11.9: Proteins

Foods high in protein include meat and fish, egg whites, milk, beans, and peas. Proteins from animal products contain all 20 types of amino acids; plant proteins often lack or have very little of one or more types. Since different plant foods lack different amino acids, a complete set of amino acids can be obtained from plants by eating specific combinations of plant foods (e.g., rice and beans, corn and beans).

Uses

Digestive enzymes break dietary protein molecules into individual amino acids, which are then absorbed into the blood. Body cells use fragments from some amino acids for energy and to build nonprotein molecules. Other amino acids are used to build body proteins.

Body cells require large supplies of all 20 types of amino acids to build proteins. If even one type is missing or is in very short supply, synthesis of body proteins virtually stops. Although 11 of the 20 types can be manufactured in abundance by the liver and other cells, adequate amounts of the other nine types cannot be made in the body and must be supplied by dietary proteins. These amino acids are called essential amino acids.

Body proteins made from dietary amino acids have many uses. Many are used to build cell membranes, keratin (epidermis), collagen and elastin fibers (dermis, ligaments, cartilage), and muscle cell microfilaments. Others are used as enzymes to control chemical reactions; hormones to send messages; antibodies to fight infections; buffers to regulate acid/base balance; clotting factors to stop bleeding; blood proteins to help transport lipids (lipoproteins); and blood proteins to control the movement of water through capillary walls. Body protein molecules are broken down and their amino acids are used for energy only under starvation conditions.

In people of all ages body proteins such as keratin, muscle cell microfilaments, and digestive enzymes are continuously lost or broken down. Since the body has very limited stores of amino acids, a regular intake of dietary proteins is necessary for the continuous replacement of these structures so that their functions, and therefore homeostasis, are maintained.

Recommended Dietary Intakes

The RDA of 0.8 to 1.0 gram of protein per kilogram of body weight per day for elderly people should be increased at least to 1.0 gram per kilogram per day. Even higher protein intakes may be appropriate for individuals who must produce extra proteins, such as people recovering from a serious injury or disease and those engaged in very strenuous physical training. Diets with a reduced protein content may be recommended for individuals with certain kidney diseases because such diets yield less urea. Diets containing mostly animal proteins rather than plant proteins also generate less urea because a higher proportion of the amino acids from animal proteins can be used to build body proteins. With reduced urea production, less urea must be eliminated by the kidneys and a urea buildup in the body is avoided. This is important because excess urea can harm body cells.

Protein Deficiencies

Since proteins contribute to many body structures and perform many functions, diets low in one or more amino acids needed for protein production cause many problems. Examples include structural and muscle weakness; slowed body reactions; increased risk of infection; loss of acid/base homeostasis; excess bleeding; edema; and poor recovery from injury.

Protein Excesses

Extra amino acids from excess dietary proteins can be used for energy or converted into other useful materials and stored fat. Therefore, diets containing more than the recommended amounts of protein usually cause no difficulty except the positive energy imbalance that may develop. However, individuals with kidney disease may accumulate harmful levels of urea.