When a nutrition label states one servings has x amount of protein, not all of those grams are treated the same way in the body. That's because not all proteins are the same. Protein is the building block of muscle and if you don't get enough or the right kind, your body won't be able to build any. This is especially true if you follow a tough strength training schedule.
Proteins are made up of amino acids. When the amino acids combine, they form proteins. Not all proteins are alike. The protein's characteristics are determined by what order the amino acids are arranged in and which ones are included. Not all proteins contain the same amino acids.
Amino acids can be separated based on whether the body can make them or not. If the body can't make an amino acid, it's called an essential amino acid. If we don't eat it, there's no way the body will get it. If the body can make it, it's called a non-essential amino acid.
Protein's main role in the body is to build new tissue. For a protein to be able to do this, it must contain all the essential amino acids. If a protein contains all the essential amino acids, it's called a complete protein. If it's missing any of the essential amino acids, it's called an incomplete protein. Since your body can't use incomplete proteins to build new muscle, you want to find foods that are high in complete proteins.
Complete proteins are found only in animal products (with the exception of soy). That means proteins that come from milk, meat, cheese, eggs and fish are all complete. Unfortunately, most of these sources are also high in saturated fat which has been shown to increase the risk of cardiovascular disease.
Your goal should be to eat foods high in complete proteins and low in saturated fats. Some good examples are chicken, turkey, lean cuts of beef, lean ground beef (or drained ground beef), seafood, low fat dairy, egg substitutes.
Where does this leave vegetarians? Since complete proteins almost always come from animal sources, can vegetarians ever build any muscle? The answer is yes. When you eat an incomplete protein, it's not as if your body throws it out. When a protein is digested, it gets disassembled into the individual amino acids and gets combined with amino acids from other foods.
If you eat an incomplete protein that is lacking in amino acid X at the same time as eating another incomplete protein that is lacking in amino acid Y, your body will use the amino acids from both proteins to create a complete one. If you're a vegetarian, as long as you have a diet that is high in variety, your body will take care of collecting all the essential amino acids from different foods.
While protein is the only macronutrient that can build muscle, eating too much of it isn't a good idea. If you eat more than you need, your body doesn't use it to build more muscle. Excess protein, just like excess fats and carbohydrates get converted into body fat and stored for later use. The body doesn't store excess protein as protein, it stores it only as fat. Once it's converted into fat, it can't be changed back into protein. You should also remember that protein by itself won't build any muscle. To build muscle, you also need to be engaged in a strength training routine.
Guide to Protein