What Carbohydrates Do For Your Body.
Muscle mass-building requires energy, through carbohydrate intake. To gain mass, you have to ensure you're getting enough protein to rebuild muscle tissue damaged through training, but you also have to eat a lot of carbohydrates because gaining size requires you to fill your muscles with glycogen.
Glycogen is the collection of carbohydrates stored in muscles that powers your training and affects anabolism; think of it as an energy stockpile. To maintain that supply as you're trying to gain muscle, you need a positive intake in carbohydrates at each meal that is, you have to eat more carbohydrates than you burn.
What everyone needs to remember though is there is a difference in carbohydrates. Simple vs complex.
Complex carbohydrates take more time for the body to break down and use. This, in turn, provides you with a more even amount of energy. Complex carbs are things like whole grains and legumes.
Simple carbohydrates are faster to break down so the energy given doesn't last as long. They are also higher in sugar and low in fiber which is why they are so fast to break down.
When the body doesn't have carbohydrates to burn it goes to fat. This can be a good thing but over long periods of time your body will start to burn muscle, which in turn can cause damage. Remember some muscle damage can be irreversible. Heart damage is a good example as your heart is a muscle.