FRIDAY, Dec. 23 (HealthDay News) -- Children who are overweight or obese during early childhood have a greater risk of having asthma at age 8 than normal-weight kids, a new study finds.

Researchers in Sweden followed more than 2,000 children for eight years, using preschool and school health records to track their height and weight at ages 1 year, 18 months, 4 years and 7 years. Parents completed questionnaires about their child's health, including asthma and allergy status.

Children who had persistently high BMI (body mass index) -- in the 85th percentile or above -- throughout early childhood, or who were normal-weight toddlers but gained weight and had a high BMI at age 7, were more likely to have asthma than kids who had a normal body weight.

However, kids who had a high BMI at an early age -- at 18 months or 4 years -- but slimmed down by age 7 were not at higher risk of asthma than other kids.

"If the children are only overweight during the early period before 4 years of age we do not see an increased risk of asthma during school age," said lead study author Jessica Magnusson, a Ph.D. student at the Institute of Environmental Medicine in Stockholm. "However, if they are persistently overweight, or overweight at a later age -- age 7 -- then there is an association with asthma at age 8."

Asthma, characterized by inflammation of the airways, may cause wheezing, coughing, chest tightness and trouble breathing.

The study is in the January issue of Pediatrics.

At age 8, about 6 percent of the kids in the study had asthma. Those overweight at age 4 and age 7 had a nearly 2.5 times greater risk of having asthma.

Researchers excluded kids who'd had early symptoms of wheezing or had been diagnosed with asthma prior to age 2.

Researchers also took into account parental history of asthma. A high BMI was associated with an increased risk of asthma only in kids without parental history of the disease, according to the study.

Researchers pointed out that their study does not show that being overweight or obese causes asthma. However, the march upward in childhood obesity rates has coincided with an increase in asthma rates, leading some to speculate that the two may be linked biologically.

One theory is that leptin, a hormone found in fat tissue, may contribute to an inflammatory immune response that could trigger asthma, which is a chronic inflammation of the airways.

A prior study found higher leptin levels in overweight children, and that even among overweight children with similar BMIs, kids with asthma tended to have higher leptin levels.

The current study also found an association between being overweight at age 7 and sensitization to airborne allergens. Sensitization, or the presence of certain antibodies in the blood, often indicates an allergy to a particular substance, but researchers did not track actual symptoms.

Getting control of a child's weight is important to prevent asthma and other conditions that are showing up more in kids, including diabetes and high cholesterol, said Nancy Copperman, director of public health initiatives in the Office of Community Health at North Shore-LIJ Health System in Great Neck, N.Y.

And obesity and asthma can feed off each another. Children experiencing asthma symptoms and having difficulty breathing may be less apt to participate in physical activity, while parents may worry about their asthmatic kids and not allow them to do certain things, such as run outside in the cold, Copperman said.

"What this study argues for is prevention," she said. "The kids who were heavier and got leaner didn't have the increased incidence of asthma, while those who were lean and got heavier or were heavy from the beginning did ... Obesity is not a cosmetic problem. It has real health consequences."

More information

The American Academy of Child & Adolescent Psychiatry has more on childhood obesity.