Many diets geared toward bodybuilders and powerlifters stress how vital protein intake is. The fact that carbohydrates play just as important of a role is many times overlooked or ignored.
The reason carbs are so clutch when it comes to a muscle building diet is, first and most important, carbs provide energy to your muscles (simple as that). The Carbohydrates that you eat are stored in your muscles and the liver. During training, your body coverts these stored carbohydrates into ATP molecules and use them as energy. Let's face it, simple or complex carbohydrates are a must to reach your maximum potential in the gym.
Here is the rub. The proper carb intake can prevent pointless muscle loss. If you do not intake enough carbs and you hit the gym for an intense session and you burn up your stored carbs the body has to start looking for an energy source, and that source will be proteins. Here is the problem, the body cannot tell the difference between food protein (the protein you eat) and muscle tissue protein. It will burn what it needs to fuel you through your workout.
Most people are very concerned with insulin levels post workout. It is not a must to run out and buy some humulin-R and inject slin after a workout. We can bring on a spike in insulin simply by ingesting simple carbohydrates after your weight training workouts. Eating "clean" complex carbs is a must, but "simple" sugars or carbs are very important post workout, do this and you get a very nice raise in body produced insulin. I suggest either a weight gainer protein shake or just a non-diet soda or Power-aid will do the trick.
Carbs do not only fuel muscle fibers and cells. The central nervous system also uses carbohydrates as a source of fuel. During workouts, your nervous system is what actually tells your muscles fibers when, and and what intensity to contract. This is how one becomes overtrained...too much lifting will wear you down mentally. Proper carb intake keeps our CNS running properly by providing the correct amount of fuel to our muscles and this, in turn, prevents overtraining.
From what I've read and experinced 40 - 55% of you daily caloric intake should come from carbohydrates. The majority of which should be complex carbs. This is just a % I'm giving to help set you on the right track...just like anything we do in weight training each person should experiment and find out what is right for them.