In affluent countries, up to one half of all adults are overweight, as determined by a body mass index (BMI = weight in kilograms divided by height in meters squared) greater than 25 kg per m². Although asthma and obesity may not be causally related, the high prevalence of obesity results in many asthmatic patients being obese. In theory, weight loss should benefit lung function by reducing the tendency of airways to collapse, stimulating adrenal function and reducing dietary allergens and salt content in the diet. Stenius-Aarniala and colleagues studied the effect of weight loss on lung function, morbidity, symptoms and health status in obese, asthmatic patients.

Asthmatic persons with a BMI of 30 to 42 kg per m² were recruited through newspaper advertisements. Inclusion criteria included persons between 18 to 60 years of age who had a previous diagnosis of asthma, a bronchodilator response of 15 percent or more, and were nonsmokers or had stopped smoking for at least two years or more before 50 years of age. Exclusion criteria included pregnancy, eating disorders, serious medical conditions or allergies to foods used in the study protocols. After a complete physical assessment, a three-week run-in period was initiated during which lung function and biochemical baselines were established. After two weeks of baseline measurements, the 38 participants were randomly assigned to either the treatment or control group. Participants in the treatment group attended 12 group sessions during a 14-week period that focused on weight control; this segment included eight weeks during which the participants took a very-low-calorie dietary preparation. Participants in the control group also attended group sessions at the same intervals as the treatment group; discussion topics were chosen by the participants. At the end of the first year, both groups had received the same amount of education about asthma and allergy.

During the study, all participants received regular medical care. During the diet period, they measured daily morning and evening pre- and post-bronchodilator peak expiratory flow rates (PEF). Forced vital capacity (FVC) and forced expiratory volume in one second (FEV₁) were measured at baseline, at the end of the dieting period, after the 14-week study, and at six and 12 months. Participants recorded asthma symptoms using a visual analog scale and tracked their use of rescue bronchodilator medications. Serum cortisol and diurnal urine cortisol excretion concentrations, blood and urine concentrations of sodium, potassium, calcium and magnesium, triglycerides and cholesterol levels were measured at baseline, after dieting and after 14 weeks. Health status was monitored on four occasions during the year using a standardized respiratory questionnaire.

At the end of the diet period, patients in the treatment group had a mean weight loss of 14.2 kg (31.2 lb) or 14.5 percent of body weight, and at one year they maintained an average weight loss of 11.2 kg (24.6 lb) or 11.3 percent. In the control group, the mean weight loss after eight weeks was 0.3 kg (0.66 lb) and an average weight gain of 2.3 kg (5.1 lb) or 2.2 percent after one year. At the end of the dieting period, the treatment group showed a 7.2 percent significant improvement in predicted FEV₁ compared with the control group. The difference in FVC change between the two groups was also statistically significant. These differences in pulmonary function remained statistically significant after one year. Subjectively, participants in the treatment group reported a median reduction in dyspnea of 13 mm on the visual analog scale compared with 1 mm in subjects in the control group. In the treatment group, a reduction of 1.2 doses of daily rescue medication compared with 0.1 doses in the control group was reported. Participants in the treatment group showed significant improvement in health status, and this improvement was maintained at the one-year interval. Over the follow-up period, the number of exacerbations and the use of steroids were reduced in the treatment group compared with the control group.

The authors conclude that weight reduction in obese persons with asthma leads to significant improvement in pulmonary function, symptoms and health status.