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Meta-study

Obesity may protect against COPD

Chronic obstructive pulmonary disease, commonly caused by smoking, may be mitigated by overweight and obesity. Photo: Flickr/SuperFantastic
Obese and overweight had lower mortality rates after pulmonary disease diagnosis
Meta-analysis studied a total of 21,150 individuals
Misdiagnosis suggested as one possible confounding factor

The overweight and obese may be more resilient against chronic obstructive pulmonary disease (COPD) than those of a normal weight, according to a new meta-analysis study.

The paper, “Body Mass Index and Mortality in Chronic Obstructive Pulmonary Disease”, analysed the results of 22 studies of the disease, and found that higher BMIs were associated with lower mortality rates.

While lead investigator Chao Cao, of the Huazhong University of Science and Technology, Wuhan, China, stated that the pathophysiological basis for the phenomenon was “unknown”, he hypothesised that one possible confounding factor was obese patients being misdiagnosed with COPD.

Results

The 22 papers investigated a total of 21,150 individuals over periods ranging from 1-17 years, with 11 of the papers having a medium follow-up length over five years. The studies were analysed to investigate risk ratios for COPD among different BMI groups.

Cao et al found that overall, underweight patients were associated with a higher COPD mortality than normal weight patients (risk ratio 1.34, 95% confidence interval = 0.33-0.68, while overweight and obese patients had a lower mortality (RR 0.47, 95% CI 0.33-0.68 and RR 0.59, 95% CI 0.38-0.91, respectively).

The investigators stated that current indices to measure mortality risk for COPD included BMI as a variable, but in most – for example, the BODE index (BMI, Obstruction, Dyspnea, Exercise capacity) – BMI is valued at either 0 or 1, and the inflection point is 21 kg/m2. They suggested that their research showed that the BMI category associated with the lower risk of mortality was 25 or more, and that therefore a new scoring system taking this into account might be a more effective predictor.

Although the reason for the lower observed mortality rate was not established, Cao et al said that one study had hypothesised that obesity in itself contributes to low forced expiration volume tests, which may lead to some patients being classified as having severe COPD despite not having a severe decline in lung function.

Cao et al also said that the data was insufficient to assess the relationship between BMI and mortality in different classes of obesity, and that there may be a “ceiling BMI”, beyond which the lower mortality is no longer observed.

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