Asthma and obesity

I am enjoying having opportunity this week to catch up on some of the talks from the American Thoracic Society conference, which was held in New Orleans just over two weeks ago. I’m impressed that they are available online already.

One symposium looked at the interaction between obesity and respiratory / critical care medicine. I’ll probably blog more about this as I work through the talks.

Dr Stephanie Shore, PhD, from the Harvard School of Public Health, gave an excellent lecture on obesity and asthma at that session. The following is a summary of what I got out of that talk.

Firstly, we do know that asthma and obesity interact in the following ways:
- there is an increased prevalence of asthma amongst obese individuals;
- as people become more obese, the likelihood of new asthma developing increases;
- obesity leads to reduced control of asthma;
- obesity leads to increased severity of asthma;
- weight loss helps improve asthma control;
- and obese mice…..

What have mice got to do with it? Well, many data relevant to human health were initially derived from mice, so it turns out that obese mice have something to teach us. And obese mice have a tendency to develop asthma. But more of that later.

Why is there more asthma amongst people who are obese? Is this a new sort of asthma? Does obesity give some people asthma? Or is it the same old sort of asthma, with obese individuals simply predisposed.

It looks more like the former suggestion – ie obese individuals with asthma have a particular sort of asthma. Obese individuals with asthma have less eosinophils in bronchoalveolar lavage fluid (fluid that is washed through the smallest airways at bronchoscopy). They also are less sensitive to the impact of corticosteroid. All of this suggests that they do not have ‘allergic asthma’ of the sort that occurs almost universally in children with asthma, but less frequently in older asthmatics.

What’s going on there? There is ‘more thought than data’ when it comes to how obesity may contribute to asthma development. The ideas include the suggestions that there may be:
1. Common aetiologies. The same processes that contribute to the development of obesity may contribute to the development of asthma. These may be congenital / in-utero issues; dietary factors; genetic abnormalities or acquired exposures
2. Co-morbid problems. People with obesity suffer from more lipid abnormalities, gastro-oesophargeal reflux, obstructive sleep apnoea, diabetes and hypertension. Do these associated problems contribute to the development of asthma. GORD and OSA have been shown to. The others, maybe.
3. Obesity impacts on lung mechanics, leading to reduced functional residual capacity and reduced tidal volume. Either of these mechanisms may lead to airway hyperresponsiveness – and asthma.
4. Systemic inflammation or other systemic consequences of obesity may contribute to development of asthma.

With regards to this last issue, in particular, Dr Shore embarked on a little tutorial on adiponectin. I didn’t know too much about this hormone, so was helped by the tute. Like leptin, adiponectin is produced in fat cells (adipocytes) and circulates in several different forms in the blood stream. Its levels are reduced in obesity. It sensitizes to the effect of insulin (so low levels contribute to insulin resistance) and it has anti-inflammatory properties (so low levels in obesity can lead to more inflammation). Dr Shore’s lab had researched the impact of this compound in the lungs of mice. In mice given adiponectin, allergen –induced airway hyperresponsiveness (AHR) and inflammation is reduced. In adiponectin deficient mice there is increased allergen induced AHR and inflammation. Transgenic mice that overexpress adiponectin demonstrate reduced airway inflammation. The suggestion (idea rather than data) is that adiponectin may be a mediator of airway inflammation in people who are obese – not just in mice – contributing to the development of asthma.

The important clinical information for now is that weight loss helps. There have been, in the last 10 years, 10 studies looking at patients with bariatric-surgery – induced weight loss and 4 looking at patients with dietary weight loss, with regards to the impact of weight loss on asthma control. They have demonstrated:
- improved lung function parameters;
- reduced medication use;
- reduced symptoms;
- improved asthma control;
- improved asthma-related quality of life;
- reduced severity of asthma and reduced frequency of hopitalisation ….

….as a consequence of weight loss in people with asthma.

So, there is evidence that obese patients with poorly controlled asthma should be encouraged to lose weight. It will help bring their asthma under control.

Andrew