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I feel I should always post this, as it's good to understand how your body is processing foods.
As you eat, the following happens:
1. Digestion starts immediately in the mouth, where the amylase enzyme starts breaking down starch into glucose. Simple carbs are converted to glucose almost immediately after they are eaten. Complex carbs first need to be broken down into simple carbs before they are converted to energy. Glucose is converted to energy in the form of ATP, as your body requires.
2. After you have met your immediate energy requirements, your body's next priority is to replenish glycogen stores. Glycogen is a way of storing glucose where it can be made immediately available in the case of exertion, such as sprinting or lifting heavy weights, or endurance exercise. Insulin stimulates the action of the glycogen producing enzyme, glycogen synthase.
3. Protein is first broken down into short chains of amino acids by the enzyme pepsin, and then into single amino acids by pancreatic enzymes such as trypsin. Amino acids cannot be stored in the body, and will form an "amino acid pool" in the body, which can be drawn from when muscle synthesis is required, or when glycogen stores are depleted. Once the 'amino acid pool' is full, excess amino acids get converted to sugars (carbs) and fatty acids.
4. Once glycogen stores are depleted, your body will use up the 'amino acid pool' as fuel before turning to fats. This is why it is popular to do cardio on an empty stomach if your goal is to lose weight - so that both the glycogen stores and amino acid pool are quickly used up, forcing the body to turn to fat for fuel. That topic is highly debatable however. Also, this is why you should not do cardio if your goal is to build muscle mass, since this amino acid pool will then be used up as fuel, and will no longer be available to draw amino acids from for muscle building.
5. Once all available glucose, glycogen, and free amino acids are used up, your body will start using fat as a fuel.
6. If there is an excess of carbs, these will be converted to fat as soon as the glycogen stores are full (roughly 30-40 minutes after eating). An excess of protein depends on your total protein consumption during the day, and is independent of the amount of carbs/fat consumed, but will depend on your amino acid pool as described above.
(An interesting consequence of this is that if your amino acid pool is full, every gram of protein you eat will turn to fat. Conversely, if your amino acid pool is being used up faster than you can refill it, no matter what your daily calories, you could eat almost unlimited (pure) protein without putting on a single gram of fat from it.) - This is also why it is important to eat your protein in many small meals during the day! Your body cannot normally handle more than 25-40g of protein at a time, and will convert the rest to sugars and fatty acids, whereas eating it spread out over the day will give you a non-fattening, endless source of amino acids for your muscle building!
7. Fats which you eat are broken down mainly into fatty acids, which are stored as fat, but also as glycerol, which is used to help break down glucose into energy. Thus, too much dietary fat will result in fat storage. Too little fat will result in too little glycerol, resulting in inefficient glycolysis (carb digestion), which will mean your body will use other resources such as muscle for fuel, something you most definitely don't want.
I should clarify that things in here that say "muscle building" are equally important for muscle RETENTION when you're losing weight.
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