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Can Selenium Curb Cancer?

Selenium may protect women from bladder cancer; Men may not get prostate protection
by Craig Weatherby

Selenium is a trace element critical to basic immune and brain functions in the human body.

But selenium intake varies widely around the U.S. and the world.

Grains are not high in selenium but constitute the major selenium source for most people.

Accordingly, most people's selenium blood levels depend largely on the levels of selenium in the soil in which the grains they most commonly consume are grown.

Grains grown in the American Midwest tend to be rich in selenium, so deficiency is rare in the U.S.

But the best selenium sources by far are ocean fish, including salmon (ODS 2008).

And it makes good sense to achieve blood levels above the minimums required to prevent deficiency, given the growing evidence that selenium helps prevent cancer and can block the effects of dietary mercury.

Excellent sources of selenium include button mushrooms, shiitake mushrooms, cod, shrimp, snapper, tuna, halibut, calf's liver, and salmon.

(See our accompanying article, “Whole Salmon Favored over Fish Oil Pills,” in which researchers cite high selenium and omega-3 content as key salmon attributes.)

Seafood is rich in selenium
Salmon is certainly a rich source of selenium, but some of our selection offers even more of the mineral… though not as much vitamin D or omega-3s.

All figures are in micrograms (mcg) and come from USDA data tables for cooked seafood, or canned where noted.

The US RDA is 55 mcg, and few other food sources even come close to these levels per serving:
  • Selenium per 3.5 oz serving
  • Albacore Tuna (canned) 60
  • Sardines (canned) 52.7
  • Mackerel (canned) 51.6
  • Halibut 46.8
  • Sablefish 46.8
  • Pollock 46.8
  • King Salmon (chinook) 46.8
  • King Crab 40
  • Shrimp/Prawns 39.6
  • Silver Salmon (coho) 38
  • Sockeye Salmon (red) 37.8
  • Cod 37.6
  • Scallops 27.9
Scientific understanding of selenium's roles in the basic biology of cancer has grown rapidly during last two decades.

Basic research and studies in animal and humans strongly support the protective role of selenium against various types of cancer.

Epidemiological studies link inadequate selenium intake to increased risk of cancer, and selenium is essential to formation of “selenoproteins” such as glutathione peroxidase and thioredoxin reductase, which form key parts of the body's internal immunity/antioxidant system.

Now, a new study suggests that diets rich in selenium may reduce a woman's risk of bladder cancer by 34 percent.

And the Dartmouth Medical School researchers behind the new research results also reported significant risk reductions among moderate smokers, and cancer patients with a cancer- related gene called p53 (Wallace K et al. 2008).

Prior studies find an association between selenium and bladder cancer among women, but the new results are the first to show an association between selenium and p53-positive bladder cancer.

Selenium and cancer: the background
The protective actions of selenium flow from its synergistic ability to induce apoptosis (cancer cell “suicide”), inactivate pro-inflammatory immune system agents (transcription factors and lipoxygenases), induce Phase II detoxifying liver enzymes, down-regulate androgen receptors, up-regulate anti-cancer transcription factors, neturalize free radicals and toxic metals, and prevent DNA damage.

Selenium levels have been falling in Europe since the EU imposed levies on wheat imports from the US, where soil selenium levels are high.

As a result, average intake of selenium in the UK has fallen from 60 to 34 micrograms per day, leading to calls from some to enrich soil and fertilizers with selenium to boost public consumption (Selenium-enriched fertilizers are used in Finland).

The US RDA for selenium is 55 mcg (60 mcg during pregnancy and 70 mcg while nursing). The European Commission's recommended daily intake (RDI) is 65 micrograms.

Selenium can be harmful in excess, but selenium toxicity is rare in the U.S., and the few reported cases have been associated with industrial accidents and a supplement manufacturing error.

The Institute of Medicine of the National Academy of Sciences has set a tolerable upper intake level (UL) for selenium at 400 micrograms per day for adults: a conservative limit that includes a generous buffer.

Selenium for smokers, the genetically susceptible, and the fair sex
Lead researcher Margaret Karagas and her co-workers measured selenium levels in the toenails of 767 people newly diagnosed with bladder cancer (76 percent male, average age 62) and 1,108 people from the general population (61 percent male, average age 61).

While no associations were found between selenium levels and bladder cancer risk for the whole population group, significant reductions in risk were found for women (34 percent), moderate smokers (39 percent) and cancer patients with the p53 gene (43 percent).

The researchers hope to replicate these findings on a larger scale in order to examine the connection between selenium and bladder cancer in women and those with p53 tumors, as well as with patient prognosis.

Selenium shows no prostate-prevention effect
Another new investigation, based on data from the European Prospective Investigation into Cancer and Nutrition (EPIC) study, indicates that higher blood levels of selenium may not reduce the risk of prostate cancer.

The researchers, led by Naomi Allen from the University of Oxford, performed a study involving 959 men with prostate cancer and 1,059 healthy men who acted as the controls for comparison.

The concentration of selenium in the subjects' blood did not correlate with their risk of developing prostate cancer.

  • Allen NE, Appleby PN, Roddam AW, Tjonneland A, et al. Plasma selenium concentration and prostate cancer risk: results from the European Prospective Investigation into Cancer and Nutrition (EPIC). American Journal of Clinical Nutrition. December 2008, Volume 88, Pages 1567-1575.
  • Brigelius-Flohé R. Selenium compounds and selenoproteins in cancer. Chem Biodivers. 2008 Mar;5(3):389-95. Review.
  • Navarro-Alarcon M, Cabrera-Vique C. Selenium in food and the human body: a review. Sci Total Environ. 2008 Aug 1;400(1-3):115-41. Epub 2008 Jul 26.
  • Office of Dietary Supplements / National Institutes of Health (ODS). Dietary Supplement Fact Sheet: Selenium. Accessed online December 14, 2008 at
  • Wallace K, Kelsey KT, Schned A, Morris JS, Andrew AS, Karagas MR. Selenium and Risk of Bladder Cancer: A Population-Based Case-Control Study. Cancer Prevention Research. December 2008, doi:10.1158/1940-6207.CAPR-08-0046