Do Allergies Raise Risk of Mental Illness? New Study Finds No Causal Link

Do Allergies Raise Risk of Mental Illness? New Study Finds No Causal Link
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A new study might provide some comfort to those who suffer from allergies. The research found little evidence that having allergies directly raises the risk of developing mental health problems, or vice-versa, something that previous research had suggested might be the case. Though it’s still possible that treating allergies can improve some people’s mental state, there doesn’t seem to be a cause-and-effect relationship between being allergic to something and mental illness, the researchers say.

Allergies and mental health conditions like depression or severe anxiety are abundant ailments of humanity. Some studies have suggested that people diagnosed with either one are more likely to be diagnosed with the other. Based on that observational data, scientists have argued that there might be common underlying risk factors to both, and/or that developing one of these conditions (usually allergies) predisposes us to the other. For instance, people with allergies have overactive immune responses to their triggers, leading to symptoms caused by inflammation. And chronic inflammation is also thought to play some role in mental illness.

A common refrain in science, though, is that correlation doesn’t necessarily equal causation. Unfortunately, it’s not always possible to conduct randomised and controlled experiments that can show causation, like the clinical trials used to show a drug works. But researchers have other methods to try untangling cause-and-effect from large population studies. One of these methods is known as mendelian randomization, which relies on studying people’s genetic biomarkers — variations of genes that are suspected of increasing our risk of a certain condition.

Researchers at the University of Bristol turned to data from the UK Biobank, a long-running project started in 2007 that collected the (anonymous) genetic information of 500,000 volunteers in the country and which has been tracking their health ever since. They focused on whether having genetic biomarkers for either allergy or mental illness was linked to a higher risk of being diagnosed with the other. Because a person’s genes can’t be affected by unrelated outside factors, any link you do find is more likely to represent a true cause-and-effect, according to study author Ashley Budu-Aggrey, a epidemiologist and medical geneticist at Bristol.

As other studies have done, Budu-Aggrey and her team found a strong association between being diagnosed with allergic diseases like asthma as well as mental health conditions like depression. But they only found very weak evidence of a genetic link between allergies and mental illness in either direction, indicating that no overall causal relationship exists, particularly for allergies causing mental illness. The study’s results were published Wednesday in Clinical and Experimental Allergy.

“Our findings suggest that the risk of allergic disease does not increase the risk of mental health problems,” Budu-Aggrey said.

Like any research, the study does have its limitations. Outside of the genetic data, much of the information collected from volunteers on their health is self-reported, so there’s always the possibility of some misreporting. And they did find a stronger genetic link between hay fever and bipolar disorder, suggesting that there could be some direct relationship between these two ailments specifically.

Another caveat is that the team only focused on whether the onset of an allergic disease could raise someone’s risk of later mental illness. But it’s still possible that having an allergy go untreated or grow worse over time could directly impact someone’s risk of mental illness. Conversely, as some research has suggested, successfully treating a severe allergy might improve a person’s mental health.

The authors say that these are all issues that future studies should explore. And mendelian randomization in general should be more widely used by researchers to help settle questions about cause-and-effect relationships brought up by other observational studies, Budu-Aggrey said. Past studies using this method have raised doubt about the suspected fear that cannabis use causes schizophrenia, as well as supported the theory that being more educated can heighten our risk of nearsightedness.

Ultimately, though, the results do suggest that it’s not going to be possible to hit two birds with one stone in this particular case. Preventing mental illness and allergies are both worthy goals, but we probably shouldn’t expect to reach one by tackling the other, according to Budu-Aggrey.

“Interventions to prevent allergic disease are not likely to improve mental health,” she said.