Fuel Economy

Your body is a highly efficient machine. It will burn off the easiest fuel first, in order to cover its energy requirements and stash whatever is left over as a reserve for tough times when pie is scarce.
You want your body to be inefficient. Like a car, in other words you want your body to have to burn more fuel for less energy, but it wont!
Man used to hunt his food. He would expend vast amounts of energy to catch and kill his prey. Now he hunts his pie by sitting on his arse the whole day, eating copious amounts of high calorie fuel and concentrating on not being propelled of his chair by spontaneous pie combustion, otherwise known colloquially as gas.
We have it the wrong way round. We eat too much and exercise too little. In this age of abundance, we need to learn to fight our primal instinct (which compells us to consume) be conservative, and control what we put in. Our bodies have had the benefit of aeons of evolution. They will happily run for a day off the calories we can ingest in a few mouthfulls!
Check labels! Be aware what you eat! Be careful of your drinks! A sugar laden soft drink or a couple of beers will tripple the energy value of any meal you eat!