Slowed glycogen utilization enhances exercise endurance in patients with heart failure.

OBJECTIVES The objective of the study was to investigate the impact of alteration of glycogen stores and metabolism on exercise performance in patients with heart failure. BACKGROUND In normal subjects, muscle glycogen depletion results in increased exertional fatigue and reduced endurance. Skeletal muscle biopsies have revealed reduced glycogen content in patients with congestive heart failure (CHF). Whether glycogen depletion contributes to reduced endurance and abnormal ventilation in these patients is unknown. METHODS Bicycle exercise tests with measurement of respiratory gases were performed following dietary manipulations to induce glycogen depletion (60% protein, 40% fat) and slow glycogen utilization (60% carbohydrate, 30% fat, 10% protein) in 13 patients with CHF (left ventricular ejection fraction 22+/-6%; age 48+/-9 years) and 7 control subjects (age 45+/-5 years). Maximal exercise, exercise at 75% of peak workload until exhaustion and 1-min cycles of supramaximal exercise at 133% of peak were performed on three occasions over a two-week period. RESULTS Significant changes in resting respiratory quotients (RQs) in normal (Baseline: 0.78+/-0.03; Depleted: 0.69+/-0.05) and CHF subjects (Baseline: 0.84+/-0.05; Depleted: 0.72+/-0.05) were observed (both p<0.05). Peak Vo2 (oxygen consumption) in both groups was unchanged. The ventilatory response to exercise was analyzed by correlating CO2 production (V(CO2)) to minute ventilation (VE) in each test. The slopes of these correlations were not affected in either group. With glycogen depletion, exercise endurance was reduced from 17 to 6.1 min (57+/-19%) in normal subjects versus a reduction of 9.4 to 8.1 min (11+/-19%) in patients (p<0.05). With slowed glycogen use, CHF patients increased exercise endurance from 9.4 to 16.5 min (65%) versus 17 to 20.6 min (18%) in normal subjects (p<0.05). CONCLUSIONS Glycogen depletion minimally affects maximal exercise performance, endurance or ventilation in CHF patients, whereas slowed glycogen utilization markedly enhances exercise endurance. Therapeutic interventions that increase or slow use of glycogen stores may have clinical benefit.