runners who’d loaded up on carbohydrates the day before the race ran faster than those who had eaten fewer carbohydrates. The difference was especially striking beginning at about the 18-mile mark, just when many runners famously “hit the wall” and feel their energy flag. The carbo-loaded runners jauntily maintained their pace. The others did not.
In both studies, carbohydrates eaten at breakfast on race day, during the race itself or on days earlier in the week were relatively unimportant. It was primarily what people ate on the day before the race that mattered."