Background
Chronic obstructive pulmonary disease (COPD) is associated with dyspnea and exercise intolerance, two major impediments to quality of life. Although low body weight[
1] and muscle wasting[
2] have traditionally been the focus of nutritional management in COPD, recent data indicate that obesity is becoming frequent in this disease[
3]. On one hand, a high body mass index (BMI) appears to convey a survival advantage to patients with COPD[
1,
4]. On the other hand, obesity by itself may compromise lung function[
5], decrease exercise tolerance particularly during weight bearing activities[
6,
7], and quality of life[
8], leading to greater disability[
9,
10].
The effects of obesity in combination with COPD on exercise tolerance and dyspnea have received little attention. In one study, obese patients with COPD had a greater peak exercise capacity and reduced dyspnea perception at a standardized ventilation during incremental cycling exercise compared to their lean counterparts[
11]. These counterintuitive beneficial effects of obesity were felt to be related to reduced operating lung volumes during exercise in the obese individuals. Other studies showed that the 6-min walking distance [
3], but not constant exercise cycling test time[
12], was reduced in obese patients with COPD compared to non-obese patients highlighting the importance of taking into account the exercise testing modality before concluding about the impact of obesity on exercise capacity in COPD. Whether overweight may also influence exercise capacity in COPD has not yet been addressed.
Pulmonary rehabilitation addresses the systemic consequences of COPD, beyond the impairment in lung function. As summarized in a recent meta-analysis[
13] pulmonary rehabilitation improves dyspnea, exercise tolerance and quality of life. Because of the growing prevalence of weight excess in COPD[
14], it is important to learn about the impact of overweight and obesity on pulmonary rehabilitation. A retrospective study[
3] showed that obesity did not adversely affect rehabilitation outcomes, although data obtained prospectively would be useful to confirm these findings.
Based on the existing data suggesting that overweight and obesity may interact with COPD, our hypothesis was that increasing BMI in COPD would reduce exercise tolerance, increase exertional dyspnea and reduce functional status during walking but not cycling and compromise the response to pulmonary rehabilitation in patients with COPD. This study was thus undertaken to investigate the effects of overweight and obesity combined with COPD on 1) resting pulmonary function; 2) 6-min walking distance and endurance time during a constant workrate cycling exercise test (CET time) 3) health-related quality of life and 4) improvement of these parameters following pulmonary rehabilitation. To address these issues, we took advantage of a prospective cohort of patients with COPD entering pulmonary rehabilitation in Canada.
Discussion
This study reports on the impact of obesity and overweight in a large prospective cohort of patients with COPD participating in pulmonary rehabilitation. The results can be summarized as follow: i) obese and overweight patients had higher FEV1, lower static lung volumes and higher peak incremental exercise capacity at baseline, ii) despite this, their CET time was not longer than that of patients with normal BMI, iii) obese patients had a reduced walking capacity compared to overweight and normal BMI patients, iv) BMI did not seem to affect SGRQ scores in the present population, finally v) overweight and obesity did not reduce the magnitude of improvement in exercise capacity and quality of life after pulmonary rehabilitation and BMI had no effect on outcomes on univariate or multivariate regression analyses taking account differences in baseline pulmonary function.
It is interesting to observe that, in this cohort, the proportion of overweight and obese patients was greater than normal BMI patients, a likely reflection of the obesity epidemic that afflicts industrialized countries [
32,
33]. These results underscore that the study of the impact of obesity and overweight in patients with chronic respiratory disorders will be a topic of interest in the coming years.
Obese and overweight patients had higher FEV
1 and FEV
1/FVC ratio than their lean counterparts, an observation that was previously reported [
3,
4,
34,
35]. One possible explanation is that patients with weight excess tend to be more dyspneic for a given FEV
1[
36] as illustrated by the higher Borg dyspnea and leg fatigue scores found in the obese patients during 6MWT. Therefore, obese patients with COPD might attract medical attention at an earlier stage of their disease. It is intriguing to consider that obesity may influence the natural history of COPD; in a subanalysis of the TORCH trial[
37], BMI >25 kg/m
2 was associated with a slower decline in FEV
1. Another possibility for the differences in baseline lung function relates to the influence of obesity on ventilatory function. Decreased chest wall and lung compliance in obesity[
11] would tend to increase expiratory flows and decrease resting lung volumes.
At baseline, resting hyperinflation was reduced and the IC/TLC ratio increased in the obese population. This finding is consistent with those of Ora et al.[
11]. One novel finding of our study is that overweight was also associated with reduced lung volumes in comparison with patients with normal BMI.
We found that obesity had a significant impact on walking capacity but not on the endurance time during cycling exercise. This is likely the result of the increase in energy expenditure associated with weight bearing exercise[
38] as shown by higher body weight-walking distance product[
25]. From a functional point of view, walking better represents daily activities than cycling. Taken together, these data suggest that obese COPD patients might have more important functional impairments. It would be interesting to study the impact of weight reduction strategies on walking capacity in obese patients with COPD.
As indicated by similar SGRQ total scores, there was no difference in health status between groups, both at baseline and post-rehabilitation, even though patients in obese group had a more limited walking capacity. This could be related to the fact that obese patients might compensate by adapting their environment and diminishing the amount of activity they perform.
In a retrospective analysis, Ramachandran and colleagues[
3] reported that the improvement in 6-min walking distance and quality of life improved to a similar extent after rehabiliation in obese patients with COPD when compared with patients with a BMI < 30 mg/kg
2. One limitation of that study is that it did not include overweight patients. This appears to be relevant given that overweight is even more common than obesity. Our prospective study therefore adds to this information in showing that dyspnea, quality of life and exercise tolerance improve as much in the obese and overweight COPD patients as their normal BMI counterparts. BODE scores improved significantly in our population within each group to an extent that is consistent with the literature[
39].
We did not observe significant reductions in BMI after pulmonary rehabilitation. Exercise in itself is usually not sufficient to adequately manage obesity[
40] and it should be done in conjunction with nutritional counseling which was not offered here. In the future, it will be important to learn how to intervene efficiently with COPD patients in their goal of loosing excess fat.
This study provides some novel information. First, it is, to our knowledge, the only study looking prospectively at the effect of BMI on pulmonary rehabilitation outcomes. We also report on improvements in terms of MCID for 3 different outcomes, the 6 MWT, CET and SGRQ. Although it is generally suspected that walking capacity is compromised in obese COPD patients, this study is the first to systematically investigate the impact of obesity on specific exercise modalities. Finally, the number of patients enrolled in our trial also provides sufficient statistical power to make valid conclusions.
The impact of comorbid conditions on rehabilitation outcomes is currently being investigated [
41,
42]. In general, it is felt that comorbidities do not prevent pulmonary rehabilitation from being effective although some conditions such as metabolic diseases and osteoporosis may reduce the chances of success [
41,
42]. The present study extends these results by showing that obesity reduces the likelihood of a patient achieving the MCID of improvement in distance walked during the 6MWT after rehabilitation. In contrast, the proportion of patients reaching the MCID for cycle exercise and SGRQ was not influenced by BMI. The proportion of our patients reaching the MCID for the SGRQ is similar to what has been reported [
41]. Although we did not record the amount of aerobic and resistance training that was performed during rehabilitation, BMI was not a factor in the choice of the training strategies and modalities (home versus hospital-based) used in the three groups. It is thus unlikely that intrinsic differences in the design of the training programs were the main factors in explaining the lower proportion of obese patients reaching the MCID for the 6MWT.
Although the improvement in 6MWT following pulmonary rehabilitation was less than typically reported[
41,
43], we felt that the CET data was reassuring about the exercise enhancing effects of our rehabilitation programs. Measuring the cycling endurance time is a better test of the functional effect of pulmonary rehabilitation than the 6MWT[
16]. The modest gain in the distance covered during the 6MWT probably reflects our program's emphasis on the bicycling component of the training intervention since the training modality is known to impact on specific outcomes. For example, patients training solely on stationary bicycle have less improvement in walking capacity compared to patients performing walking exercises[
44].
Our study has potential limitations. First there were only 3% of morbidly obese patients (BMI >40 kg/m
2), it would be important to see how such patients fare in the context of pulmonary rehabilitation. Secondly, it is well recognized that reduced fat free mass is associated with muscle weakness[
45], decreased exercise tolerance[
46] and poorer survival[
47,
48] in COPD. Reduced fat-free mass may occur despite normal BMI[
49]. Thirdly, the CET and 6MWT dyspnea and leg fatigue Borg scores were collected only at the end of exercise. Dyspnea and leg fatigue scores obtained at isotime would have been useful to assess the effects of pulmonary rehabilitation on these variables in a more complete fashion. Finally, the impairment in baseline FEV
1 was greater in patients normal BMI and the question may be raised as to whether this difference in disease stage between groups could explain our results. We do not believe that this is the case for the following reasons:
i) the main analysis consisted in an analysis of covariance that took into account any differences in FEV
1 and lung volumes at baseline;
ii) since obese patients had milder airflow obstruction and resting hyperinflation, their performance during the 6MWT should have been better, not worse; and
iii) the magnitude of improvement following pulmonary rehabilitation is independent from baseline lung function[
50].
Competing interests
L. Laviolette is recipient of a research training award from the Centre de Recherche de l'Institut Universitaire de Cardiologie et de Pneumologie de Québec. J. Bourbeau is the recipient of a John R. & Clara Fraser Memorial Award from the faculty of Medicine, McGill University. This work was supported by the Respiratory Health Network of the FRSQ and by a grant from GlaxoSmithKline, Canada. F. Maltais holds a GSK/CIHR Research Chair on COPD at Université Laval.
Authors' contributions
FS and LL carried out the analysis of the database and drafted the manuscript. SB and MJB recruited the patients, performed the exercise tests and the questionnaires and help revising the draft. JB and FM conceived of the study, and participated in its design and coordination and helped to draft the manuscript. All authors read and approved the final manuscript.