Researchers found that men with asthma were 29 percent less likely to develop aggressive prostate cancer. And they were 36 percent less likely to die from the disease, according to the study.
However, the findings do not show that asthma protects men from prostate cancer, according to Elizabeth Platz, professor of epidemiology at Johns Hopkins Bloomberg School of Public Health, and co-leader of the Cancer Prevention and Control Program at Johns Hopkins Kimmel Cancer Centre in Baltimore.
"We don't know yet whether the association we see in this observational study is a case of cause and effect," Platz said in a Hopkins news release.
The researchers analysed data from nearly 48,000 American men between the ages of 40 and 75. None had been diagnosed with cancer prior to 1986. Their health was followed from 1986 through 2012.
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The link between asthma and reduced risk of deadly prostate cancer remained even after the researchers accounted for factors such as whether men took asthma medications or whether their asthma was diagnosed early or later in life.
The findings are surprising because some studies have suggested that prostate cancer is linked to the kind of inflammation associated with asthma, according to Platz.
She and her colleagues also found a reverse association involving hay fever and prostate cancer. Men with hay fever were 10 to 12 percent more likely to have aggressive or fatal prostate cancer.
The study was published online in the International Journal of Cancer.
The researchers looked at the connection between asthma and prostate cancer based on work in mice showing the immune cell response to prostate cancer, which they called Th2 inflammation.
"Asthma is often considered to be a disease of chronic inflammation, particularly Th2 inflammation. And cancer is often thought of as mediated by Th2 inflammation. So what we expected was that asthmatics would have a higher incidence of prostate cancer," said Dr. Charles Drake, co-director of the Prostate Cancer Multidisciplinary Clinic at Johns Hopkins Kimmel Cancer Centre.
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"It's possible that the Th2 inflammation that drives asthma is not the same as the Th2 inflammation that drives cancer," he said in the news release.
Or it may be that people with asthma have higher levels of other immune cells that attack tumour cells, Drake added.
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Image: Mature man with asthma pump from Shutterstock