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(Chest. 1999;115:371-375.)
© 1999 American College of Chest Physicians

Body Composition Analysis by Dual Energy X-ray Absorptiometry and Exercise Performance in Underweight Patients With COPD*

Masanori Yoshikawa, MD; Takahiro Yoneda, MD; Atsushi Kobayashi, MD; Akihiro Fu, MD; Hideaki Takenaka, MD; Nobuhiro Narita, MD and Kunimoto Nezu, MD, FCCP

* From the Second Department of Internal Medicine and Department of Surgery III, Nara Medical University, Nara, Japan.


   Abstract
TOP
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 References
 
Study objectives: The aim of this study was to examine the effect of body composition on maximal exercise performance in patients with COPD.

Methods: The study was carried out on 27 patients with COPD and was confirmed by pulmonary function testing. Body composition was measured by dual energy x-ray absorptiometry (DXA). Exercise performance was conducted on a cycle ergometer and was measured as maximal work rate (WRmax) and maximal oxygen uptake (O2max). Bone mineral content (BMC), lean mass (LEAN), and fat mass (FAT) were assessed by DXA and were expressed as a percentage of ideal body weight, BMC, LEAN, and FAT.

Results: LEAN% correlated significantly with O2max (r = 0.66, p = 0.0002) and WRmax (r = 0.70, p < 0.0001). No significant correlation was found between FAT% and exercise performance. By stepwise regression analysis, variables significantly contributing to WRmax and O2max were LEAN% and the maximal voluntary ventilation. Total variance explained in these models was 81% for WRmax and 82% for O2max.

Conclusion: Lean mass was an important determinant of maximal exercise performance in patients with COPD.

Key Words: body composition analysis • COPD • dual energy x-ray absorptiometry • exercise performance • lean body mass • malnutrition


   Introduction
TOP
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 References
 
Exercise limitation is a common manifestation of COPD. Although exercise limitation has generally been due to airflow limitation, several studies have demonstrated that exercise limitation could not be predicted accurately by resting pulmonary function, including FEV1 and the diffusing capacity of the lung for carbon monoxide (DLCO).1 ,2 ,3

Malnutrition is a common problem in patients with COPD, and a significant percentage of patients were reported to be underweight.4 ,5 Although weight loss has been reported to be associated with decreased lung function5 ,6 ,7 and respiratory muscle strength,5 ,8 its impact on exercise performance has not been clearly demonstrated. Some studies have shown that submaximal exercise performance has no association with body weight as a percentage of ideal weight in patients with COPD.9 ,10 ,11 Lean body mass (LEAN), which represents skeletal muscle mass, has been reported to decrease in malnourished patients with COPD,4 ,5 ,7 although the precise mechanism for the reduction has not been identified.12 Schols et al13 have reported significant correlation between fat-free mass (FFM), evaluated by bioelectrical impedance analysis, and the distance walked in 12 min. However, anthropometric measurements and bioelectrical impedance analysis were reported to overestimate FFM.14 ,15 Furthermore, the relation between maximal exercise performance and body composition in patients with COPD has not been described.

Newly developed dual-energy x-ray absorptiometry (DXA) makes it possible to analyze body composition, bone mineral content (BMC), LEAN, and fat mass (FAT) in an accurate and reproducible fashion with very low radiation exposure.16 ,17

The aims of this study were to measure the body composition by DXA, to explore the relationship between these body composition measures and maximal exercise performance, and to evaluate whether they can be the predictors of maximal exercise performance, using multiple regression analysis in patients with stable COPD.


   Materials and Methods
TOP
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 References
 
Subjects
Twenty-seven ambulatory male patients with COPD were recruited to participate in this study. The criteria for diagnosis of COPD were based on the standards of the American Thoracic Society.18 The entry criteria included the following: (1) clinical diagnosis of COPD; (2) clinically stable condition (no recent infection or cardiac complaints); (3) absence of other pathologic conditions, including neuromuscular, metabolic, and malignant diseases; and (4) not receiving oral corticosteroids. All patients had a history of cigarette smoking and evidence of COPD by spirometry (an FEV1 < 80% predicted with a reduction of the FEV1/FVC ratio to < 70%).

The experimental protocol was approved by the Committee for Protection of Human Subjects, Nara Medical University, according to the Declaration of Helsinki. All the subjects gave their informed consent prior to the initiation of the study.

Pulmonary Function Tests
All patients underwent pulmonary function testing. Vital capacity, FVC, FEV1, maximal voluntary ventilation (MVV), residual volume (RV), and total lung capacity (TLC) were measured using a pulmonary function instrument with computer processing (FUDAC 50; Fukuda Denshi; Tokyo, Japan), and the ratio of FEV1/FVC was calculated. The values obtained were compared to the normal values of Berglund et al.19 RV and TLC were determined by the helium gas dilution method, and DLCO was measured by the single-breath method.

Body Composition Analysis
Body composition was measured by DXA with a total body scanner (Lunar DPX; Lunar Radiation Corp; Madison, WI) that uses a constant-potential x-ray generator and a K-edge filter (cerium) to separate the beam into high- and low-energy regions. The attenuated x-rays that passed through the subjects were measured with an energy-discriminating detector. The differential attenuation of the two energies was used to estimate the BMC and soft-tissue mass. BMC and soft-tissue mass, partitioned into FAT and LEAN, were calculated separately based on the difference in mass attenuation coefficients. Patients were in a supine position on a pad as they were scanned in a rectilinear manner from head to toe. The entire analysis was usually completed within 15 min. Each body composition value was expressed as a percentage of ideal body weight, BMC (BMC%), FAT (FAT%), and LEAN (LEAN%), because no ideal values were available.

Exercise Performance
All patients underwent maximal exercise tests on a cycle ergometer (STB 1350; Nihon Kohden; Tokyo, Japan). After 1 min of unloaded pedalling, the workload was increased by 10 W every minute in a ramp protocol until exhaustion. Gas exchange was monitored during the exercise test with a computerized metabolic cart (MMC Horizon System 4400tc; SensorMedics Corp; Yorba Linda, CA). Minute ventilation, oxygen uptake, and carbon dioxide output were measured by the breath-by-breath method.

Statistical Analysis
Values obtained were expressed as the mean ± SD. The relationships between static lung function, body composition, and exercise performance were analyzed by linear regression analysis. Stepwise multiple regression analysis was used to determine the best predictors of maximal exercise capacity, maximal oxygen uptake (O2max) and maximal work rate (WRmax) among selected independent variables. The level of statistical significance for each test was set as p < 0.05.


   Results
TOP
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 References
 
Anthropometric and Pulmonary Function Data
Anthropometric characteristics and static pulmonary function data of the patients are described in Table 1 . Body weight was expressed as an absolute value and as a percentage of ideal body weight (%IBW).20 Nineteen of 25 patients (76%) were underweight (%IBW < 90), and no patients with %IBW > 100% were included. Patients had a wide range of airflow limitations, including reduced diffusing capacity and mild to moderate hyperinflation.


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  		Table 1. Anthropometric and Pulmonary Function Data of Patients with COPD*

 
Body Composition
The DXA values for body composition are shown in Table 2 .


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  		Table 2. Body Composition in Patients with COPD*

 
Exercise Performance
Maximal exercise capacity on the cycle ergometer test is summarized in Table 3 . O2max was expressed as an absolute value and as a percentage of the predicted value.21


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  		Table 3. Physiologic Variables During Maximal Exercise on Cycle Ergometry*

 
The relationship between LEAN% and exercise performance is shown in Figure 1 . LEAN% correlated significantly with O2max and WRmax (p = 0.0002 and p < 0.0001, respectively). No correlation, however, existed between FAT% and exercise performance.



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  		Figure 1. The effects of LEAN% on O2max and WRmax.

 
Determinants of Exercise Performance
Correlation coefficients between exercise performance, pulmonary function, and body composition obtained from linear regression analysis are shown in Table 4 . WRmax correlated significantly with FEV1 (p < 0.01), DLCO (p < 0.05), RV/TLC (p < 0.01), MVV (p < 0.01), and LEAN% (p < 0.01). O2max also correlated significantly with FEV1 (p < 0.01), DLCO (p < 0.01), RV/TLC (p < 0.01), MVV (p < 0.01), and LEAN% (p < 0.01).


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  		Table 4. Relationship Between Exercise Performance and Pulmonary Function and Body Composition in a Linear Regression Analysis*

 
Stepwise multiple regression analysis was performed to determine the best combination of predictors for WRmax and O2max (Table 5 ). Although FEV1 correlates closely with MVV, MVV was selected as an independent variable because it accounted for 71% and 74% of the variance in WRmax and O2max, respectively, and, thus was the best single predictor of WRmax and O2max. MVV and LEAN% appeared to be significant determinants of WRmax and O2max. The best regression equation to predict WRmax was:

and also to predict O2max was:

The total variance explained in these models was 81% for WRmax and 82% for O2max. The standardized partial regression coefficient of MVV was larger than that of LEAN%, and that of LEAN% was similar in the models for WRmax and O2max.


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  		Table 5. Results of Stepwise Multiple Regression Analysis for WRmax and O2max*

 

   Discussion
TOP
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
References
 
Body weight loss is a common finding in patients with severe COPD and has been reported to be associated with impairment of pulmonary function,5 ,6 ,7 reduction of respiratory muscle strength,5 ,8 and poor prognosis.6 Although several studies have examined whether nutritional impairment contributes to reduced exercise performance in COPD patients,6 ,9 ,10 ,11 ,12 the effect of body composition on exercise performance has not been clearly demonstrated. No difference was found in submaximal exercise performance, as measured by the 6-min walking test, between underweight (%IBW < 90) and normal weight patients in two studies.10 ,11 By contrast, a positive association between maximal exercise performance and %IBW has been described previously.6 ,10 ,22 %IBW correlated significantly with O2max, as evaluated by incremental cycle ergometry,10 ,22 and maximal workload was significantly decreased in underweight patients.6

LEAN, which is mostly muscle, has been reported to be reduced in malnourished patients with COPD.4 ,5 ,7 Muscle wasting can affect muscle function, including pulmonary function5 and respiratory muscle strength.5 ,23 ,24 Thus, it is essential to evaluate body composition as well as body weight in patients with COPD.

In the present study, it was clearly documented that LEAN correlated significantly with maximal exercise performance. Thurlbeck25 demonstrated, using autopsy specimens, that diaphragm weight was proportional to body weight. Similarly, Arora and Rochester26 found that alterations in body weight and muscularity profoundly affect diaphragm muscle mass. In two animal studies,27 ,28 nutritional depletion has been associated with a reduction in diaphragmatic muscle mass as a result of decrease in the cross-sectional area of muscle fibers. The depletion is greater for fast fibers than for slow fibers. These data may explain in part the correlation between LEAN and O2max.

The reduction of exercising muscle mass could provide another explanation for the correlation of LEAN with O2max. Lower limb fatigue is an important factor contributing to exercise intolerance in deconditioned COPD patients29 and is due to a variable degree of muscle atrophy resulting in poor muscle strength and endurance. Furthermore, O2max correlated significantly with isometric quadriceps force despite the lack of association with PImax.30 We found that LEAN correlated closely not only with O2max, but also with WRmax.

We found no association between FAT and maximal exercise performance. The patients studied who were underweight or normal weight appeared to have low or normal FAT. In the present study, only a modest range of FAT was observed. To clarify the effect of FAT on exercise performance, obese patients with COPD should be investigated. In addition, it is unclear whether the gender difference in the effect of body composition on exercise performance may be found, because female patients were not included in our subjects.

In the present study, stepwise multiple regression analysis was performed to predict O2max and WRmax in terms of resting pulmonary function and LEAN. We found that the MVV and LEAN were strong predictors of maximal exercise performance in patients with COPD. Several published studies1 ,2 ,3 ,31 have used multiple regression techniques to examine how different combinations of physiologic variables can predict O2max and maximum workload. However, an analysis that includes LEAN has not previously been reported. Schols and coworkers13 have previously reported good correlation between FFM measured by bioelectrical impedance analysis and the 12-min walking distance. They also described how FFM, independent of airflow obstruction, was an important determinant of exercise performance in patients with severe COPD. Although our result appeared to be in accordance with their study, independent variables in stepwise regression analysis in their paper did not include values for lung volumes, gas exchange capacity, and ventilatory capacity. Furthermore, the determinants of maximal exercise performance are not necessarily the same as those of submaximal exercise.30

We conclude that LEAN, measured by DXA, is an important determinant of maximal exercise performance in patients with COPD.


   Footnotes
 
Correspondence to: Masanori Yoshikawa, MD, Second Department of Internal Medicine, Nara Medical University, 840 Shijo-cho, Kashihara, Nara, Japan 634-0813

Abbreviations: BMC = bone mineral content; BMC% = percentage of BMC; DLCO = diffusing capacity of the lung for carbon monoxide; DXA = dual energy x-ray absorptiometry; FAT = fat mass; FAT% = percentage of FAT; FFM = fat-free mass; %IBW = percentage of ideal body weight; LEAN = lean mass; LEAN% = percentage of LEAN; MVV = maximal voluntary ventilation; RV = residual volume; TLC = total lung capacity; O2max = maximal oxygen uptake; WRmax = maximal work rate

Received for publication February 17, 1998. Accepted for publication August 26, 1998.


   References
TOP
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 References
 

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