Obesity is a major cause of morbidity that has been increasing in prevalence over the past few decades. The parallel trends in the increase of asthma and obesity may indicate a potential link between the conditions. There is increasing evidence relating body-mass index to the prevalence and incidence of asthma in both children and adults, although most consistently in adolescent girls. It is unlikely that the association is attributable to reverse causation — that is, that people with asthma exercise less because exercise induces symptoms of asthma. Rather, weight gain can antedate the development of asthma. Weight loss by patients with asthma tends to improve lung function. Potential explanations for the association of obesity and asthma are that the development of both conditions is determined in early life, that mechanical factors promote symptoms of asthma, or that gastroesophageal reflux resulting from obesity induces asthma. Physical inactivity may promote both obesity and asthma. A British study found that trends in overweight and obesity did not explain the increase in the incidence of asthma from 1982 to 1994. The association between asthma and obesity may there- fore be of recent origin, suggesting that recent changes in lifestyle and diet are now associated with both asthma and overweight. Studies have examined dietary intakes of fruits, vegetables, cereals and starches, various fatty acids, vitamin A, vitamin C, vitamin E, minerals (sodium, magnesium, copper, zinc, and selenium), and antioxidants for possible associations with asthma. Diet is complex and difficult to measure, and standardized tools are still lacking. Variation in methods of determining the frequency of intake, which individual foods are consumed, eating habits, and serum nutrient levels can introduce substantial misclassification, and the close correlations among the intakes of different nutrients make it difficult to identify independent effects. In cross-sectional surveys, a wide range of nutrients appear to have an effect on asthma outcomes. The evidence from prospective studies and randomized clinical trials, however, is far less consistent and conclusive. Maternal nutrition during pregnancy may have a role, but data on this subject remain scarce. Intervention studies promoting avoidance of cow’s milk and eggs during pregnancy have failed to protect the infants from asthma, and breast-feeding also does not protect children from asthma. Recent studies showing a positive association between breast-feeding and asthma84 may reflect adherence to recommendations for children at risk for asthma rather than breastfeeding’s being a causal factor for asthma.