October 2 (HealthDay News) -- The inhaled steroids that are often used to treat asthma don't work as well in the overweight or obese, new research shows.

In fact, the treatments are 40 percent less effective in these patients than in those of a healthy weight, said study author Dr. E. Rand Sutherland, an associate professor of medicine at National Jewish Health in Denver.

"The combination of obesity and asthma appears to do something to limit the pathways by which steroids reduce inflammation," he said.

"We had seen these preliminary reports that inhaled steroids might not work as well [in overweight patients]," he said. So, his team decided to try to see if that was so, and to find out why.

About 20 million Americans have asthma, said Sutherland, and nearly half are overweight or obese, having a body-mass index (BMI) of 25 or higher.

His team looked at 33 adults with asthma and 12 without. "We took blood cells and airway cells" from all, he said, and measured the response of the cells to the glucocorticoid called dexamethasone.

The steroid works by interfering with inflammatory signaling pathways by raising the level of a molecule called MAP kinase phosphatase-1 (MKP-1).

When the cells were exposed to the steroid, the levels of MKP-1 increased by 5.27 times in the lean asthma patients but increased by 3.11 times -- 41 percent less -- in those who were overweight or obese.

Why the difference occurs is not known, Sutherland said. "We can speculate. Obesity has been associated with increasing inflammation in the body. It may be that the same processes that increase inflammation leading to diabetes and cardiovascular disease may also be implicated in modifying steroid effects."

The study is well-done, said Dr. John E. Heffner, past president of the American Thoracic Society and chair of medical education at the Providence Portland Medical Center, in Oregon.

It shows that the more obese a person with asthma is, the less the expression of MKP-1 -- and the more inflammation.

The practical advice, Heffner said, is to suggest overweight patients with asthma lose weight. However, he said, "We don't know if you can regain your responsiveness to glucocorticoids if you lose weight."

Sutherland cautions those with asthma not to stop taking medications. "This [study ] doesn't mean the drug won't work, it just means the drugs might not work as well" in overweight patients with asthma. Physicians may want to increase the dose, however.

The study was published in the first October issue of the American Journal of Respiratory and Critical Care Medicine.

SOURCES: E. Rand Sutherland, M.D., M.P.H., associate professor, medicine, National Jewish Health, Denver; John E. Heffner, M.D., past president, American Thoracic Society, and chairman, medical education, Providence Portland Medical Center, Ore.; October 2008, American Journal of Respiratory and Critical Care Medicine