The body uses 3 energy systems in the following order:

1. Anaeroboic Phosphogen - Normally used up in 10sec sprints and doesn't require oxygen.  Consists of ATP and creatine phosphate (hence why creatine is handy to have as a supplement to create more ATP).

2. Glycolysis - Utilises glucose (carbs) to produce ATP. Normally used up in 2-3min of exercise. 

3. Aerobic - Initiated after 2-3min of exercise. Utilises carbs and oxygen (leaves lactic acid as waste product). When carbs is depleted the body utilises fat as energy. When fat is used up protein from muscle fibres is then used. 

Carbs is stored in the body in 3 ways:

> Blood Glycogen - 40cal
> Muscle Glycogen - 1600 cal
> Liver Glycogen - 800 cal

So something to think about when losing weight. However for muscle gainers, it is absolutely important that there is carbs replenished in the muscles for them to grow.  

A lot of people ask me this at the gym. How do you get abs bro? I heard this so many times and the answer never changes "Its all in the diet. You can work as hard as you want but you will never see them if you don't get rid of that fat covering it. Everyone has abs. I only train them once or twice a week. Ask any bodybuilder or personal trainer, they'll say the same thing".