Obesity is known to be a risk factor for developing asthma but a new study shows that the reverse is also true: people with asthma are more likely to go on to become obese.

The new research, presented at the European Respiratory Society International Congress, indicates that those who develop asthma as adults and those who have non-allergic asthma are at the greatest risk of obesity. The team behind the research say it suggests the relationship between asthma and obesity is more complex than previously thought and more research is needed to better understand and tackle these two growing health challenges.

The research was presented by Dr Subhabrata Moitra, a European Respiratory Society research fellow at ISGlobal – the Barcelona Institute for Global Health (a centre supported by the "la Caixa" Foundation), Spain. He explained: "We already know that obesity can be a trigger for asthma, perhaps via a physiological, metabolic or inflammatory change.

"Until now there has been very little research on whether the reverse is true – whether asthma can lead to obesity. In this study, we have enough people and we have followed them for long enough to observe the relationship between these two conditions."

The research was part of the European Community Respiratory Health Survey and included 8,618 people from 12 countries who were not obese at the start of the research. This means they all had a body mass index of less than 30 kg/m².

Participants were considered to have asthma if they reported ever having asthma and had an asthma attack or were woken up by an attack of shortness of breath in the previous 12 months, or if they were currently taking asthma medication.

The study began recruiting in the 1990s, and participants were followed up after ten years and again after 20 years. The researchers examined the relationships between having asthma at the start of the study and the likelihood of being obese ten years later. They also studied people who had developed asthma after ten years in the study and their risk of obesity by 20 years. Researchers took other risk factors into consideration, including age, sex, country and physical activity.

They found that 10.2% of people with asthma at the start of the study had become obese ten years on. Among people who did not have asthma, 7.7% were obese ten years later.

The increase in the risk of obesity was even greater in people whose asthma began in adulthood. It was also greater in people who had asthma but did not suffer with allergies.

Dr Moitra added: "By following a large number of study participants over two decades, we have been able to observe how having asthma increases a person's risk of going on to become obese, especially if their asthma begins in adulthood or if they have asthma but no allergies.

"Our findings suggest the relationship between the two conditions is more complicated than we previously realised. It's important that we do more work to pick this apart. For example, we do not know why having asthma increases the risk of developing obesity or whether different asthma treatments have any effect on this risk."

Professor Guy Brusselle from Ghent University in Belgium is Chair of the European Respiratory Society Science Council and was not involved in the study. He said: "With the right medication, many people with asthma gain good control of their symptoms. However, there is no cure for asthma and there is still a lot we do not know about its causes and its effects on the rest of the body. This research is an important step in helping us untangle the relationship between obesity and asthma but it also raises new questions about why the two are linked and what can be done to help patients."