Without protein, the human body would not be able to survive. Protein performs four very important functions.

Function
The body uses protein for:

• Growth and repair of new and damaged tissues. Skin, muscles, hair, finger nails, and blood clots are all made of protein.
  
• Regulating all body functions through the actions of enzymes, hormones, and other functional molecules.
  
• Transporting other nutrients and oxygen throughout the body.
  
• Supplying energy when adequate amounts are not supplied by carbohydrates and fat. Providing immune system defenses; antibodies are made of proteins.

Protein is an organic macromolecule comprised of compounds called amino acids. Amino acids are often referred to as the building blocks of protein. They consist of an amino group (H2N-), a carboxyl group (-COOH), a hydrogen (-H), and what is called a “side group” (usually denoted chemically as “R”) attached to a central carbon atom. There are 22 different amino acids; they differ by the type of “R” group attached.

Thirteen of the 22 amino acids can be manufactured by the body. The remaining nine amino acids – often called essential amino acids – must by supplied by the diet. People in developing countries may suffer from diet-related diseases and other health problems because of the shortage of protein foods.

Protein foods that supply all nine of the essential amino acids are called complete proteins. Foods that supply only some of the nine essential amino acids are called incomplete proteins. Two incomplete protein foods can be eaten together to form a complete protein source. Most generally, animal proteins are complete protein sources and plant proteins are incomplete protein sources. However, animal proteins also provide more fat and calories than plant proteins. It is a wise dietary practice to consume combinations of plant proteins to fulfill some of the body’s need for complete proteins. Some examples of combining incomplete proteins to form complete proteins are:

• Legumes (dried beans, lentils, split peas) and rice
• Pinto beans and corn tortillas
• Peanut butter sandwich (peanuts are a legume).

The amino acids are joined together by peptide bonds to form polypeptides. A protein consists of one or more of the polypeptide chains. Enzymes are globular proteins that catalyze chemical reactions within the body. For enzymes and all proteins, shape determines function – and the shape is determined by the sequence of the different amino acids.

Denaturation is the disruption of the bonds and the three-dimensional shape of a protein. This is often accomplished by changes in pH or temperature. To see denaturation in process, cook an egg white. The visible differences (moving from translucent to opaque, from watery to rubbery) are due to protein denaturation caused by heat.

It is recommended for adults that 10-35% of calories come from protein; for teenagers and children over the age of four, it is recommended that 10-30% of calories come from protein. Additional protein is needed by women during times of pregnancy and lactation. People should consult the Dietary Reference Intake charts for their gender and age group for specific protein requirements.