Add asthma to the list of health problems that obesity can cause in young women.

Many parts of the world have seen an explosion in the obesity epidemic and increases in asthma. Studies have shown several possible links between the two, but an understanding of the issue remains incomplete, say researchers.

In this new study, researchers estimate that more than one in four new cases of asthma among girls and women aged 9-26 years is due to excess weight.

They found no connection between obesity and asthma in boys or young men. That could explain the fact that more boys have asthma before puberty, but girls are more likely to develop asthma as they mature.

Why the difference? The study doesn’t settle that question. It may be partly due to women’s increased body fat after puberty. Or perhaps female sex hormones play a role, write the researchers.

They found that higher BMI (body mass index) increased girls’ risk of developing asthma after age 9. Before then, girls’ weight wasn’t related to asthma. Children with asthma also weren’t more likely to become obese adults.

To see the link between asthma and obesity, consider the girls and women who developed asthma between ages 9 and 26. Of those, 34% were obese, with a BMI higher than 30. Nineteen percent were overweight, with a BMI of 25-30. Only 11% had a BMI under 25.

Overall, weight problems were responsible for an estimated 28% of new asthma cases in girls and women aged 9-26 years, says the study.

The data came from a study of about 1,000 children born in New Zealand from April 1972 to March 1973. The kids’ BMIs were tracked over the years, along with cases of asthma and wheezing.

Family histories of asthma and smoking were also noted, along with breastfeeding and birth order. None of that changed the results.

The study was conducted by researchers including Robert Hancox of the Dunedin School of Medicine at New Zealand’s University of Otago. It appears in the March 1 issue of the American Journal of Respiratory and Critical Care Medicine.

SOURCES: Hancox, R. American Journal of Respiratory and Critical Care Medicine, March 1, 2005; vol 171: pp 440-445. News release, American Thoracic Society.