Top Up Carbs to Feed Brain

From Canadian Running

So it seems pretty clear: you only need supplemental carbs for a one-hour exercise bout if you haven’t topped up your carb supplies beforehand. There are two ways your body stores glycogen: in your muscles (which is then used exclusively by your muscles), and in your liver (which feeds glucose into your bloodstream to fuel your heart and brain and keep blood sugar levels stable). When you sleep overnight, your muscle glycogen stays relatively stable, but your liver glycogen drops by more than 50 percent (because your brain and heart are still running all night). So the researchers believe that, if you don’t have a pre-exercise meal, the sports drink is needed to make up for your depleted liver glycogen stores.