Journal of Clinical Endocrinology & Metabolism, doi:10.1210/jc.2004-0890
The Journal of Clinical Endocrinology & Metabolism Vol. 90, No. 5 2675-2680
Copyright © 2005 by The Endocrine Society
Exercise Capacity and Biochemical Profile during Exercise in Patients with Glycogen Storage Disease Type I
H. R. Mundy,
P. Georgiadou,
L. C. Davies,
A. Cousins,
J. V. Leonard and
P. J. Lee
Charles Dent Metabolic Unit, National Hospital for Neurology and Neurosurgery (H.R.M., A.C., P.J.L.), London WC1N 3BG, United Kingdom; Department of Cardiology, Royal Brompton Hospital (P.G., L.C.D.), London, SW3 6NP United Kingdom; and Department of Biochemistry and Metabolism, Institute of Child Health (J.V.L.), London WC1N 1EM, United Kingdom
Address all correspondence and requests for reprints to: Dr. Helen Mundy, Charles Dent Metabolic Unit, National Hospital for Neurology and Neurosurgery, Queen Square, London WC1N 3BG, United Kingdom. E-mail: ceri{at}davies151.freeserve.co.uk.
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Abstract
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Glycogen storage disease type I (GSD-I) is an inherited disorder of carbohydrate metabolism. Hepatic glucose-6-phosphatase is deficient, leading to impaired gluconeogenesis and glycogenolysis. Patients prevent fasting hypoglycemia by frequent feeds of low glycemic index foods. Normal muscle does not contain glucose-6-phosphatase, and GSD-I is usually classified as a hepatic glycogenosis. However, clinical experience has suggested that patients have decreased cardiovascular fitness, but this had not been formally investigated. This paper reports the results of maximal treadmill cardiopulmonary exercise testing in adult patients with GSD-I. It documents a major reduction in exercise capacity in these patients and demonstrates biochemical aspects of exercise that are different from those of normal controls. All patients showed a reduction in exercise capacity, but there was a wide range of exercise tolerance. Additional work needs to address whether improved adherence to or intensification of therapy in adulthood will ameliorate exercise intolerance.
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Introduction
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GLYCOGEN STORAGE DISEASE type I (GSD-I; McKusick 232200) is an autosomal recessive inborn error of carbohydrate metabolism caused by defects of the glucose-6-phosphatase (G6Pase) complex. G6Pase hydrolyzes glucose-6-phosphate to glucose and has a central role in both glycogenolysis and gluconeogenesis. Patients suffer hypoglycemia whenever exogenous glucose is exhausted. Secondary metabolic disturbances of hyperlacticemia, hypercholesterolemia, hypertriglycideremia, and hyperuricemia occur due to increased flux through the intact glycolytic and pentose phosphate pathways. Before successful dietary treatment, patients suffered marked hepatomegaly, wasting, and severe growth retardation. Treatment with either frequent feeding of low glycemic index foods or continuous nasogastric drip feeding has dramatically improved the clinical outcome for these patients. Survival to adult life is now the norm. However, increasingly complications that are presumably due to the more minor, but chronic, metabolic disturbances are being recognized (1).
G6Pase is not present in normal muscle. Historically, untreated patients suffered muscle wasting, but with routine treatment from infancy, this problem is not seen (2). Clinically, adult patients appear less cardiovascularly fit than their peers, but formal evaluation has not previously been reported. To evaluate muscle function, we performed maximal treadmill cardiopulmonary exercise test in a group of adult patients with GSD-I and matched control subjects and measured biochemical profiles throughout exercise and recovery.
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Patients and Methods
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Patients were recruited from the metabolic clinic at the National Hospital for Neurology and Neurosurgery. To be included they had to be over 14 yr of age with a diagnosis of GSD-Ia or GSD-Ib. This was proven by biochemical analysis of G6Pase activity in intact or disrupted liver microsomes or by genetic analysis of the G6Pase catalytic subunit (GSD-Ia) or glucose-6-phosphate transporter (GSD-Ib) genes. For each patient, an age- and sex-matched healthy control was recruited (Table 1
). There were no significant differences in age (P = 0.721), height (P = 0.083), or weight (P = 0.599) between patients and controls. In addition, the controls were matched for self-reported exercise levels (sedentary, active, or very active). Patients have widely varying fasting tolerances and so followed their normal dietary regimens and refrained from eating for 1 h before exercise only. The same conditions were applied to control subjects.
All patients and control subjects gave fully informed consent, and the study was approved by the ethics committee of the National Hospital for Neurology and Neurosurgery (London, UK).
Cardiopulmonary exercise tests
The patients and controls underwent symptom-limited cardiopulmonary exercise testing on a motorized treadmill. Three minutes of resting data were collected to perform baseline measurements before exercise. The rest period was prolonged at the discretion of the investigator if additional time was required for adjustment to the mouthpiece and for stabilization of physiological variables. A modified Bruce exercise protocol (3) was used with an additional stage 0 (3 min; speed, 1 mph; 5% gradient). Ventilation (VE), oxygen uptake (VO2), and carbon dioxide production (VCO2) were monitored continuously, breath by breath, at rest, during exercise, and for 10 min of recovery after exercise, using a respiratory mass spectrometer (Amis 2000, Innovision, Odense, Denmark). Data were then averaged over 15-sec intervals. Calibration was performed before each study. Patients and controls were encouraged by the supervising physician to exercise to the limit of their symptoms. Peak oxygen consumption (VO2 peak) was recorded, and the average ventilatory equivalent for carbon dioxide (VE/VCO2 slope) throughout exercise was calculated by linear regression analysis using the entire exercise period. Blood pressure was recorded using a mercury sphygmomanometer at rest, at the end of each 3-min stage, and at peak exercise. The electrocardiograph was monitored continuously. Maximum heart rate (Max HR) was recorded and used to calculate the heart rate reserve (HRR), where HRR is the predicted Max HR – HR at maximum exercise. The predicted Max HR is estimated by the formula 210 – 0.65 x age (years). The respiratory gas exchange ratio (RER) was calculated as VCO2/VO2. The oxygen uptake efficiency slope (OUES) was calculated by linear regression of VO2 (milliliters per minute) and log10VE (liters per minute) (4). The anaerobic threshold was quantified by the V-slope method (5).
Blood analyses
An indwelling catheter was inserted into a vein on the dorsum of the hand or arm before exercise. After 15 min of rest, blood samples were taken into lithium heparin and sodium fluoride tubes and immediately separated by centrifugation at 3000 rpm for 5 min. The supernatant was then placed in plain tubes and stored on ice. In addition, blood was taken into heparinized syringes and placed on ice for measurement of blood acidity at baseline, peak, and 10 min after exercise. Blood pH and bicarbonate were measured using a commercial pH monitor (ABL700, Radiometer A/S, Copenhagen, Denmark). The cannula was flushed throughout with normal saline.
Samples were analyzed for glucose and lactate using the commercial Vitros 250 slide system (Johnson&Johnson Clinical Diagnostics) and for ammonia using the Vitros 750 slide system (Johnson&Johnson Clinical Diagnostics, Amersham Little Chalfont, Buckinghamshire, UK). Nonesterified fatty acids (NEFA) were condensed with coenzyme A (CoA) to form CoA esters, then oxidized by acyl CoA oxidase to form H2O2. The concentration was assayed colorimetrically with a napthelene dye system (Mira Plus Analyzer, COBAS, Basel, Switzerland). ß-Hydroxybutyrate was assayed by absorbance spectrophotometry after oxidation with concomitant reduction of NAD+ to NADH. For measurement of the precision of these estimates, see Table 2. Plasma amino acids were assayed by HPLC. all biochemical assays were performed in the biochemical laboratories of Great Ormond Street Hospital (London, UK). Patients ate after the monitored recovery period and remained resting under medical supervision until they had fully recovered from exercise.
Statistical methods
All comparisons between the patient and control groups were made using the Mann-Whitney rank-sum test. Descriptive and comparative statistics and linear regression analyses were performed using SPSS 12.0 for Windows (SPSS, Inc., Chicago, IL).
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Results
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Experimental subjects
All subjects successfully completed exercise uneventfully. Blood sampling was possible in seven of eight patients.
Cardiopulmonary data
Table 3 indicates the responses for patients and controls at peak exercise. There was no significant difference in median HRR between patient and control groups (P = 0.328). The median exercise time was significantly increased in the control group (P = 0.016), as was the median VO2 peak (P < 0.001). The median patient VO2 peak as a percentage of control was 64%. Individual VO2 peak as a percentage of the individual control VO2 peak is shown in Fig. 1.
The patient median OUES at 2733 ml/min·kg was significantly (P < 0.05) smaller than the control OUES at 3549 ml/min·kg.
The median VE/VCO2 for the patient group was 28 (interquartile range, 26–29) and was identical to that of the control group (interquartile range, 25–31).
Figure 2 shows the mean RER for patients and controls across a range of relative exercise intensities (i.e. normalized to percentage of VO2 peak). Significant differences were found at 50% (P = 0.02) and 90% (P = 0.027) of VO2 peak.
The anaerobic threshold was not clearly identifiable in three of the patients and one of the controls. Therefore, no additional analysis of this variable was possible.
Biochemical data
Glucose.
The mean blood glucose levels for patients and controls were not significantly different at baseline. The patient mean then showed a progressive decrease throughout exercise and recovery. The control mean, in contrast, rose and showed a hyperglycemic response during recovery, such that control and patient values were significantly separated during recovery (P = 0.026). One patient did show a hyperglycemic recovery phase response.
Lactate.
The patient mean blood lactate level was significantly higher at baseline than the control mean (P = 0.001). Although the mean of both groups rose throughout exercise, the rate of increase was greater for the control group, so that at peak exercise, there was no significant difference between the group means.
NEFA.
The patient mean NEFA was significantly higher than the control at baseline (P = 0.03). Both patient and control means showed the same pattern of decrease throughout exercise, followed by an increase in recovery to above baseline.
Alanine.
The mean patient alanine level was greater than the control value throughout the entire profile, but the difference was only significant at 6 min of exercise due to the wide range of patient values.
Changes in the patient median and control mean levels of glucose, lactate, alanine, and NEFA are shown at rest, after 6 min of exercise, at peak exercise, and at 5 and 10 min of recovery (Fig. 3).
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FIG. 3. Patient ( ) and control (•) values at baseline (0), after 6 min of exercise (6), at peak exercise (P), and 5 and 10 min postexercise (5 and 10). *, Significant difference between patient and control values (P < 0.05).
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Acid-base.
There was no difference between patient and control mean hydrogen and bicarbonate ion concentrations at baseline. One patient was acidotic before starting exercise. The bicarbonate concentrations remained very similar throughout the profile. The control group developed a significantly greater acidosis than the patient group at peak exercise. Venous hydrogen ion and bicarbonate concentrations are shown at rest, peak exercise, and 10 min postexercise (Fig. 4).
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FIG. 4. Patient () and control (•) values at baseline (0), peak exercise (P), and 10 min postexercise (5 and 10). *, Significant difference between patient and control values (P < 0.05).
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Discussion
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Glycogen storage diseases are a heterogeneous group of inherited disorders of carbohydrate metabolism caused by enzyme deficiency within either liver or muscle. They have been traditionally divided into muscle and hepatic forms depending upon the site of enzyme deficiency and the clinical phenotype. GSD-I is deficiency of the enzyme G6Pase. It is a disorder of hepatic gluconeogenesis and glycogenolysis. Because there is no expression of G6Pase within normal muscle, it is traditionally assigned to the hepatic group (1). Indeed, in a recent review of complications of GSD-I by the European Study on GSD-I, there is no mention of myopathy (6).
This study shows that exercise capacity, as measured by VO2 peak during maximal cardiopulmonary exercise testing, is significantly reduced in this group of GSD-I adults compared with normal controls. This includes patients who were treated from infancy in accord with current recommendations. VO2 peak can be influenced by patient motivation, but this observation was confirmed by the reduced patient OUES, which is a valid measure, even with submaximal effort. Comparison can be made with patients with pure muscle glycogenosis, myophosphorylase deficiency, or McArdle’s disease. This group of patients has an illness characterized by muscle pain and easy fatigability. Due to their interesting muscle physiology and biochemistry, they are a relatively well studied group. In a comparable study, seven McArdle’s patients underwent treadmill cardiopulmonary exercise testing with sex-, age-, and weight-matched controls (7). Figure 5 shows the individual exercise capacity of each patient in the two groups, expressed as the percentage of their control peak VO2. Although the median percentage is lower for McArdle’s patients (53% vs. 64%), there is no significant difference between the two groups.
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FIG. 5. Exercise capacity of the GSD I patients and of published historical McArdle’s (GSD V) patients expressed as a percentage of their control VO2peak. P, GSD I patient in the present study; M, McArdle patient from the study by Riley et al. (7 ).
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Patients with GSD-I do not suffer from primary cardiomyopathy. They can develop hypertension, but no patient in this study had electrocardiographic evidence of left ventricular hypertrophy. Respiratory disease is not a feature of GSD-I. The explanation would therefore seem to lie in skeletal muscle function.
Patients with GSD-I cannot hydrolyze glucose-6-phosphate. Therefore, in situations in which counterregulation is stimulated, one would expect a failure of hepatic glucose production and that the increasing concentrations of glucose-6-phosphate are channeled through glycolysis and the pentose-phosphate pathway. This is certainly the case in fasting. Patients develop hypoglycemia and increased blood lactate. To extend the period of euglycemia, patients regularly ingest low glycemic index foods, most commonly uncooked corn starch.
In normal subjects, exercise creates a nonsteady state in which increasing glucose requirement is associated with a prompt increase in production and delivery to the contracting muscle. In moderate exercise this is regulated by the glucagon/insulin ratio, but in high intensity exercise there are marked elevations in plasma catecholamine levels (8). There was no significant difference between the median blood glucose levels of patients and controls at the onset of exercise. However, as would be expected with failure of hepatic glucose production, there was a steady fall in the patients’ median blood glucose levels throughout exercise. Although our control subjects showed the expected hyperglycemic response after exercise to exhaustion, the patients’ median blood glucose continued to fall. Although, interestingly, one of the patients (P4) did, in fact, show a hyperglycemic effect postexercise. This suggests that for this patient acute endogenous glucose production is possible. This phenomenon has been documented previously in adults with GSD-I, who seemed resistant to fasting and had a documented hyperglycemic response to administered glucagons. Enhanced prior glycogen synthesis and increased glycogen to lactate to glycogen cycling with liberation of free glucose by debrancher enzyme has been postulated to be the cause. However, recently, a new, more widely expressed enzyme with G6Pase activity has been described, and this may be contributing to glucose homeostasis (9).
GSD-I patients had significantly elevated basal blood lactate concentrations compared with the control group, and there was a wide variation (range, 2.1–10.9 mmol/liter). However, at peak exercise there was no significant difference in lactate concentration, indicating that the rate of increase was much greater in the control subjects. This suggests that the control subjects were able to perform more anaerobic energy production within the muscle than were GSD-I patients. Lactate efflux from the muscle is primarily carrier-mediated in a HCO3–-lactate antiport. It is highly sensitive to the HCO3– concentration of the muscle perfusate (10). It may be that the lactic acidosis present at rest in these GSD-I patients would cause lowering of blood bicarbonate sufficient to reduce lactate efflux and inhibit anaerobic metabolism. In one patient (P1), there was a notably low resting bicarbonate concentration (14.8 mmol/liter). This was associated with a high venous lactate level (10.9 mmol/liter). His ventilatory equivalent for CO2 was 42, much higher than that of any other patient or control. One could reasonably postulate that in this patient, the acute acid-base disturbance was partly responsible for his extremely poor exercise capacity. However, as a patient group, the median bicarbonate concentration at rest or throughout exercise was no lower than that of the control group. In addition, the ventilatory equivalent for the patient group was not significantly different from the control value (P = 0.798). Therefore, a reduction in muscle lactate production may reflect a primary decrease in anaerobic metabolism.
In the liver of patients with GSD-I, glucose-6-phosphate is channeled through glycolysis, causing increased production of acetyl-CoA with a concomitant increase in the production of fatty acids and cholesterol. At the same time, hepatic fatty acid oxidation is down-regulated by the inhibition of carnitine palmitoyltransferase-1 by the increased malonyl-CoA. This explains the elevated plasma NEFA profiles previously reported in GSD-I patients. Our patients also have a significantly increased NEFA concentration at the onset of exercise compared with our control population. When exercise is initiated, catecholamine concentrations increase, and insulin concentrations decrease. During the first 15 min of exercise, plasma NEFA concentrations usually decrease because the rate of uptake by the muscle exceeds the rate of appearance from adipolysis. During recovery, there is a rebound increase in NEFA level as reduction in lipolysis lags behind the decrease in muscle uptake (8). These patterns were confirmed in our control subjects and were also mirrored by the patients’ results, albeit at a rather higher median value throughout. The Randle glucose fatty acid cycle predicts that increased NEFA concentrations would lead to impairment of muscle glucose utilization (11). The main features of this model are that increased fat oxidation in muscle would inhibit both pyruvate dehydrogenase and phosphofructokinase by accumulation of acetyl CoA and citrate, respectively. This leads to an inhibition of insulin-stimulated glucose uptake. This work was extended by Shulman (12), who proposed that fatty acids additionally caused insulin resistance through inhibition of insulin receptor substrate protein-1 signaling. In this study, GSD-I patients have a significantly lower RER than their controls during moderate intensity exercise (50% of VO2 peak), indicating increased fatty acid oxidation and supporting Randle’s hypothesis.
Ideally for this study we would have performed a series of exercise tests to allow familiarization with the equipment used. However, GSD-I is a very rare disease, so to maximize the number of patients recruited, we opted to perform just one test, but to apply the same conditions to control subjects. A larger number of patients may have enabled us to comment on the anaerobic threshold. Patients and controls were asked to self-report exercise levels as sedentary, active, or very active, but it may be that this masks a degree of variation in physical activity, particularly among the sedentary group, which could contribute to the reported differences. In this study, subjects were never in a steady state, but, rather, were constantly adapting to changes in exercise intensity. It would be instructive to perform additional studies with exercise at a submaximal level, closer to levels of physical activity that may be performed daily.
This study has documented a major reduction in exercise capacity in patients with GSD-I. It has demonstrated some biochemical aspects of exercise that are different from those of normal controls. Although all patients showed a reduction in exercise capacity, there was a wide range of exercise tolerance. Additional work needs to address whether improved adherence to or intensification of therapy in adulthood will ameliorate exercise intolerance.
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Footnotes
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Dr. H. Mundy was supported by a grant from the Dromintee Charitable Trust (UK).
First Published Online January 25, 2005
Abbreviations: CoA, Coenzyme A; G6Pase, glucose-6-phosphatase; GSD-I, glycogen storage disease type I; HRR, heart rate reserve; Max HR, maximum heart rate; NEFA, nonesterified fatty acid; OUES, oxygen uptake efficiency slope; RER, respiratory gas exchange ratio; VCO2, carbon dioxide production; VE, ventilation; VO2, oxygen uptake; VO2 peak, peak oxygen consumption.
Received May 12, 2004.
Accepted January 17, 2005.
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References
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Abstract
Introduction
