The relationship between selenium and cancer

Selenium is a nonmetallic element with atomic number 34, symbol “Se,” located in period 4 of the periodic table. It is one of 13 trace elements , meaning it is necessary for our body but is present in extremely small amounts. Daily requirements for these trace elements are low, but they are essential for cellular enzyme reactions in our body.

It is present in soil in inorganic form (as selenite or selenit), in plants in organic form (as selenomethionine and selenocysteine), and in animals as selenomethionine. Selenium forms part of 24 different selenoproteins. Selenoproteins participate in biological processes such as reproductive system function, thyroid hormone metabolism, DNA synthesis, and defense against infections and DNA damage.

Which foods contain selenium?

Brazil nuts contain the most selenium. Sunflower seeds, shellfish, fish, turkey, chicken and red meat are also rich in selenium. Eggs, mushrooms, onions, garlic and grains contain selenium as well. Because it is found in many foods, people with a normal diet do not develop selenium deficiency and do not need supplementation.

Selenium’s potential chemopreventive (cancer-preventing) effect has been investigated in different cancers, especially prostate cancer. Although some studies suggest selenium reduces prostate cancer risk , randomized controlled studies of selenium intake have not observed any relationship.

Prostate cancer and selenium - scientific evidence

• A meta-analysis published in 2010 reported that selenium supplements had no effect on prostate cancer incidence.
• A multicenter, double-blind study in the United States examining selenium and vitamin E in cancer prevention included men over 50, totaling 35.533 participants. Results showed no significant benefit of vitamin E or selenium intake in preventing prostate cancer.
• In 423 patients with a precursor lesion that could develop into prostate cancer (high-grade prostatic intraepithelial neoplasia), the SWOG S9917 study found, after 3 years, no difference in disease progression between people taking selenium and those not taking it.
• In a double-blind, randomized, placebo-controlled phase 2 study, selenium supplementation was observed to make no difference to PSA levels in patients with early-stage (localized) prostate cancer under active surveillance.
• However, the double-blind Nutritional Prevention of Cancer study reported that selenium supplementation was protective against prostate cancer.

Bladder cancer and selenium - scientific evidence

A randomized, double-blind phase 3 study found selenium supplementation did not help prevent recurrence of nonmetastatic bladder cancer . The SELECT study also found no protective effect of selenium in bladder cancer.

Lung cancer and selenium - scientific evidence

Two separate randomized controlled studies in America and China reported that selenium supplementation did not have a protective effect against lung cancer .

Skin cancer and selenium - scientific evidence

Various studies found no relationship between selenium supplementation and the incidence of basal and squamous cell skin cancers. However, a double-blind randomized controlled study found an increased risk of squamous cell skin cancer and non- melanoma skin cancer among selenium users. In a study called SUVIMAX, with participants aged 35 to 60 totaling 12.741, selenium use was observed to increase skin cancer risk in women.

Colorectal cancer and selenium - scientific evidence

In the Selenium and Celcoxib (Sel/Cel) study, patients aged 40 to 80 who had polyps removed by colonoscopy numbered 1621. Selenium’s protective effect against large bowel (colon and rectal) cancer was analyzed in these patients. Results showed that selenium had no effect on preventing colorectal cancer.

Conclusion

In conclusion, studies examining the relationship between selenium and cancer have found no association for many cancer types. In addition, some studies report an increased risk of skin cancer. Current knowledge shows that selenium supplementation is not effective in preventing any cancer.

There are also reported cases in which selenium taken as a supplement caused poisoning; therefore, we do not recommend supplements such as selenium medication or selenium tablets. However, if low blood levels are demonstrated, we recommend eating selenium-rich foods.