if your body thinks its going to have food in a few hours or so its going to use that food quick, if it knows its going to have to wait 6 or 7 hours before it gets any more fuel its going to slow down...think about how professional free divers (no scuba tank) slow their breathing, and in turn their heart rates down, so they can last longer, they are starving their body for air, same principle with your body and food.