food gets burned by metabolically active cells which possess mitochondria (which turn food into ATP). under normal conditions, they work on glucose. when glucose shortage occurs (after waking up for example), glucose will be spared for the brain. mitochondria must adapt to burning fat for energy. and since the main users of energy are the liver and muscles, wouldn't it make sense to train at maxiumum intensity in the morning before eating (for burning fat, that is)?