We hypothesized that when plasma glucose availability is maintained by carbohydrate (CHO) ingestion, trained cyclists can utilize plasma glucose at very high rates during the later stages of prolonged exercise (10). To test this hypothesis, a well-trained male cyclist was studied during exercise to fatigue at 70% when ingesting glucose throughout exercise. A primed continuous infusion of [U-13C]glucose was begun after 60 min of exercise to measure rates of plasma glucose appearance (), disappearance (), and oxidation (). and rose progressively throughout exercise, peaking at 6.85 and 6.99 mmollmin, respectively, at fatigue (i.e., 133 min). Most (93%) of this glucose was oxidized; during the final 30 min of exercise, , averaged 6.10 mmollmin and accounted for approximately half of total CHO oxidation. These results support the hypothesis that trained cyclists can oxidize plasma glucose at very high rates during the later stages of prolonged exercise when fed CHO.
A.R. Coggan is with the Exercise Physiology Laboratory at The Ohio State University, Columbus, OH 43210. R.J. Spina, W.M. Kohrt, and J.O. Holloszy are with the Section of Applied Physiology, and D.M. Bier is with the Metabolism Division, Washington University School of Medicine, St. Louis, MO 63110.