It still complicated, though. For example, eat 1500 calories a day while your metabolism uses 1800 or so. Sounds foolproof? What if those 1500 calories are all sugar? There goes your metabolic rate and your body fat will go through the roof. What if you eat 60% protein, 30% fiber, 10% fat? You will probably wind up looking like a career athelete, even if you never exercise. The composition of the calories has a huge effect on their usage (through the mechanism of you hormone feedback systems). It may not be rocket science, but the body is complex to the point of being a black box for most people.