RESEARCH TRIANGLE PARK, N.C.--Researchers are bringing up concerns regarding the outcome of a study published in late 2003 in which selenium was found to promote the risk of squamous cell carcinoma and total nonmelanoma skin cancer (J Natl Cancer Inst, 95, 19:1477-81, 2003) (http://jncicancerspectrum.oupjournals.org).
In the study, data was analyzed from the Nutritional Prevention of Cancer Trial, a double blind, placebo-controlled clinical trial designed to test whether 200 mcg/d of selenium (as selenized yeast) would prevent nonmelanoma skin cancer among 1,312 patients who previously had the disease. The dose was seen to inhibit prostate cancer but increase squamous cell carcinoma and total nonmelanoma skin cancer risk.
"I am not sure what is going on with this agent in the skin," the study’s lead author told INSIDER at that time. "Clearly, it does not appear to offer protection against skin cancer. Whether it actually increases risk is not as clear. ... Our best estimate is that selenium does not increase risk by much."
James Huff, Ph.D., from the National Institute of Environmental Health Sciences, wrote in the Feb. 18 Journal of the National Cancer Institute (NCI) (96, 4:333-4, 2004) that the adverse effects of selenium should be taken seriously--especially since selenium sulfide was seen to promote liver and lung tumors in rat models during the 1980s. “In addition to causing skin cancers in humans, these experimental carcinogenic results should caution us that long-term selenium intake may be more hazardous than previously realized,” stated Huff, adding researchers should stay on their toes regarding the possible carcinogenic effects from selenium-containing supplements.
Huff, however, didn’t totally dismiss the protective benefits of selenium in overall incidence of prostate cancer, since this is where the mineral has shown the most promise. “Although, the effect was restricted [in the study] to those with lower baseline prostate-specific antigen and plasma selenium concentrations,” he wrote.
Huff concluded that as NCI currently investigates the use of selenium and vitamin E for prostate cancer prevention (the SELECT trial, slated to end in 2014), the study volunteers should be closely monitored for signs of other types of cancer development.
In response to Huff, the October 2003 study’s authors reported that it is true--the toxicity, carcinogenicity and anti-cancer activities of selenium vary from compound to compound. "The biologic activity of selenium is very much dependent on its chemical form," the authors wrote. "Nonetheless, the fact that one selenium compound has deleterious effects does not mean all others do."
Industry suppliers noted the study results raise interesting questions. "Selenium, although known for having positive benefits in fighting certain cancers--particularly prostate--can have pro-cancer effects when taken in large doses, especially when taking an inorganic source such as sodium selenite," said Max Motyka, director of the human products division at Albion Advanced Nutrition in Clearfield, Utah. "Theoretically, any of the antioxidant-involved minerals would be helpful in programs to fight against cancers, including manganese, copper and zinc, in addition to the aforementioned selenium."
Motyka added that the pro-carcinogenic effects of selenium may be more complicated than at first glance. “Recently, it was brought to the public’s attention that there was a data grouping that suggested women who had taken a very large number of antibiotics for infections been seen to have a higher incidence of cancer. One thing that we should keep in mind here is that the immune system, aside from genetics and environmental causes, protects us against many things, not just infection. It helps fight off things like cancer. Could it be that the repeated bouts of infection, which required antibiotic therapy, were pointing out a weakness in the immune system that was the actual precursor to the cancers involved?"