The most important consideration when bodybuilding is taking in adequate energy or enough calories. Loads of studies have shown that consuming an additional 2270 - 3630 calories a week (approximately 500 extra calories a day), along with appropriate weight training, will result in one pound of muscle gain.
Muscles rely on glycogen (the energy they use for fuel) to perform work. When bodybuilders replace carbohydrate with protein in their diet, they have lower muscle stores of glycogen. For that reason, a high protein/low carbohydrate diet cannot provide enough glycogen for our muscles, so they feel weak, tired, and fatigue quickly. (FYI, three grams of water are needed for each gram of glycogen we store in our muscles.)
In addition, our bodies need to convert any excess protein we eat in order to burn it as energy or to store it as fat. This process helps us to get rid of the nitrogen contained in amino acids (the building blocks of protein), which we then urinate out; however, this makes us dehydrated, causes muscle cramping, and stresses our liver and kidneys.
As a result, it's important for a bodybuilder to take in enough carbohydrates. They are used to fuel the muscles that you need to help build lean body mass. Lots of extra protein by itself will not add muscular bulk -- carbohydrates actually spare protein so that it can be used to repair muscle tissue, which is how we build muscular strength and size.
How much protein do bodybuilders need? First figure out how many calories you need. If you're not taking in enough calories, you can't build muscle tissue efficiently. That's because your body will be burning most of your calories, not using them to repair muscle tissue. For example, take a 180 pound guy -- if he's moderately active, he probably needs about 2700 calories a day (plus or minus a few) to maintain his weight. Besides his moderate daily activity level, he could burn about 500 calories during an hour of heavy weightlifting. If he wants to add one pound of muscle weight per week, he needs approximately 500 extra calories per day plus about 500 more to make up for the energy deficit from intense weightlifting. This makes his grand total to be around 3700 calories a day.
So how can we translate this number to his protein needs? The RDA for protein has been established at 0.8 grams/kg of body weight for adults. This is not enough to build muscle mass for intense athletes. Although it's difficult to pinpoint a specific number because you have to take into account many variables, research has determined an acceptable range: even at the very high end, the top protein intake needs to be 1.5 - 2.0 g/kg of body weight. For our 180 lb. (divided by 2.2 = 82 kg) lifter, this would be 122 - 164 grams of protein per day. Since protein has 4 calories per gram, then this amount of protein would comprise 13 - 18 percent of his daily caloric intake of 3700 calories; the usual recommendation is about 12 - 15 percent. As you can see, a huge excess of protein is not needed.
It's easy to get enough protein from food -- and quite easy for many people to overdo it. If too much high-fat protein is taken in, and not used, gains may be seen in fat tissue rather than in muscle tissue.