Studies that have reported an association between asthma and obesity have mostly been single cross sectional studies, although an increased incidence of asthma in participants who were overweight at baseline has been reported in several longitudinal studies. The latter have contributed to the debate about the nature of the association, showing that the reverse causality explanation—that decreased physical activity as a result of asthma leads to increased weight—is not plausible. Cohort studies have the potential to determine whether increased symptoms of asthma are an immediate consequence of weight gain or, if not, to establish the lag, but frequent follow up would be required. However, pure cohort studies cannot answer the question of how much of the increase in the prevalence of asthma in a population can be explained by an increase in obesity, as age and period are completely confounded. Repeated cross sectional studies in the same population are needed to answer this question.

POPULATION SURVEYS

Some countries have regular cross sectional population health surveys, such as the National Health and Nutrition Examination Surveys (NHANES) in the United States and the Health Survey for England. Such surveys routinely include height and weight, as increased body mass index (BMI) in adults is a risk factor for a number of diseases, but the Health Survey for England has included respiratory symptoms only in selected surveys. Increases in asthma and in overweight (BMI 25–<30 kg/m²) or obesity (BMI ⩾30 kg/m²) have been reported many times over the last 20 years, but rarely in the same study. This …