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1 From the Departments of Respiratory Medicine (MPKJE, EFMW, and AMWJS) and of Surgery (NEPD), Maastricht University, Maastricht, Netherlands, and the Asthma Center Hornerheide, Horn, Netherlands (RM).
2 Supported by a research fellowship from the European Society of Parenteral and Enteral Nutrition and a University Hospital Maastricht grant. 3 Address reprint requests to MPKJ Engelen, Department of Respiratory Medicine, University Hospital Maastricht, PO Box 5800, 6202 AZ Maastricht, Netherlands. E-mail: m.engelen{at}pul.unimaas.nl.
ABSTRACT
Background: Exercise is known to improve physical capacity and muscle mass in patients with chronic obstructive pulmonary disease (COPD). However, recent evidence suggests that exercise may also negatively influence metabolism in COPD.
Objective: The objective was to investigate whether exercise influences whole-body protein metabolism differently in COPD patients and control subjects and to elucidate the effect of the specific underlying lung disease.
Design: Whole-body protein synthesis and breakdown and urea synthesis were measured by using stable-isotope methods in 14 male patients with severe COPD (forced expiratory volume in 1 s: 37 ± 12% of predicted) and in 8 male control subjects during and after 20 min of exercise. Subjects were normal weight [body mass index (in kg/m2) of COPD patients and control subjects: 25.8 ± 3.9 and 25.7 ± 4.4, respectively]. The COPD group was selected to include patients with (Emph+, n = 7) and without (Emph-, n = 7) emphysema. Absolute workload was 35 ± 5 W, corresponding to 17 ± 2%, 33 ± 9%, and 52 ± 14% of the maximal obtained workload in the control, Emph-, and Emph+ groups.
Results: Exercise induced a 9% increase in protein synthesis and breakdown in the Emph- and control groups, which normalized postexercise. In the Emph+ group, protein turnover did not change significantly during exercise but decreased postexercise (± 10%). Exercise did not change net protein breakdown (protein breakdown - synthesis) or urea synthesis, except in the Emph+ group, which showed a 14% reduction in urea synthesis postexercise (P < 0.05).
Conclusion: Low-intensity exercise suppresses whole-body protein and urea turnover in COPD patients with emphysema and needs to be considered when maximal anabolism is targeted through a combination of exercise and nutrition.
Key Words: WORDSExercise • chronic obstructive pulmonary disease • COPD • protein metabolism • urea synthesis • stable isotopes • men
INTRODUCTION
It is generally accepted that physical exercise is an important element in the pulmonary rehabilitation of patients with chronic obstructive pulmonary disease (COPD). Exercise training, often combined with nutritional support, has been shown to increase body weight in patients with COPD and to result in significant increases in fat-free mass, muscle function, and exercise capacity (1, 2). However, recent evidence suggests that exercise may also have negative metabolic consequences in COPD. Low-intensity, constant cycle exercise induced significant alterations in plasma and skeletal muscle amino acid concentrations in COPD patients in whom fat-free mass was preserved (3), suggesting that exercise may be an important (protein) metabolic stressor even in patients with normal nutritional status.
Severe intrinsic alterations have been found in the skeletal muscle of COPD patients at rest. In COPD patients with emphysema, elevated concentrations of inosine monophosphate were found (4), suggesting an imbalance between ATP resynthesis and utilization. Moreover, a low proportion of type I fibers was found in emphysema patients (5), which is illustrative of a severely reduced oxidative capacity. Exercise in emphysema is often accompanied by accelerated anaerobic metabolism (6) and by arterial hypoxemia (7). All these factors may directly or indirectly influence protein metabolism through alterations in substrate metabolism.
The hypothesis of the present study was that physical exercise influences protein metabolism differently in COPD patients, particularly those with emphysema, than in healthy control subjects. Earlier studies in healthy subjects showed that protein metabolic changes often take place during recovery from exercise (8). This suggests that besides the actual exercise period, the postexercise (recovery) response to exercise is important to the overall state of protein metabolism in COPD. Studies of the effects of exercise on whole-body protein metabolism in patients with COPD may be clinically relevant because they indirectly provide information on the protein requirements of these patients in daily life and during pulmonary rehabilitation. If a catabolic effect of exercise is present as a result of altered substrate requirements, nutritional supplementation may be warranted to reach the expected anabolic effect of exercise in COPD patients.
In the present study, the metabolic effects of exercise in COPD were investigated by examining whole-body protein synthesis and breakdown in COPD patients and in healthy control subjects. Because urea is the predominant end product for the excretion of the nitrogen resulting from the catabolism of amino acids (and thus protein), whole-body urea synthesis was simultaneously determined. To elucidate the effect of the specific underlying lung disease, we studied COPD patients with and without radiologically proven emphysema who were able to preserve muscle mass.
SUBJECTS AND METHODS
Study population
Fourteen COPD patients with moderate to severe airflow obstruction and 8 healthy age-matched volunteers were studied. The COPD group was carefully selected to include 7 patients with macroscopic emphysema (Emph+) and 7 without (Emph-) on the basis of high-resolution computed tomography (9). All subjects were men. All patients had COPD according to the American Thoracic Society guidelines (10) and chronic airflow limitation, defined as a measured forced expiratory volume in 1 s (FEV1) < 70% of reference FEV1. Furthermore, the patients had irreversible obstructive airway disease (< 10% improvement in FEV1 predicted baseline after inhalation of a ß2agonist), were clinically stable, and had not had a respiratory tract infection or exacerbation of their disease for 4 wk before the study. Exclusion criteria were malignancy; cardiac failure; distal arteriopathy; recent surgery; severe endocrine, hepatic, or renal disorder; and use of anticoagulant medication. Also, subjects who had used oral corticosteroids within 3 mo of the start of the study were excluded. The maintenance treatment of the studied COPD patients consisted of inhaled ß2-agonists, inhaled anticholinergics, inhaled corticosteroids, and oral theophylline. The COPD patients and control subjects were sedentary persons who were retired. Although most of the COPD patients were former smokers (80%), the current smoking status of the subjects was 3 in the patient group (21%) and 1 in the healthy control group (13%). Written informed consent was obtained from all subjects, and the study was approved by the medical ethical committee of the University Hospital Maastricht.
Study protocol
After fasting overnight, the subjects remained in a supine position for 3 h. A catheter was placed in an antecubital vein of the arm for infusion of the tracers. Each subject was given a priming dose, followed by continuous infusion until the end of the experiment. The following isotope infusion rates and priming doses were used (where FFM is fat-free mass): L-[ring-2H5]phenylalanine, infusion rate = 0.054 µmol • kg FFM- 1 • min- 1, priming dose = 3.68 µmol/kg FFM; L-[ring-2H2]tyrosine, infusion rate = 0.014 µmol • kg FFM- 1 • min- 1, priming dose = 0.95 µmol/kg FFM; L-[ring-2H4]tyrosine, priming dose = 0.3 µmol/kg FFM; [15N2]urea, infusion rate = 0.186 µmol • kg FFM- 1 • min- 1, priming dose = 44.42 µmol/kg FFM. The tracers were obtained from Cambridge Isotope Laboratories (Woburn, MA).
A second catheter was placed in a superficial dorsal vein of the hand of the contralateral arm, which was placed in a thermostatically controlled hot box (internal temperature: 60 °C), a technique designed to mimic direct arterial sampling (11). Arterialized venous blood samples were taken at 2, 2.5, and 3 h during the infusion to ensure an isotopic steady state.
Subsequently, all subjects performed a submaximal exercise test on an electronically braked cycle ergometer (Cornival 400; Lode, Groningen, Netherlands) for 20 min, which is comparable to a single exercise (training) session used in patients with severe COPD. The work rate for each subject was calculated as 20% of the predicted maximum workload for height, age, and weight, according to the equations of Jones (12). The pedaling frequency was selected by the subjects between 60 and 70 rpm and was held constant throughout the test. An infrared electrode was placed on a finger to measure oxygen saturation (Fasttrac; Sensor Medics Co, Anaheim, CA). Heart rate was measured throughout the test with the use of a sport tester (PE3000; Polar Electro company, Kempele, Finland). Arterialized venous blood was sampled at 10, 15, and 20 min of exercise and at 15, 30, 45, and 60 min during recovery.
Analysis of arterialized venous blood
Arterialized venous blood was collected in a heparin-containing syringe, immediately put on ice, and subsequently centrifuged at 3120 x g and 4 °C for 10 min to obtain plasma. Plasma was deproteinized with sulfosalicylic acid (5%) and was stored at –80 °C until analyzed. Concentrations of phenylalanine and urea were analyzed by using an HPLC technique (13), and tracer-tracee ratios were analyzed by using a liquid chromatography–mass spectrometry system (14).
Calculations
We used a single pool model in which the whole-body amino acid pool is assumed to be homogeneous, with a constant exchange of amino acids entering and exiting from a metabolic pool of amino acids, because all proteins are constantly being synthesized and simultaneously degraded. Flux or protein turnover is defined under steady state conditions as the total flux into or out of the active metabolic amino acid pool. In our case, the influx into the metabolic pool is from the breakdown of body protein. The efflux from the metabolic pool includes the amino acids used for synthesis of protein and for hydroxylation (or oxidation). All other metabolic pathways are considered minor. Thus, in the postabsorptive state: protein turnover = protein breakdown = protein synthesis + hydroxylation (15, 16).
The following equations were used for the calculations in the postabsorptive state:
RESULTS
Age, height, body weight, fat-free mass, and fat mass did not differ significantly between the COPD patients and the control group (Table 1). No recent involuntary weight loss was present in the patients or control subjects. On average, the studied COPD group had moderate to severe airflow obstruction (FEV1: 37 ± 13% of predicted) and mildly to moderately reduced diffusing capacity (71 ± 25% of predicted). Stratification of the COPD group into the Emph+ (n = 7) and Emph- (n = 7) subgroups resulted in large differences in pulmonary function between the groups. The Emph+ group had more severe airflow obstruction and air trapping than did the Emph- patients (P < 0.05); there was no significant difference between the subgroups in arterial oxygen pressure. In the control group, all lung function values were within the normal range.
View this table:
TABLE 1 . General characteristics of the study population1
The absolute work rate used in the exercise test did not differ significantly between the study groups (Table 2) but when expressed as a percentage of a previously performed incremental exercise test was higher in both COPD subgroups than in the control subjects (P < 0.01). Moreover, the relative work rate was even higher in the Emph+ than in the Emph- group (P < 0.05). Transcutaneous oxygen saturation at rest and postexercise was lower in both COPD subgroups than in the control group. Moreover, oxygen saturation was lower postexercise in the Emph+ group than in the Emph- patients (P < 0.05). Heart rate did not differ significantly between the 3 groups at rest or postexercise.
View this table:
TABLE 2 . Exercise characteristics of the study population1
As reported earlier (21), rates of protein turnover at rest were higher in the patients with COPD than in the healthy control subjects. Whole-body protein breakdown (Figure 1A) and protein synthesis (Figure 1B) increased (to a maximum of 9%) after the start of exercise in the control group. During recovery from exercise, protein synthesis and breakdown immediately normalized in the control group. Net protein breakdown (Figure 1C) did not change significantly during exercise or recovery in the control group.
FIGURE 1. . Mean (± SE) changes in whole-body protein breakdown (A), whole-body protein synthesis (B), and whole-body net protein breakdown in the healthy control group (; n = 8), the patients with chronic obstructive pulmonary disease without emphysema (Emph- group; ; n = 7), and the patients with chronic obstructive pulmonary disease and emphysema (Emph+ group; ; n = 7) during 20 min of exercise and 1 h of recovery. FFM, fat-free mass. A: Two-factor ANOVA showed a significant group effect during exercise (control compared with Emph+, P < 0.01; Emph- compared with Emph+, P < 0.01) and recovery (control compared with Emph+, P < 0.001; Emph- compared with Emph+, P < 0.05). There was no significant time effect during exercise or recovery and no significant group x time interaction. B: Two-factor ANOVA showed a significant group effect during exercise (control compared with Emph+, P < 0.05; Emph- compared with Emph+, P < 0.05) and recovery (control compared with Emph+, P < 0.001; Emph- compared with Emph+, P < 0.05). There was no significant time effect during exercise or recovery and no significant group x time interaction. C: There was no significant group or time effect during exercise or recovery and no significant group x time interaction.
The protein metabolic response to exercise differed between the COPD subgroups. In the Emph+ patients, protein breakdown (Figure 1A) and protein synthesis (Figure 1B) did not change significantly during exercise and decreased below baseline values during recovery. This decrease (to a maximum of 10%) remained until at least 1 h into recovery. In contrast, the response in protein breakdown and synthesis of the Emph- group was not significantly different from that of the control group. The change in whole-body protein breakdown was significantly different during exercise and recovery between the Emph+ and the control groups (P < 0.01 and P < 0.001, respectively), and between the Emph+ and Emph- groups (P < 0.01 and P < 0.05, respectively).
A significant difference was also found for the change in whole-body protein synthesis during exercise (control compared with Emph+, P < 0.05; Emph+ compared with Emph-, P < 0.05) and recovery (control compared with Emph+, P < 0.001; Emph+ compared with Emph-, P < 0.05). There was a significant time effect for protein breakdown and synthesis during exercise (control compared with Emph-, P < 0.05) and recovery (control compared with Emph+, P < 0.05). Net protein breakdown (Figure 1C) did not change significantly during exercise or recovery in either COPD subgroup.
In line with the results for net protein breakdown, whole-body urea turnover (Figure 2) did not change significantly throughout the study in the Emph- and control groups. However, urea synthesis decreased from baseline after 30 min postexercise in the Emph+ group (to a maximum of 14%). The change in urea synthesis during recovery from exercise differed significantly between the control and Emph+ groups (P < 0.05). Plasma phenylalanine and urea concentrations did not change significantly during exercise or recovery in the control or COPD subgroups (data not shown).
FIGURE 2. . Mean (± SE) changes in whole-body urea turnover in the healthy control group (; n = 8), the patients with chronic obstructive pulmonary disease without emphysema (Emph- group; ; n = 7), and the patients with chronic obstructive pulmonary disease and emphysema (Emph+ group; ; n = 7) during 20 min of exercise and 1 h of recovery. FFM, fat-free mass. Two-factor ANOVA showed a significant group effect during recovery (control compared with Emph+, P < 0.05) but not during exercise. There was no significant time effect during exercise or recovery and no significant group x time interaction.
DISCUSSION
The results of the present study indicate that exercise at a constant low work rate does not induce net protein catabolism in clinically stable, weight-stable patients with COPD but suppresses whole-body protein and urea turnover (by 10–14%) in those with emphysema. Until now, only a limited number of studies were available examining the acute effects of cycle exercise on whole-body protein metabolism in healthy subjects. To our knowledge, no studies are available concerning other chronic wasting diseases. In the present study, we chose to examine the response to exercise in COPD patients and control subjects by using absolute work rate values because in real-life situations, subjects perform absolute and not relative work. Moreover, relative exercise, which places an identical metabolic strain on COPD patients and control subjects, is difficult to obtain because COPD patients stop cycling during maximal exercise tests mostly as a result of shortness of breath.
The increase in whole-body protein breakdown during low-intensity exercise found in the Emph- and control group agrees with some previous studies in healthy subjects (22–24) but is in contrast with others (25). Conflicting results have been reported regarding whole-body protein synthesis during endurance exercise in healthy subjects: both decreased (22, 26) and unchanged (23, 25) values have been observed. The differing findings are at least partly related to the different exercise protocols (intensities and durations) and study groups (mostly young and physically active) used. Remarkably, the response in whole-body protein turnover to exercise was similar between the Emph- group and the control group in the present study despite the difference in relative work intensity.
Protein turnover during exercise
Whole-body protein turnover was not increased in the Emph+ group during exercise. Several factors may theoretically contribute to this observation. Previously, we observed reduced muscle amino acid concentrations at rest in COPD patients with emphysema relative to those in patients without emphysema (27). When this indicates a reduction in amino acid availability, it may contribute to the fall in protein synthesis during exercise (28, 29). Moreover, significantly lower transcutaneous oxygen saturation was found during exercise in the Emph+ group than in the Emph- group. In fact, plasma lactate concentrations were higher at the end of exercise in the Emph+ group than in the Emph- group (3.8 compared with 3.0 mmol/L). When tissue hypoxia accompanies exercise in COPD patients with emphysema, the whole-body protein synthesis rate is expected to decrease (30). Moreover, the increase in protein turnover during exercise as seen in the Emph- group is an energy-consuming process (formation of peptide bonds, amino acid transport, RNA turnover, and protein breakdown). Recently, activity-related energy expenditure was found to be elevated in patients with emphysema (31), likely as the result of reduced skeletal muscle oxidative capacity (5). Moreover, the efficiency of breathing is decreased in COPD patients during exercise. In particular, patients with emphysema breathe at a higher inspiratory level as the result of dynamic hyperinflation of the diaphragm and therefore have to also use their shoulder-girdle muscle for breathing. Therefore, it can be hypothesized that patients with emphysema preferentially use their substrates (glucose and fat) for ATP production related to the enhanced work of breathing and anaerobic skeletal muscle contraction during exercise and less for protein turnover.
Protein turnover postexercise
Whole-body protein synthesis and breakdown returned to baseline values immediately in the Emph- group but decreased significantly below baseline values (by on average 10%) in the Emph+ patients. The reduced values in the Emph+ group remained until at least 1 h into recovery, which is remarkable because no energy is needed for muscle contraction. Although all COPD patients were ventilatory limited, the relative work intensity was slightly higher in the Emph+ than in the Emph- group. Previous studies in healthy subjects of whole-body protein synthesis and breakdown after high-intensity endurance exercise reported both unchanged (32, 33) and increased (22, 33) values. The latter was explained by the elevated free intracellular amino acid pool, which resulted from an increased transport of amino acids into skeletal muscle (29) and to skeletal muscle proteolysis immediately after exercise (29, 34). To our knowledge, none of the studies observed a reduction in protein turnover postexercise. This suggests that the suppressed response in protein turnover to exercise in the Emph+ group was related to the underlying lung disease.
Urea synthesis during and after exercise
Net whole-body urea synthesis was not increased in the control and Emph- groups. Carraro et al (35) found in healthy subjects that low-intensity [40% of maximal oxygen uptake (
It is still possible that net protein breakdown is enhanced in the skeletal muscle of COPD patients despite whole-body protein balance. Recently, a severe reduction in muscle amino acid concentrations and an increase in plasma amino acid concentrations were observed in stable patients with severe COPD after low-intensity exercise (3). This suggests an enhanced nitrogen efflux from muscle, indicating a potential catabolic effect of exercise. In line with this, studies in healthy subjects found an increased nitrogen efflux from muscle during low-intensity exercise (36). However, evidence also exists that the gut is a main source of the increased Ra during exercise (37). Quantification of the rate of protein synthesis and breakdown in the skeletal muscle of Emph+ patients will be an important step forward in the understanding these subjects’ altered protein metabolic response to exercise.
Implications of the present findings
Earlier studies in healthy subjects showed that protein supplements taken immediately postexercise have greater anabolic effects than when taken preexercise, which is likely due to an accelerated amino acid transport into cells that increases the substrate availability of free amino acids for protein synthesis (8). The suppressed response in whole-body protein turnover in the Emph+ group in the present study could mean that amino acid transport into cells is reduced and that protein supplements taken postexercise are less effective in stimulating muscle protein synthesis in Emph+ than in Emph- patients. This means that the timing of feeding in Emph+ patients may be important in relation to muscle mass gain or preservation. Although the studied COPD patients were both clinically and weight stable at the time of measurement, an optimal anabolic stimulus of exercise and nutrition may be of particular importance during periods of involuntary weight loss and when factors are involved that are known to cause protein unbalance (ie, an acute inflammatory state). In conclusion, a suppressed response in protein turnover to exercise is present in patients with emphysema and needs to be considered in therapeutic interventions when the goal is to achieve maximal anabolism.
ACKNOWLEDGMENTS
MPKJE contributed to study design, data collection, data analysis, and writing of the manuscript; NEPD contributed to study design and data analysis; RM contributed to data collection; EFMW contributed to study design; and AMWJS contributed to study design and writing of the manuscript. None of the authors had any financial or personal interest in any company or organization sponsoring the research, including advisory board affiliations.
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