It's not that simple. For one, you want to lose fat weight, not muscle, bone, etc. weight. How does your body consume or store fat? It depends on available calories, the caloric composition, and your hormone feedback systems. Arguably, the hormones exert more control than anything else.
It gets more complicated from there: hormone's respond to what you eat. Eat more sugar, it raises your insulin (a hormone) which will stop the conversion of fat to energy, and indirectly will cause the conversion of blood sugar to fat. If your insulin is improperly regulated, you'll have difficult following the simple formula above. The same goes for thyroid hormones. They regulate your metabolism. Another hormone involved is cortisol which reponds to stress levels and can increase your blood sugar levels (cascading over into insulin, for example). Note that you can run into the non-intuitive case of over exercise increasing your cortisol causing excessive blood sugar causing fat storage. It's not easy, but it is possible.
One way to look at it simply is to eat foods that control your blood sugar, and manage your stress levels to avoid excessive cortisol. Excessive protein and fiber calories won't cause fat to build, going against the above formula. In fact, protein and fiber are excellent blood sugar regulating foods and are therefore good at controlling fat building.