WEDNESDAY, March 15, 2006 (HealthDay News) -- Women who are heavier and have high levels of a hormone produced by fat cells seem to be at an increased risk for having asthma.
The hormone, called leptin, had already been shown to be associated with asthma in children, and there have been reports that asthma and obesity are related. However, in this case, the role of leptin seems to be independent of obesity.
"A lot of people have been wondering for many, many years why asthma is rising across the world, particularly in the developed world, and the obesity epidemic has been a near parallel to the asthma epidemic," said study author Dr. Akshay Sood, an associate professor of medicine at the University of New Mexico School of Medicine in Albuquerque. "A lot of epidemiologists have been wondering whether there is really a connection or just a strange coincidence."
Sood conducted this research while at the Southern Illinois University School of Medicine, in Springfield, Ill.
Indeed, previous studies have demonstrated an association between obesity and asthma, and the correlation appears to be stronger in women than in men.
And adipose, or fat tissue, is turning out to be more complicated than once thought. "For a while, adipose tissue was thought to be an inert organ that just stored fat and no one wanted it," Sood said. "Now it seems from a lot of animal experiments that adipose tissue is a very pro-inflammatory organ. It seems that fat cells are constantly producing pro-inflammatory and anti-inflammatory cytokines [proteins related to the immune system] and imbalances may result in a variety of immunological disorders, although this is only a hypothesis."
This study, which appears in the March 2 online issue of Thorax, looked at information on almost 6,000 American adults who had participated in the Third National Health and Nutrition Examination Study (NHANES). There was information on levels of leptin, and on who had asthma.
The NHANES survey included blood leptin levels and self-reported definitions of asthma. Self-reported diagnoses of conditions are usually considered a weakness in research.
"It's certainly not definitive," said Dr. David Taylor, section head of pulmonary/critical care medicine at the Ochsner Clinic Foundation, in New Orleans.
In this sample, leptin levels were significantly higher among men and women who had been diagnosed with asthma vs. those who had never had the disease. The mean leptin concentration was 11.1 micrograms per liter for those who had never had asthma and 13.7 for those who currently had asthma, a statistically significant difference, according to Sood.
The association was stronger in women than in men, and stronger still in women who were premenopausal.
Leptin seemed to have an association with asthma that went over and above that of obesity, suggesting that it may have an independent role to play.
"We've shown that leptin seems to have an association with asthma, particularly in women," Sood said. "We also showed that even though body-mass index [BMI] is associated with asthma, the effect of BMI cannot be completely explained by leptin, so it seems that leptin might have an independent effect on asthma."
"We think of leptin as related to obesity but it may be that it does something else, an inflammatory or pro-inflammatory effect," Taylor said. "At a science level, that may be the most interesting part to this study, that there may be another role for leptin other than obesity. We know that asthma is an inflammatory disease. It would make more sense than to blame it on obesity per se."
The findings mean little for patients, however, other than to consider the well-worn message: Maintain a healthy body weight.
"You don't need any more science to say you need to lose weight," Sood said.
For more on adult asthma, visit the American Academy of Allergy, Asthma and Immunology.