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Available Evidence Shows Selenium Supplements Not Useful for Preventing Cancer But May Cause Harm

Worst Pills, Best Pills Newsletter article February, 2020

Dietary supplements containing the trace mineral selenium for many years have been widely touted for the prevention of cancer.[1] Such claims were based on findings from early laboratory research and observational studies in people that suggested selenium might have anticancer properties.

But in 2018, the Cochrane Collaboration (a nonprofit organization that specializes in analyzing evidence from multiple studies) published an updated systematic review of data from randomized,...

Dietary supplements containing the trace mineral selenium for many years have been widely touted for the prevention of cancer.[1] Such claims were based on findings from early laboratory research and observational studies in people that suggested selenium might have anticancer properties.

But in 2018, the Cochrane Collaboration (a nonprofit organization that specializes in analyzing evidence from multiple studies) published an updated systematic review of data from randomized, controlled clinical trials and observational studies that evaluated the effects of selenium supplementation on the risk of developing cancer.[2] The review showed — with a high level of certainty — that selenium supplements are not beneficial for reducing cancer risk.

What are dietary supplements, and how are they regulated?

Used regularly by at least half of all Americans, dietary supplements are defined by the 1994 Dietary Supplement Health and Education Act (DSHEA) as any products intended to supplement the diet that contain a vitamin, mineral, herb or other botanical; an amino acid; or “a dietary substance for use by man to supplement the diet by increasing the total dietary intake.”

The use of dietary supplements has grown steadily since 1994 and is now widespread in the U.S. The DSHEA clarified that supplements were to be regulated as foods, not drugs, and thus were exempt from the much tougher regulations accorded to drugs, such as the requirement to prove that they are both safe and effective.

No supplement has been demonstrated to be safe and effective under the rigorous standards that the FDA applies to drugs. Furthermore, while drug companies have to report any serious or unexpected adverse event they learn about to the FDA, there is no such reporting requirement for manufacturers of dietary supplements.

Still, manufacturers are permitted to aggressively promote dietary supplements. Although a supplement manufacturer is prohibited under FDA regulations from making health claims when promoting a supplement, it may make structure or function claims. For example, a supplement manufacturer, without any supporting evidence whatsoever, can assert that its product “promotes prostate health” (a structure or function claim) but is precluded from claiming that it “treats the symptoms of an enlarged prostate” (a health claim).

About selenium[3]

Selenium is an essential nutrient for humans. It plays a key role in reproduction, thyroid gland function, DNA synthesis and protection from infections.

Selenium is found naturally in soil and many types of food. According to the National Academy of Medicine (formerly the Institute of Medicine), the recommended dietary allowance for selenium for nonpregnant, nonlactating adults is 55 micrograms per day. The richest food sources of selenium are Brazil nuts, seafoods and organ meats. Other sources include muscle meats, cereals, grains and dairy products (see Table, below, for the estimated selenium content of selected food sources).

Estimated Selenium Content of Selected Food Sources

Food Source, Serving Size Micrograms of selenium per serving
Brazil nuts, 1 ounce (6-8 nuts) 544
Tuna, yellowfin, cooked, dry heat, 3 ounces 92
Halibut, cooked, dry heat, 3 ounces 47
Sardines, canned in oil, drained solids with bone, 3 ounces 45
Ham, roasted, 3 ounces 42
Shrimp, canned, 3 ounces 40
Macaroni, enriched, cooked, 1 cup 37
Beef steak, bottom round, roasted, 3 ounces 33
Turkey, boneless, roasted, 3 ounces 31
Beef liver, pan fried, 3 ounces 28
Chicken, light meat, roasted, 3 ounces 22
Cottage cheese, 1% milkfat, 1 cup 20
Rice, brown, long-grain, cooked, 1 cup 19
Beef, ground, 25% fat, broiled, 3 ounces 18
Egg, hard-boiled, 1 large 15
Puffed wheat ready-to-eat cereal, fortified, 1 cup 15
Bread, whole-wheat, 1 slice 13
Baked beans, canned, plain or vegetarian, 1 cup 13
Oatmeal, regular and quick, unenriched, cooked with water, 1 cup 13
Spinach, frozen, boiled, 1 cup 11
Milk, 1% fat, 1 cup 8

Source: National Institutes of Health, Office of Dietary Supplements, Selenium Fact Sheet for Health Professionals, https://ods.od.nih.gov/factsheets/Selenium-HealthProfessional/.

Most people in the U.S. consume more than adequate amounts of selenium in their daily diets. As a result, selenium deficiency is very rare in the U.S. However, patients with end-stage kidney disease undergoing hemodialysis, which removes selenium from the blood, and certain patients with HIV infection often have deficient levels of selenium.

Cochrane review findings[4]

The authors of the 2018 Cochrane systematic review carefully searched scientific databases for all randomized clinical trials that compared the effects on cancer risk of selenium supplementation at any dose for at least four weeks with those of a placebo or no supplement use in adults age 18 or older. They excluded trials in which selenium was one of multiple supplements given to subjects, unless the trial also had a group that received only selenium. Their search identified 10 clinical trials meeting their criteria that together enrolled more than 27,000 subjects.

The primary outcomes of interest assessed in the review were the incidence of any cancer, the incidence of specific types of cancer (for example, lung cancer and prostate cancer) and the proportion of subjects who died from any cancer and from specific types of cancer. The review authors also assessed the rates of certain adverse events.

The analyses of the randomized clinical trials found that selenium supplements had no statistically significant effect on the risk of developing or dying from cancer overall. Likewise, analyses for specific types of cancers showed that selenium supplements did not reduce the risk of developing colorectal, non-melanoma skin, lung, breast, bladder or prostate cancer. The certainty of the evidence was rated as “high” for all these analyses except for those related to breast and non-melanoma skin cancer, for which the certainty of the evidence was rated as “moderate.”

On the other hand, the results from some of the randomized clinical trials raised safety concerns about the use of selenium supplements. For example, data from the largest randomized clinical trial called the Selenium and Vitamin E Cancer Prevention Trial (SELECT), which was funded by the National Cancer Institute,[5] showed that selenium supplementation increased risks of hair loss and dermatitis (skin rash or inflammation). Moreover, SELECT demonstrated that for subjects who had the highest baseline selenium exposure, selenium supplementation increased the risk of high-grade prostate cancer. Finally, some of the trials showed that selenium supplementation slightly increased the risk of type 2 diabetes.

The review authors also analyzed data from 70 observational studies that assessed baseline selenium intake in more than 2.3 million cancer-free adults and then measured subsequent health outcomes. Although the analyses for these studies showed that higher selenium exposure overall was associated with lower rates of cancer and cancer deaths, the studies had major weaknesses related to their design and other factors. As a result, the certainty of the evidence from these observational studies was rated as “very low.” Such weak evidence should be disregarded in the face of the strong evidence from the much more rigorous randomized clinical trials.

The authors of the Cochrane review concluded that overall, there is no evidence to suggest that increasing selenium intake through diet or supplementation prevents cancer in humans.

What You Can Do

You should not take selenium supplements in an effort to reduce your risk of cancer or for any other reason unless your doctor has instructed you to do so because you have been diagnosed with selenium deficiency or are at high risk of selenium deficiency.
 



References

[1] Vinceti_M, Filippini_T, Del Giovane_C, et al. Selenium for preventing cancer. Cochrane Database of Systematic Reviews. 2018; Issue 1:CD005195.

[2] Ibid.

[3] National Institutes of Health, Office of Dietary Supplements. Selenium. October 17, 2019. https://ods.od.nih.gov/factsheets/Selenium-HealthProfessional/. Accessed November 8, 2019.

[4] Vinceti_M, Filippini_T, Del Giovane_C, et al. Selenium for preventing cancer. Cochrane Database of Systematic Reviews. 2018; Issue 1:CD005195.

[5] Lippman SM, Klein EA, Goodman PJ, et al. Effect of selenium and vitamin E on risk of prostate cancer and other cancers: the Selenium and Vitamin E Cancer Prevention Trial (SELECT). JAMA. 2009;301(1):39-51.