Proteins are known as the building blocks of the body. They are essential for growth, for building body tissues, and for basic body functions. They can also be used for energy if the diet does not contain enough carbohydrates and fats.
Proteins consist of substances called amino acids. The body is able to manufacture many of them, but there are nine amino acids it cannot manufacture and must get from foods. A food protein that contains all nine essential amino acids is called a complete protein. Meats, poultry, fish, eggs, and dairy products contain complete proteins.
Proteins that lack one or more of these essential amino acids are called incomplete proteins. Foods high in incomplete proteins include nuts, grains, and dried beans and other legumes. Foods that, if eaten together, supply all the amino acids are called complementary proteins. For example, cornmeal tortillas topped with chili beans supply complete protein because the corn supplies the amino acids lacking in the beans. Beans and rice is another example of a food combination supplying complementary proteins.
Complementary proteins are especially of interest to vegetarians, especially vegans.
The average adult needs about 50 to 60 grams of protein a day. For most North Americans, getting enough protein daily is not a problem; most get about twice as much as they need. Greatly excessive protein in the diet can lead to a variety of health problems, including kidney and liver damage.
[source: professional cooking sixth edition]