The purpose of the present study was to examine the effects of a high- or low-carbohydrate (CHO) diet on performance, aerobic and anaerobic contribution, and metabolic responses during supramaximal exercise. Six physically-active men first performed a cycling exercise bout at 115% maximal oxygen uptake to exhaustion after following their normal diet for 48 h (∼50% of CHO, control test). Seventy-two hours after, participants performed a muscle glycogen depletion exercise protocol, followed by either a high- or low-CHO diet (∼70 and 25% of CHO, respectively) for 48 h, in a random, counterbalanced order. After the assigned diet period (48 h), the supramaximal cycling exercise bout (115% maximal oxygen consumption) to exhaustion was repeated. The low-CHO diet reduced time to exhaustion when compared with both the control and the high-CHO diet (-19 and -32%, respectively, p < 0.05). The reduced time to exhaustion following the low-CHO diet was accompanied by a lower total aerobic energy contribution (-39%) compared with the high-CHO diet (p < 0.05). However, the aerobic and anaerobic energy contribution at the shortest time to exhaustion (isotime) was similar among conditions (p > 0.05). The low-CHO diet was associated with a lower blood lactate concentration (p < 0.05), with no effect on the plasma concentration of insulin, glucose and K(+) (p > 0.05). In conclusion, a low-CHO diet reduces both performance and total aerobic energy provision during supramaximal exercise. As peak K(+) concentration was similar, but time to exhaustion shorter, the low-CHO diet was associated with an earlier attainment of peak plasma K(+) concentration.