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

Abstract

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 (Pl) 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˙O_{2 max}) and the velocity associated with ventilatory threshold (ν_VT). The second and the third sessions were composed of two submaximal treadmill runs (20 min long, 0% grade, performed at ν_VT), before and after an 80-min overground run, also conducted at ν_VT. During these submaximal tests, the subjects ingested (in a random order) either a Pl 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˙O_{2 max} and 16.5 (0.6) km · h⁻¹. A significant effect of exercise duration was found on C_r (ΔC_r) at the end of the run, whatever the solution ingested (ΔC_r= 5.7% and 7.01% for CHO and Pl, respectively). A reduction in the respiratory exchange ratio (from 0.98 to 0.90) was observed only at the end of the Pl 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 Pl, 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˙O_{2 max} in well-trained subjects can be explained mainly by alterations in neuromuscular performance, which lead to a decrease in muscle efficiency.