The influence of dietary carbohydrate and pre-exercise glucose consumption on supramaximal intermittent exercise performance

The present study examined whether a pre-exercise consumption of glucose by subjects having adhered to a 3-day low carbohydrate (CHO) or normal CHO diet would influence supramaximal intermittent exercise performance. Sixteen moderately active men volunteers (mean(s.d.) age 20.0(1.3) years) agreed to undertake three exercise tests over an 8-day period; in addition to completing a VO2max test, the subjects performed two identical maximal interval tests (MIT(1) and MIT(2)). Periods of 3 days separated each of the three tests. The interval tests involved five 60-s 'all-out' cycling bouts working against a resistance of 0.075 kg kg(-1) body mass; each bout was separated by 5 min of passive recovery. For 3 days preceding the first interval test (MIT(1)), all subjects adhered to a 'moderate' CHO diet which comprised 59.1% (approximately 4.1 g kg(-1) body mass) of the daily energy intake as CHO. Following MIT(1) and for 3 days before MIT(1) subjects were randomly assigned to follow either a moderate CHO diet (60.8%) or a low CHO diet (14.4% or 1.1 g kg(-1) body mass). All food and drink consumed during the experimental period was weighed and recorded for later dietary analysis. One hour before MIT(2), eight subjects were administered (in single blind fashion) a 15% glucose solution (1 g kg(-1) body mass) while the other eight subjects consumed a low-energy sweetened placebo. During both interval tests, values of work, exercise VO2, plasma glucose, plasma lactate and venous blood pH were statistically analysed. No changes in performance between MIT(1) and MIT(2) across conditions were found (P > 0.05). However, those subjects who consumed the glucose solution before MIT(1) (irrespective of their dietary CHO intake) consumed significantly less oxygen during exercise than those who had been given the placebo solution (P < 0.05). While these findings question the ergogenic potential of consuming glucose before supramaximal exercise, the VO2 data implicate a possible shift in substrate utilization during repeated sprint exercise after pre-exercise glucose ingestion