Men and women taking selenium supplements for 10 years had 41 percent less total cancer than those taking a placebo, a new study by Cornell University and the University of Arizona shows. "Although more than a hundred of animal and dozens of epidemiological studies have linked high selenium status and cancer risk, this is the first double-blind, placebo-controlled cancer prevention study with humans that directly supports the thesis that a nutritional supplement of selenium, as a single agent, can reduce the risk of cancer," said Gerald F. Combs Jr., a nutritional biochemist and Cornell professor of nutritional sciences.
Combs and a group of co-authors reported their findings in the Jan. 1, 1997 issue of The Journal of the American Medical Association. The senior author is epidemiologist Larry Clark, who was at Cornell at the onset of the study and is now at the Department of Family and Community Medicine at the University of Arizona School of Medicine.
The other primary authors are Cornell biostatisticians Bruce W. Turnbull and Elizabeth Slate, professor and assistant professor, respectively, in the School of Operations Research and Industrial Engineering, and David S. Alberts, M.D., of the College of Medicine, Arizona Cancer Center.
In 1983, the researchers recruited 1,312 randomized patients with histories of skin cancer at seven dermatology clinics located in low-selenium areas of the United States (Augusta and Macon, Ga., Columbia, S.C., Wilson and Greenville, S.C., Miami, and Newington, Conn., where consumers ingest an average of about 100 micrograms of selenium a day). The patients were given either a placebo or a 200-microgram daily supplement of selenium (twice the average amount these Americans consume in their diet, thereby tripling their selenium intake).
Skin cancer patients were chosen because they have a 25 percent annual chance of a recurrence, and skin cancer is easy to diagnose and can be quickly treated. The researchers set out to determine whether they could reduce the average recurrence rate with selenium supplements.
Ironically, 10 years later, the results were not significant for skin cancer. However, they were "compelling" for overall cancer incidence and mortality rates, Combs stressed. Of the selenium group, 69 developed some type of cancer compared with 116 of the placebo group; 28 of the selenium patients died of cancer compared with 58 from the placebo group.
"Overall, the selenium group experienced 18 percent less mortality than the placebo group, and almost all of that difference was due to some form of cancer," said Combs, who credits Cornell with having the longest history of research in selenium nutrition research in the world. "This is the first time anyone has shown that any single nutrient can result in such a reduction in cancer risk. The fact that we saw a pattern in lower incidence and mortality rates across all the clinics gives us even greater confidence in these findings."
Prostate, esophageal, colorectal and lung cancer rates were among the most dramatic: patients in the selenium group had 71 percent, 67 percent, 62 and 46 percent reductions in cancer rates, respectively, than the placebo group.
The current Dietary Guidelines now recommend that men consume 70 micrograms of selenium a day and women 55 micrograms; Americans typically get between 100 to 160 micrograms a day in their diets, Combs said.
Selenium blood levels vary widely in populations. Even Americans with the lowest selenium intake of 60 to 80 micrograms per day -- those living along the Southeastern seaboard and in the Pacific Northwest -- ingest two to five times more than citizens of New Zealand and 10 to 20 times more than people living in some areas of China. Selenium blood levels vary among populations largely because of wide differences in soil, agronomic practices, food availability and preferences and methods of food preparation.
Although a 1995 Harvard University study of more than 62,000 nurses reported no anti-cancer benefits of selenium, Combs said that the researchers measured selenium in the toenail clippings of the nurses. "This method presumes that the concentration of selenium in nails accurately correlates with metabolic selenium status in the body," Combs pointed out. "There is no evidence for that."
The University of Arizona-Cornell research team reported in 1991 that low selenium levels in the blood were linked to increased risk of neoplastic polyps in the colon, a precursor to colorectal cancer. And in other studies at Cornell, colleagues of Combs' reported in 1995 that animals fed diets high in selenium had 50 percent fewer tumors than those fed diets of average selenium content.
Combs is not recommending the use of oral supplements of selenium; however, he does emphasize the importance of consuming low-fat diets that are adequate in selenium and are balanced with respect to other essential nutrients. The most important dietary sources of selenium are meats, fish and cereals; dairy products and eggs contribute significant amounts. Some nuts can also be high in selenium if they were grown in high-selenium areas.
Of the 40 nutrients currently recognized as essential for human nutrition, selenium was the last to be recognized in 1957. A key component for at least two essential enzymes, selenium provides the body with antioxidant protection in concert with vitamin E and is required for normal thyroid hormone metabolism.
The study was funded in part by grants from the American Institute of Cancer Research, the American Cancer Society and the National Institutes of Health.