prostate cancer, particularly in older men whose selenium levels tend to be lower.

Researchers from Stanford University conducted a clinical study involving approximately 150 men registered in the Baltimore Longitudinal Study of Aging. The patient’s ages ranged from 49 to 91 years old and 52 men had been diagnosed with prostate cancer. Selenium levels in plasma (the clear portion of blood in which cells are suspended) were measured an average of 3.83 years before prostate cancer was diagnosed.

Researchers found that low plasma selenium levels are associated with a four- to five-fold increase in the risk of prostate cancer. They also noted that plasma selenium levels decrease with age, possibly explaining why older men are more likely to develop prostate cancer than younger men. These results suggest that taking supplemental selenium may reduce risk of prostate cancer. However, clinical trials are needed to confirm that supplemental selenium can help prevent the development of prostate cancer.6

References:

  1. Dietary Supplement Fact Sheet: Selenium. National Institutes of Health, Office of Dietary Supplements website. Page last updated November, 2009.
  2. Dennert G, Zwahlen M, Brinkman M, Vinceti M, Zeegers MPA, Horneber M. Selenium for preventing cancer. Cochrane Database of Systematic Reviews 2011, Issue 5. Art. No.: CD005195.
  3. Journal of the National Cancer Institute, Vol 92, No 21, pp. 1753-1763, 2000.
  4. Journal of the American Medical Society, Vol 276, No 24, pp 1957-1963).
  5. Jacobs E, Jiang R, Alberts D. et al. Selenium and Colorectal Adenoma: Results of a Pooled Analysis. Journal of the National Cancer Institute . 2004; 96: 1669-1675.
  6. (Journal of Urology, Vol 166, pp 2034-2038, 2001).