Chemist's POV:
In all actuality, carbs, calories, and fats are all the same thing: different variations of sugars. Sugars, are the most simple form of energy, and usable energy is a Calorie (actually a Kilocalory, or 1000 actual calories). Your body needs calories as principal energy source, so you need them to survive. The best type of calories ar simple sugars (natural sugars, i.e. friuts/veggies, and glucose) because they require very little alteration by your body to use in the form of energy. Complex sugars are carbs. Basically, all carbs are is long chains of simple sugars. There is more energy in carbs as regular simple sugars, but your body takes longer to digest them into usable form. This is why when you are doing a very strenuous activity, it is best to eat carbs the night before, so your body has enough time to break down into usable sugars. During strenuous activity, simple sugars are best because they are ready to use immediately. Fats are simply sugar molecules that have been rearranged in a way that makes them very difficult to digest. There are many diff. types of fats as well, but without getting into it. Fats can also be used as energy, but it takes your body a lot longer to break down.
So basically its like this: your body prioritizes the easiest way to get energy at that moment: sugars first, carbs next, fats at last resort. If your body does not require the energy that you consume, it gets stored for future use. This is weight gain folks: consuming more energy or improper energy than your body needs.
The way i diet is always keeping in mind what foods to eat. Obviously calories need to be consumed, as do carbs and fat believe it or not, but i try and prioritize based on what is available.
sorry if i confused anyone.
