These findings confirm smaller studies in which researchers have suggested the same. For those who suffer from allergies and mental health issues, it's an important - and potentially validating - discovery. But for doctors who treat these conditions, it's even more significant.
Published in Frontiers in Psychiatry, the study was the brainchild of Dr. Nian-Sheng Tzeng, a psychiatrist at Tri-Service General Hospital in Taipei. An acting clinician at Tri-Service - a teaching hospital run by the country's military - Tzeng told ScienceDaily that he began the research after noticing a pattern. "I observed that some patients with the three A's appeared to suffer emotionally," Tzeng said, referring to asthma, allergies, and atopic dermatitis (eczema). "Therefore, I wanted to clarify whether these allergic diseases are associated with psychiatric disorders."
To conduct the research, Tzeng and his colleagues gathered a database of health insurance claims from within Taiwan, spanning 15 years and all age groups. Among the data, they identified 46,647 people with allergic diseases (such as the three mentioned) and 139,931 without. Of those with them, 10.8 percent developed a psychiatric condition such as anxiety or posttraumatic stress disorder. Among those without allergic diseases, the number was just 6.7 percent.
Tzeng and his team concluded from this that those who suffer from allergies are 1.6 times more likely to develop a psychiatric disorder than those who don't. More specifically, those who suffer from asthma and allergies are more likely to develop a psychiatric illness.
The research builds on earlier studies that have linked psychiatric disorders and allergies. One significant study from 2008 - published in the International Journal of Child Health and Human Development - found a prevalence of depression and anxiety among children with asthma. In that study, researchers theorized that the two conditions share underlying causes, including chronic stress, genetic factors, and inflammation.
Inflammation, according to Tzeng's team, may be the factor that connects allergic diseases and psychiatric disorders. Although still often considered an abstract term by the general public, inflammation has been gaining steam in the research world for decades. The National Center for Biotechnology Information defines it simply as "the body's immune system's response to stimulus."
This means that, on its face, inflammation is a good thing. It allows the body to ramp up its defenses in order to perform important functions such as healing wounds or fighting off bacteria. The problem is that, in too many cases, the body perceives an enemy when there isn't one - and switches on an inflammatory response that never gets turned off.
In 2006, Harvard University published a paper titled Inflammation: A Unifying Theory of Disease, in which it summarized comprehensive proof that inflammation leads to numerous illnesses - both mental and physical. Since then, even more studies have linked inflammation to major disorders that continue to spike in the U.S., including obesity, diabetes, inflammatory bowel disease (such as Crohn's and ulcerative colitis), cancer, and Alzheimer's disease.
Allergies, which affect 24 million people in the U.S., are an "overreaction of the immune system," according to the Centers for Disease Control and Prevention. But now scientists are beginning to believe that psychiatric disorders have a similar origin. Specifically, there is a growing body of research showing that depression responds to an anti-inflammatory diet - in some cases, more than it responds to pharmacological intervention.
All this serves to bolster what Tzeng's research implies - that the cause of allergies may help doctors to not only better understand psychiatric disorders, but also to better treat them. "We would like to let clinicians who care for patients with allergic diseases know that their risk for psychiatric diseases may be higher," Tzeng said to ScienceDaily. "Assessing their emotional condition and monitoring their mental health could help to avoid later psychiatric problems."
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