Carbohydrate Ingestion During Exhaustive High-intensity Intermittent Shuttle Running In The Heat

Abstract

Under certain conditions exercise capacity has been shown to improve with the ingestion of a suitable carbohydrate-electrolyte solution (CES). However, in hot environmental temperatures the fatigue process is often a consequence of dehydration and an accelerated rate of heat gain rather than carbohydrate availability; under these circumstances the volume of fluid ingested is considered more important than the carbohydrate intake. Exercise capacity may be reduced during prolonged intermittent exercise in 30°C when a CES is ingested compared with an energy-free flavoured water (FW) (Morris et al., J Sport Sci, 2003, 21:371–381). Purpose This study examines deep body temperature, energy expenditure and endurance capacity during the Loughborough Intermittent Shuttle Test (LIST) in the heat, when ingesting a 6.4% CES compared with a FW. Methods Nine male games players performed the LIST to volitional fatigue on two separate occasions at an ambient temperature of 30.4 ± 0.7 °C (mean ± SD) and relative humidity of 35.3 ± 2.5 %. Subjects ingested 6.5 ml·kg−1 body mass of test drink before exercise and 3.5 ml·kg−1 body mass during each 3 min rest period separating exercise blocks. Intestinal temperature was continuously monitored using an ingested telemetric sensor and expired air samples were collected at regular intervals during the protocol. Results No differences were detected in exercise capacity between trials: subjects ran for 86.9 (±19.9) min and 94.1 (± 28.2) min in the CES and FW trials respectively (P=0.26). Similarly no differences were apparent in intestinal temperature at the point of fatigue: CES 39.50 ± 0.50 °C; FW 39.47 ± 0.40 °C (P=0.26). Cumulative sprint time was faster when subjects ingested the CES (P=0.04) and a difference was also apparent in the total energy expenditures of 3.7 ± 0.2 MJ in the CES trial and 3.4 ± 0.4 MJ in the FW trial (P=0.02). The rate of carbohydrate oxidation was higher in the CES trail (202 ± 22 g·h−1) compared with the FW (168 ± 25 g·h−1)(P=0.04). Conclusion It appears that the ingestion of a 6.4% CES enabled subjects to perform at higher energy expenditures during the self-selected sprint phase of the exercise protocol. This increase in performance may be due to the greater availability and oxidation of carbohydrate. The increased exogenous supply of energy when ingesting the CES did not result in an increase in exercise capacity as previously reported. The deep body temperatures recorded at the point of fatigue suggest that subjects may have failed to continue exercise due to the attainment of a capacity-limiting state of hyperthermia rather than insufficient carbohydrate availability.