"Men with hay fever are more likely to have prostate cancer – but those with asthma are more likely to survive it," the Daily Mirror reports. Those were the puzzling and largely inconclusive findings of a new study looking at these three conditions.
Researchers looked at data involving around 50,000 middle-aged men and followed them up for 25 years, looking at whether asthma or hay fever at study start were associated with diagnoses of prostate cancer or fatal prostate cancer during follow-up.
The findings weren't as conclusive as the headline suggests. The researchers did find hay fever was associated with a small (7%) increased risk of prostate cancer development. There was some suggestion asthma may be associated with a decreased risk of getting prostate cancer or fatal prostate cancer. However, these links were only of borderline statistical significance, meaning there was a high risk they could have been the result of chance.
And the links between hay fever and fatal prostate cancer weren't significant at all, meaning there was no evidence that men with hay fever were more likely to die from the disease (so no need to worry if you are affected).
The possibility that inflammation, or the immune system more generally, could be associated with risk of prostate cancer is plausible, but this study tells us little about how different immune profiles could influence cancer risk.
Where did the story come from?
The study was carried out by researchers from Johns Hopkins Bloomberg School of Public Health and other institutions in the US. It was funded by grants from The National Cancer Institute and The National Heart, Lung and Blood Institute. The study was published in the peer-reviewed International Journal of Cancer.
The Daily Mirror has taken an uncritical view of the research findings and fails to make clear to its readers that the findings were mainly based on borderline statistically significant or non-significant results. These don't provide firm proof of links between asthma or hay fever and prostate cancer or lethal prostate cancer.
What kind of research was this?
This was a prospective cohort study looking into how the immune system might be involved in the development of prostate cancer.
The study authors say emerging research suggests inflammation, and the immune response in general, may be involved in the development of prostate cancer. As they say, one way to explore this is by looking at the links between prostate cancer and conditions that have a particular immune profile. Two such immune-mediated conditions are asthma and allergies, such as hay fever.
Previous studies looking at links between the conditions gave inconsistent results. This study looked at the link in a prospective cohort of almost 50,000 cancer-free men, looking to see whether they developed prostate cancer and the factors associated. Cohort studies such as these can demonstrate associations, but they cannot prove cause and effect as many other unmeasured factors may be involved.
What did the research involve?
The cohort was called the Health Professionals Follow-Up Study. In 1986 it enrolled 47,880 cancer-free men, then aged 40-75 years (91% white ethnicity), who were followed up for 25 years.
Every two years, men completed questionnaires on medical history and lifestyle, and filled in food questionnaires every four years.
At study enrolment they were asked whether they had ever been diagnosed with asthma, hay fever or another allergy and, if so, the year it started. In subsequent questionnaires they were asked about new asthma diagnoses and asthma medications, but hay fever was only questioned at study start.
Men reporting a diagnosis of prostate cancer on follow-up questionnaires had this confirmed through medical records. The researchers also used the National Death Index to identify cancer deaths.
The researchers looked at the associations between prostate cancer and reported asthma or hay fever, particularly looking at the link with "lethal" prostate cancer. This was defined as being prostate cancer either diagnosed at a later stage when the cancer had already spread around the body (so expected to be terminal), or being the cause of death.
They adjusted their analyses for potential confounders of:
- body mass index (BMI)
- smoking status
- physical activity
- family history of prostate cancer
What were the basic results?
Five percent of the cohort had a history of asthma at study start and 25% had hay fever. During the 25-year follow-up there were 6,294 cases of prostate cancer. Of these, 798 were expected to be lethal, including 625 recorded deaths.
After adjusting for confounders, there was a suggestion that having asthma at study start was associated with a lower risk of developing prostate cancer. We say a suggestion, because the 95% confidence interval (CI) of the result included 1.00. This makes it of borderline relative risk (RR) 0.89, 95% CI 0.78 to 1.00) meaning the finding may have been down to chance alone.
Hay fever, by contrast, was associated with an increased risk of developing prostate cancer, which did just reach statistical significance (RR 1.07, 95% CI 1.01 to 1.13).
Looking at lethal prostate cancer, there was again a suggestion that asthma was associated with decreased risk, but this again appeared of borderline statistical significance (RR 0.67, 95% CI 0.45 to 1.00). Hay fever was not this time significantly associated with risk of lethal prostate cancer.
The researchers then looked at ever having a diagnosis of asthma, this time not only looking at the 5% already diagnosed at study start, but also the 4% who developed the condition during follow-up. Again they found that ever having a diagnosis of asthma was associated with a decreased risk of lethal prostate cancer, but this was only of borderline statistical significance (RR 0.71, 95% CI 0.51 to 1.00).
The researchers also considered the time of diagnosis. They report that onset of hay fever in the distant past (more than 30 years ago) "was possibly weakly positively associated with risk of lethal" prostate cancer. However, this link is not statistically significant (RR 1.10, 95% CI 0.92 to 1.33).
How did the researchers interpret the results?
The researchers' conclude: "Men who were ever diagnosed with asthma were less likely to develop lethal and fatal prostate cancer." They add: "Our findings may lead to testable hypotheses about specific immune profiles in the [development] of lethal prostate cancer."
The researchers' suggestion that this research is "hypothesis generating" is the most apt. It shows a possible link between immune profiles and prostate cancer, but doesn't prove it or explain the underlying reasons for any such link.
This single study does not provide solid evidence that asthma or hay fever would have any influence on a man's risk of developing prostate cancer or dying from it, particularly when you consider the uncertain statistical significance of several of the findings.
Links suggesting asthma may be associated with a lower risk of total or lethal prostate cancer were all only of borderline statistical significance, meaning we can have less confidence that these are true links.
Links with hay fever were similarly far from convincing. Though the researchers found a 7% increased risk of developing prostate cancer with hay fever, this only just reached statistical significance (95% CI 1.01 to 1.13). The links between hay fever and risk of lethal prostate cancer that hit the headlines weren't significant at all, so they provide no evidence for a link.
Even if there is a link between asthma and allergy and prostate cancer risk, it's still possible this could be influenced by unmeasured health and lifestyle factors that have not been adjusted for.
Other limitations to this prospective cohort include its predominantly white sample, particularly given that prostate cancer is known to be more common in black African or black Caribbean men.
The results may not be applicable to these higher-risk populations. Also, though prostate cancer diagnoses were confirmed through medical records and death certificates, there is the possibility for inaccurate classification of asthma or allergic conditions as these were self-reported.
The possibility that inflammation, or the immune system more generally, could be associated with risk of prostate cancer is definitely plausible. For example, history of inflammation of the prostate gland is recognised to be possibly associated with increased prostate cancer risk. Therefore, study into how different immune profiles could have differing cancer risk is a worthy angle of research into prostate cancer.
However, the findings of this single cohort should not be of undue concern to men with hay fever or, conversely, suggest that men with asthma have protection from the disease.