The prevalence of asthma in adults in Australia is about 10%; obesity about 20%. We know that long-term oral steroid therapy often causes weight gain, sometimes obesity. Is the relationship between asthma and obesity bidirectional?
Asthma is more common in obese individuals. Weight loss has a net positive effect on lung function and asthma symptoms. Moreover, asthma is generally more difficult to control in obese individuals. It is more symptomatic for a greater percentage of time, and results in more reliever therapy usage. Obese individuals with asthma have more frequent symptoms when they gain weight; and obesity increases the odds of adults developing asthma for the first time.
Obesity does not cause airflow limitation or eosinophilia, but may increase bronchial reactivity. Most of the association is at the milder end of asthma severity spectrum. The reason(s) are not clear, however, given the phenotypic variation of both conditions, it is not surprising that there would be inconsistent results in research into the association.
There are a number of possible mechanisms to account for the increased prevalence and severity of asthma in obese individuals. Obesity is a chronic low-grade pro-inflammatory state. Potentially, systemic inflammation may affect airway inflammation, through pro-inflammatory mediators such as leptin (evidence limited).
Obesity has direct mechanical effects on lung volumes – the most common functional abnormality is reduced functional residual capacity (FRC). However, in some cases, ventilatory capacity may be reduced (i.e. a heterogeneous relationship between BMI and lung volumes).
The effect of obesity on asthma may be mediated by other co-morbidities such as gastro-oesophageal reflux and obstructive sleep apnoea (both are more common in obese individuals). The association between gastro-oesophageal reflux and asthma is only partly related to BMI. Some studies show that PPI treatment for patients with both asthma and gastro-oesophageal reflux had no effect on the frequency of asthma symptoms, but reduced asthma exacerbations and improved quality of life.
The presence of obstructive sleep apnoea (OSA) in asthmatics is associated with poorer asthma control and over 80% of severe asthmatics have at least moderate OSA. The relationship between OSA and asthma is often independent of BMI. CPAP therapy usually improves asthma symptoms in obese individuals with OSA, however, positive airway pressure not only supports a collapsible pharynx, but also intrapulmonary airways.
There are more questions than answers in the relationship between obesity and asthma, however, the association appears real.
Therefore, it is important to exercise caution in the use of high-dose intermittent or medium-long term oral steroids in managing asthma, in order to prevent weight gain. Weight reduction in obese asthmatics and management of co-morbidities is likely to lead to better asthma control and improved quality-of-life. While obesity probably reduces the effectiveness of asthma therapy, you should reconsider the diagnosis if asthma symptoms do not respond to inhaled or oral steroids.
- There is an association between asthma and obesity
- The mechanism may be via co-morbidities
- Weight loss can improve asthma control
References available on request.
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Author competing interests: nil relevant disclosures.
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