Effect of Endurance Training on Glycerol Kinetics During Strenuous Exercise in Humans Samuel Klein, Jean-Michel Weber, Edward F. Coyle, Robert R. Wolfe Glycerol kinetics were evaluated during high-intensity exercise in five untrained and five endurance-trained subjects. Glycerol rate of appearance (Ra) in plasma was determined by infusing [ZHs]glycerol during rest and 60 minutes of cycle ergometer exercise performed at 70% Voz peak. Mean plasma glycerol concentration was greater in trained than untrained subjects throughout exercise (P < .05). The average glycerol Ra during exercise and the integrated lipolytic response to exercise, expressed as total glycerol Ra above baseline, were both greater in trained (7.85 + 0.72 i~mol • kg -1 • min -1 and 289 -+ 50 /~mol • kg -1 • h -1, respectively) than in untrained (5.68 -+ 0.90 i~mol • kg -1 • min -1, and 198 -+ 31 l~mol • kg -1 • h -1, respectively) subjects (P < .05). We conclude that whole-body lipolytic rates are greater in endurance-trained athletes than in sedentary controls during high-intensity exercise performed at the same relative intensity. Copyright © 1996by W.B. Saunders Company E NDOGENOUS TRIGLYCERIDES provide an impor- tant fuel for working muscles during endurance exer- cise. The use of fat is influenced by previous exercise training and many studies have consistently documented increased fat oxidation during exercise in trained compared with untrained subjects. TM In contrast, assessment of fat mobilization, by measuring the rate of appearance (Ra) of free fatty acids (FFA) in plasma, has yielded conflicting results. FFA Ra has been found to be higher, 5 the same, 6,7 or lower 8 in trained compared with untrained subjects. One possible reason for the differences observed be- tween studies is that FFA Ra in plasma may not be an accurate measure of lipolytic activity during exercise. Glyc- erol Ra provides a better index of whole-body lipolytic rate than does FFA Ra because glycerol released during lipoly- sis cannot be metabolized by adipose tissue and must enter the bloodstream. 9 In contrast, a portion of fatty acids released during lipolysis are reesterified and thereby re- tained within adipose tissue. During high-intensity exercise, FFA Ra is a particularly poor marker of lipolysis and alterations in FFA kinetics may not even parallel alter- ations in glycerol kinetics. Romijn et al found that FFA Ra was 35% lower while glycerol Ra was 25% higher during high-intensity than during low-intensity exercise. 1° Several mechanisms may be responsible for the discrepancy be- tween FFA Ra and glycerol Ra. A portion of released fatty acids may become trapped in adipose tissue because adi- pose tissue blood flow is not sufficient to remove the large amount of released fatty acids. 1°,11 In addition, fatty acids released from intramuscular lipolysis may be oxidized locally, preventing their entry into the bloodstream. There- fore, it is possible that differences in FFA Ra observed between trained and untrained subjects may not represent true differences in lipolytic activity. We recently found that glycerol kinetics were the same in trained and untrained subjects during low-intensity exercise performed at the same absolute intensity. 6 Exercise per- formed at higher intensities should provide a greater lipolytic stimulus because of the increase in sympathoadre- nal activity. However, assessment of glycerol Ra during moderate- or high-intensity exercise in trained compared with untrained subjects has never been reported. The purpose of the present study was to evaluate the effect of endurance training on glycerol kinetics during high-intensity exercise. Glycerol Ra, measured by infusing [2Hs]glycerol, was determined during 30 to 60 minutes of cycle ergometer exercise performed at 70% of peak oxygen uptake in endurance-trained male athletes and in un- trained healthy young adult men. The relative intensity of the exercise bout was the same in both groups, but the absolute intensity was higher in trained subjects because of their higher capacity for oxygen consumption. METHODS Subjects Five untrained subjects and five endurance-trained athletes participated in this study (Table 1). The trained subjects were competitive distance runners and were actively involved in a vigorous running program for more than 3 years. The untrained subjects had not been involved in a regular exercise program for at least 1 year. All subjects were considered to be in good health after a careful medical evaluation that included a history, physical examination, and blood tests (complete blood cell count, electro- lyte panel, blood urea nitrogen, creatinine, and liver chemistries). Informed consent was obtained from each subject. This study was approved by the Institutional Review Board and the General Clinical Research Center of The University of Texas Medical Branch at Galveston, TX. Exercise Capacity Measurement of ~'o2 peak was determined during an incremen- tal cycle ergometer exercise protocol. The workload increased every minute until volitional exhaustion was reached. Oxygen consumption and carbon dioxide production were measured using a metabolic measurement cart (Sensormedics, Anaheim, CA). The From the Department of Internal Medicine, Washington University School of Medicine, St Louis, MO; Departments of,Anesthesiology, and Surgery, The University of Texas Medical Branch and Shriners Burns Institute, Galveston, TX,"and the Human Performance Labora- tory, Department of IOnesiology and Health Education, The University of Texas at Austin, `Austin, TX. Submitted May 2, 1995; accepted.August 22, 1995. Supported by National Institutes of Health Grants No. DK 49989, DK46017, and Grant No. 15849from the Shriners Hospitals. `Address reprint requests to Samuel Klein, MD, Washington Univer- sity School of Medicine, 660 S Euclid,Ave, Box 8127, St Louis, MO 63110-1093. Copyright © 1996 by W.B. Saunders Company 0026-0495/96/4503-0013503. 00/0 Metabolism. Vol 45, No 3 (March), 1996: pp 357-361 357