Carrying excess weight around the middle can impair lung function, adding to a long list of health problems associated with belly fat, French researchers say.

Abdominal obesity is already linked with diabetes, high blood pressure and heart disease as part of a cluster of health problems known collectively as metabolic syndrome.

Researchers have now shown that a large waist measurement is strongly associated with decreased lung function, regardless of other complicating factors that affect the lungs such as overall obesity and smoking.

The researchers analyzed health information of about 120,000 people in France, assessing their demographic background, smoking history, alcohol consumption, as well as lung function with respect to a measure of obesity known as body mass index, waist circumference and other measures of metabolic health.

"We found a positive independent relationship between lung function impairment and metabolic syndrome due mainly to abdominal obesity," Dr Natalie Leone of the French National Institute for Health and Medical Research wrote in the American Journal of Respiratory and Critical Care Medicine.

The researchers defined abdominal obesity as having a waist circumference of greater than 89 cm for women and 101 cm for men.

Several large studies have linked poor lung function with higher rates of deaths and hospitalization from heart disease, the researchers said.

While it was not clear from the study, the researchers think belly fat may impair the way the diaphragm and chest function. Fat tissue is also known to increase inflammation in the body, which may be playing a role, they said.

Although the reasons may not be clear, Dr Paul Enright of the University of Arizona said in a commentary that there is now enough evidence to include waist measurements as part of routine assessments of lung function.