As obesity has been increasing in recent years, so has asthma — and research in the past has shown an association between the two.

Though there may be several underlying causes behind asthma, such as respiratory infections, air pollutants, allergens, cold air, and even some medications, doctors are coming to believe that obesity may be one of them, too. Now, a new study out of the University of Bristol in the U.K. has found genetic evidence for the association.

Previous studies were observational ones, which were only able to recognize a correlation and not a causation between obesity and asthma. So researchers used Mendelian randomization, a method that combines both genetic information and observational data in order to better identify whether body mass index (BMI) has an impact on asthma. The reviewed 4,835 children who had an average age of 7.5; they studied the causal effects of BMI, fat mass, and lean mass on the participants’ asthma. The researchers found that the risk of asthma increases by 55 percent for every extra unit of BMI.

“[E]nvironmental influences on the development of asthma in childhood have been extensively investigated in epidemiological studies, but few of these provide strong evidence for causality,” the authors write in their report. “[Higher BMI in mid-childhood] could help explain some of the increase in asthma risk toward the end of the 20th century, although the continued rise in obesity but with a slowing in the rise in asthma prevalence in some countries implies that other non-BMI-related factors are also likely to be important.”

Obesity can lead to narrowing of the airways, known as bronchoconstriction. So when asthma patients develop obesity, it can make their breathing disorder worse. Overall, research has consistently shown that obesity causes diminished lung function and constricted airways.

A study completed by Columbia University Medical Center (CUMC) last year discovered that leptin, a hormone involved in energy metabolism, fertility, and bone mass, also had to do with airway diameter. The researchers found that patients with weight-related asthma could be treated with medication that monitors leptin function. This was the first explanation for why obesity is linked to asthma. “Our study started with the clinical observation that both obesity and anorexia can lead to asthma,” Dr. Gerard Karsenty, professor and chair of genetics and development at CUMC, said in a press release.

About 25 million people in the U.S. have asthma — that's about 8 percent of the population. Meanwhile, up to 35 percent of U.S. adults are obese, according to the CDC.