Taking fish-oil supplements or even eating too much fatty fish may be linked to an increased risk for prostate cancer, according to a new study from the Fred Hutchinson Cancer Research Center. The result confirms findings from an earlier study by the same team, but they are puzzling, given fish oil’s supposed anti-inflammatory effect, which would protect against cancer.
Researchers could not offer a biological reason for the link, and called for more study.
The study analyzed levels of omega-3 fatty acids — the type of oil found in some fish — in the blood of 834 men who developed prostate cancer race- and age-matched with 1,393 men who did not. Men who had the highest levels of omega-3 fatty acids had a 43 percent increase in risk for prostate cancer and 71 percent increase in risk for the high-grade prostate cancer that is the most likely to be fatal.
These results were published online Wednesday by the Journal of the National Cancer Institute. Researchers affiliated with institutions including the University of Washington, the National Cancer Institute and Cleveland Clinic were also involved.
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An initial 2011 study, which found similar results in a different group of men, surprised epidemiology professor Alan Kristal’s team at “The Hutch. “To be honest, I didn’t believe it,” Kristal said Wednesday. “It was striking enough to get it into the literature just to see if anyone would repeat it.” The team’s most recent study — and another European study — confirmed the earlier findings.
The newest data come from a study whose initial goal, when it began in 2001, was investigating the roles of selenium and vitamin E in prostate cancer.
Researchers collected blood samples from study subjects, who were not given dietary instructions for omega-3 intake.
The highest blood levels of three omega-3 fatty acids, EPA, DPA and DHA, were consistent with taking fish-oil supplements or eating at least three servings of fish per week. Those men with the highest levels were the most likely to eventually be diagnosed with prostate cancer.
However, Kristal notes that different people can have somewhat different levels of omega-3 fatty acids in their blood despite similar intake.
The link between prostate cancer and eating fatty fish or taking fish-oil supplements is far from clear. Other studies have found a protective effect, though a large analysis of many studies found that fish oil had no compelling effect on cancer risk in general
Edward Giovannucci of the Harvard School of Public Health, who was not involved in the study, noted in an email that this study looked at diagnosis but not patient outcomes.
Prostate cancers can lie dormant for decades, and the risk factors for developing a tumor may not be the same as those that cause a tumor to become fatal.
The researchers conceded they did not know of a biological mechanism to explain their findings. “If there were a compelling mechanism, that would make the findings more worrisome,” added Giovannucci.
Nevertheless, Kristal said his study should make men think twice about taking fish-oil supplements or eating more than two servings of fish per week.
Omega-3 fatty-acid supplements and enriched foods account for more than $5 billion in sales every year, according to a market-research report from Packaged Facts.
Kristal emphasizes that his study casts doubt on the health effect of dietary supplements such as fish oil and vitamins. In the same study, researchers had previously found that vitamin E was also linked to increased risk for prostate cancer.
“Humans are designed for a certain level of micronutrients, and huge doses may not be good,” Kristal said. “More micronutrients does not mean better health and sometimes means worse.”
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