The prevailing thinking is that if one increases their activity a calorie deficit would be created and a subsequent weight loss would result, however, this never seems to happen in practice, and if there is any weight or fat loss, it is short lived. So which mechanics are involved that would negate this deficit and place the body firmly back in equilibrium? There are several systems involved, but first will be that cardio increases Cortisol, and decreases in Testosterone, this results in a net loss of muscle tissue. And since muscle tissue is your engine, just like in a car, a smaller engine burns less fuel. The human body does not want to supply the wants to negate it!  


As you'll see TSH (Thyroid Stimulating Hormone) levels are lower, and this is a result of adding prolonged heat to your system. One of the thyroid's main function is as a thermostat, people erroneously think it controls metabolism, but it merely downregulates heat production in the case of prolonged high volume exercise or excessive body weight and fat storage. Where the mass to surface ratio has increased to such a degree that the individual cannot liberate heat. So the thyroid has to decrease thyroid hormone output as a way to regulate temperature so as not to damage the body. You'll notice long distance runners become very small and lose a lot muscle and overall mass, this helps to liberate heat easier. This, however, is not an ideal fat loss stratagem as you would need to do more and more high volume exercise just to maintain your weight, let alone lose any. I'm sure you've noticed, no one has ever evaporated from exercise by maintaining a constant deficit. You'll not any deficit is quickly negated. So to summarise, one needs increase the bodies requirement for nutrient by increasing muscle mass (increasing the size of the engine) as this burns significant fuel at rest, rather than trying to increase expenditure, which creates an environment of diminishing returns.