Protein

Proteins are broken down in the stomach during digestion by enzymes known as proteases into smaller polypeptides to provide amino acids for the organism, including the essential amino acids that the organism cannot biosynthesize itself. Aside from their role in protein synthesis, amino acids are also important nutritional sources of nitrogen.

Proteins, like carbohydrates, contain 4 kilocalories per gram as opposed to lipids which contain 9 kilocalories and alcohols which contain 7 kilocalories. The liver, and to a much lesser extent the kidneys, can convert amino acids used by cells in protein biosynthesis into glucose by a process known as gluconeogenesis. The amino acids leucine and lysine are exceptions.

Sources

Dietary sources of protein include meats, eggs, nuts, grains, legumes, and dairy products such as milk and cheese. Of the 20 amino acids used by humans in protein synthesis, 11 "nonessential" amino acids can be synthesized in sufficient quantities by the adult body, and are not required in the diet (though there are exceptions for some in special cases). The nine essential amino acids, plus arginine for the young, cannot be created by the body and must come from dietary sources.

Most animal sources and certain vegetable sources have the complete complement of all the essential amino acids. However, it is not necessary to consume a single food source that contains all the essential amino acids, as long as all the essential amino acids are eventually present in the diet: see complete protein and protein combining.

For Athletes...

Most Athletes believe in consuming near one's body weight in protein,

Example: If you weigh 190 lbs you would need to consume at least 190 grams of protein a day to maintain a proper muscle repairing regimen.

For Body Builders...

Since Body Building consists of breaking down muscle tissue as much as possible without causing an injury, its understandable why they consume more protein than any other athlete out there. An average pro will consume somewhere around, twice as much as his or her own body weight during the day. I would say a good portion of this amount will be consumed after a workout.