Carbohydrate ingestion does not influence the change in energy cost during a 2-h run in well-trained triathletes

The aim of this study was to examine whether the increase in the energy cost of running (C-r), previously reported to occur at the end of a prolonged run, could be influenced by the ingestion of either an artificially sweetened placebo (PI) or a 5.5% carbohydrate (CHO) solution. Ten well-trained triathletes completed three testing sessions within a 3-week period. The aim of the first session was to determine maximal oxygen uptake ((V) over dot O-2max) and the velocity associated with ventilatory threshold (nu(VT)) The second and the third sessions were composed of two submaximal treadmill rims (20 min long, 0% grade, performed at nu(VT)), before and after an 80-min overground run, also conducted at nu(VT) During these submaximal tests, the subjects ingested tin a random order) either a PI or CHO solution prior to the first submaximal run and every 20 min after that. During the first session, ventilatory threshold (VT) occurred at [mean (SD)] 81.2 (2.5)% (V) over dot O-2max and 16.5 (0.6) km . h(-1). A significant effect of exercise duration was found on C-r (Delta C-r) at the end of the run, whatever the solution ingested (Delta C-r = 5.7% and 7.01% for CHO and PI, respectively). A reduction in the respiratory exchange ratio (from 0.98 to 0.90) was observed only at the end of the P1 trial. In this study, C-r seems to be affected only to a minor extent by substrate turnover. Moreover, the increase in the demand for oxygen, estimated from the increase in ventilation, accounted for only a minor proportion of the increase in C-r (11% and 17% for CHO and PI, respectively). No correlation was found between the changes in C-r and the changes in the other physiological parameters recorded. These results suggest, indirectly, that C-r increases during a 2-h run at 80% (V) over dot O-2max in well-trained subjects can be explained mainly by alterations in neuromuscular performance, which lead to a decrease in muscle efficiency.