Muscle glycogen provides a key fuel for training and racing a marathon. Carbohydrate ‘loading’ can enhance marathon performance by allowing the competitor to run at their optimal pace for a longer period before fatiguing. For the well trained runner, this may be achieved by tapering exercise over the final days before the marathon and ensuring carbohydrate intakes of 10–12 g/kg/day over the 36–48 hours prior to the race. Sports nutrition guidelines recommend that the runner consumes sufficient carbohydrate to promote restoration of muscle glycogen between training sessions. This strategy should allow the runner to ‘train harder’ and recover optimally between workouts. A recent hypothesis suggests that runners might ‘train smarter’ by training with low glycogen stores, since this might promote greater stimulation of the training response. However, there is no evidence that a low carbohydrate diet enhances the outcomes of training or provides benefits as a depletion phase prior to carbohydrate loading. In fact, a low carbohydrate diet may even impair performance if carried out for extended periods. If there are benefits to manipulating glycogen stores for some workouts, this is likely to happen as the natural outcome of the periodisation of the high-volume programme of an elite runner.