Currently the USDA recommends 10% – 35% of calories in the human diet that come from protein. The typical American diet contains more protein than is strictly necessary. Proteins are also important in the diet for many reasons.
For example, protein is the major constituent of most cells, making up more than 50% of the dry weight. Also, protein defines what an organism is, what it looks like, and how it behaves, because the body is made of thousands of proteins. Proteins are used to produce new tissues for growth and tissue repair, and regulate and maintain body functions. Enzymes used for digestion, protection, and immunity are made of protein, and essential hormones used for body regulation require protein. Finally, proteins may be used as a source of energy when carbohydrates are not available.
Protein is found in meats, poultry, fish, meat substitutes, cheese, milk, nuts, legumes, and in smaller quantities in starchy foods and vegetables. People who consume a vegetarian diet can get plenty of protein if they keep a balanced diet.
The body breaks down protein into its building blocks – amino acids. There are 500 known amino acids, 21 of which are needed by humans. Of the 21 necessary for life, nine are considered essential since they cannot be produced by the body and must be eaten. Proteins that contain all nine essential amino acids are considered ‘high quality’ proteins. These high quality proteins tend to come from animal sources. Proteins that do no contain all nine essential amino acids are considered ‘low quality’ proteins, and tend to come from plant sources.