The thing that's missing for people who assume that exercising more while eating the same amount = weight loss is that the resting metabolism decreases (as the body gets more efficient from the improved heart/ lung condition etc.), so the increase in calories burned during exercise is offset by a decrease in calories burned at rest. I think another way the body tries to maintain a balance is by regulating the energy absorbed from food.As people have pointed out, the body adjusts to find an equilibrium and can cope with a particular range of conditions and will eventually find a plateu. Having said that, extreme levels of activity will push your body past the point where it can maintain that balance until it finds a new equilibrium, and you either lose weight rapidly or hugely increase your energy intake to maintain your weight. Blimey what do they teach in biology these days ?