Obesity is a major risk factor for asthma, but the mechanisms for the development of asthma in the setting of obesity are not known. The purpose of this article is to review the effects of obesity on airway inflammation in patients with asthma, and to discuss the effects of obesity on airway reactivity in patients with asthma.

Obesity is particularly a risk factor for non-atopic asthma. Airway eosinophilic inflammation is not increased in obesity, in fact the preponderance of the evidence suggests that airway eosinophilia is decreased in obesity. There is some preliminary data suggesting that airway neutrophilia may be increased in obesity, and that this may be particularly related to dietary fats. Obesity also alters adaptive immunity, and may suppress lymphocyte function typically associated with asthmatic airway inflammation.

Population based studies are somewhat inconsistent on the relationship between airway reactivity and asthma, however, recent studies in bariatric surgery show that weight loss surgery in severely obese patients decreases airway reactivity. One study suggested that this was particularly the case for those with low IgE (a marker of a low T_H2 asthma phenotype), suggesting there may be some heterogeneity in asthma in obesity.

There are likely to be two phenotypes of asthma in the obese: one group with early onset disease and asthma complicated by obesity, and a 2nd group with late onset disease with asthma consequent to obesity. Obesity leads to profound changes in airway function, and adaptive and innate immune responses which alter the nature of pre-existing allergic airway disease, and also cause new onset asthmatic disease.