Smokers with the highest selenium levels in their toenails who had quit smoking in the last 10 years were only 30 percent as likely to develop invasive carcinomas of the bladder compared with former smokers with low levels of the mineral, researchers said. However, nonsmokers, current smokers and smokers who had quit for more than 10 years showed no benefit associated with higher levels of selenium.
"We have found an inverse relation between selenium intake and the risk of bladder cancer among former smokers," lead investigator Maurice P.A. Zeegers, assistant professor of epidemiology at Maastricht University, told United Press International.
Smoking causes certain key molecules in cells to lose electrons, a process called oxidation. The oxidized molecules become more reactive in the body in a destructive way. Selenium counters the molecules and therefore said to have an antioxidant effect, Zeegers explained. Nonsmokers show much less benefit from selenium exposure because they have not been exposed to the oxidative stress of smoking, he said.
As reported in the November issue of Cancer Epidemiology, Biomarkers and Prevention, the research is based on a 6-year study of 121,000 men and women, ages 55 to 69, as part of broader research on cancer and diet. Risk factors for cancer -- including diet and exposure to industrial chemicals -- and smoking history were recorded for each person and toenail clippings were collected and analyzed. Toenails provide a more accurate estimate of body levels and dietary intake of selenium than blood serum levels, which can fluctuate, according to the study.
Other well-known antioxidants, such as vitamins C and E and beta-carotene, did not lower bladder cancer rates among smokers or nonsmokers, the study found.
"It's tempting to tell patients that a selenium dietary supplement might be beneficial based on these data," Mark Schoenberg, director of urologic oncology at the James Buchanan Brady Urological Institute at Johns Hopkins University in Baltimore, told UPI.
"The problem is that there are so many other things that could have potentially contributed to the results the researchers report that it would be premature to generalize these observations or incorporate this information into routine clinical practice at this time," Schoenberg said.
"This is an epidemiologic study and it is not necessarily predictive. If you as an individual have a particular selenium level it doesn't necessarily mean that you are going to have a lower or higher risk of bladder cancer," Gary Goodman, a cancer prevention researcher at The Swedish Cancer Institute and the Fred Hutchinson Cancer Research Institute, both in Seattle, told UPI.
Before drawing any general conclusions about selenium and its effects on bladder cancer, researchers need to conduct randomized studies, in which relatively similar groups either received or did not receive the mineral, Goodman and Schoenberg both said. However, previous studies have linked diets high in selenium with in lower incidences of lung, colorectal and prostate cancers.
Selenium is found in meats, fish, cereal, dairy products, eggs and some nuts. Brazil nuts in particular contain high levels. Selenium also is sold as a dietary supplement in pill form.
(Reported by Joe Grossman, UPI Science News, in Santa Cruz, Calif.)