If I recall correctly, once sugar is depleted for fuel, the body starts to burn fat and will continue to do so until carbs/sugars are once again introduced into the system. While sugar provides quick bursts of energy, fat burning offers longer, more linear energy. So a sprinter may benefit from sugar based energy, a marathon runner would do better burning fat, aka ketosis.
Many low carb/carb free diets are out there and they do work very well at losing weight by switching the body into fat burning ketosis.
I'm not sure when this switch happened in human evolution, but somewhere man became a better shopper than hunter.