Prepared by
Diane Ball Linder, EdD, LDN, RD

If you don't count water, protein is the most plentiful substance in your body

Table of Contents for Protein Introduction Food Guide Pyramid Review What You Will Learn What is Protein What Proteins Do Building Blocks Called Amino Acids Amino Acids in Protein Foods That Give Proteins Nutrition Labeling What Counts as a Serving? How Much Protein Do You Need? Summary Review questions Back to Nutrition Curriculum Lessons

Click here for the printable (Word) version of the lesson

What Proteins Do:

Some of the functions of proteins include:

• Build and repair all body tissues.
• Regulate body processes.
• Maintain fluid balance.
• Form hormones and enzymes.
• Help form antibodies to fight infection.
• Supply energy.

Protein is a part of every living cell. Many different kinds of proteins form vital parts of the body. Examples include muscles, connective tissue and other tissues such as skin, proteins in blood, enzymes, hormones and immune bodies.

If you don't count water, protein is the most plentiful substance in your body. If all the water were squeezed out of you, about half of your dry weight would be protein. About one-third of protein is in muscle, about a fifth is in bone and cartilage. About a tenth is in skin. The rest is in the other body tissues and fluids. Blood contains several dozen proteins alone. Hemoglobin, one of the proteins in blood, carries oxygen from the lungs to the tissues and brings carbon dioxide back from the tissues to the lungs. Most of the hemoglobin molecule is protein.

We need proteins all of our life for growth and maintenance. There are special times in our life when we need more protein. These include periods of rapid growth such as in infancy, childhood, teen-age years, pregnancy and when breast-feeding. Our needs for proteins increase when we are sick and when we are recovering from an injury or surgery.

Proteins in body tissues are in a constant state of exchange. Some molecules are always breaking down and others are being built as replacements. This constant turnover explains why our diet must supply adequate protein daily even when we no longer need it for growth.

Proteins regulate body processes to maintain fluid balance. Proteins in the blood called albumin and globulin help maintain the body's fluid balance by keeping water in the blood. Blood proteins have the ability to attract and keep fluid in the bloodstream. If a person doesn't eat enough protein, eventually the amount of protein in the blood decreases. Blood pressure then can force excessive fluid out of the blood vessels and into the spaces between the cells. As more and more fluid pools in these spaces, swelling or edema results. Other conditions, such as pregnancy and heart failure, can also lead to edema. If a person suffers from protein malnutrition, if they are fed protein along with other needed nutrients, their bodies can make more blood proteins. The fluid is then attracted back into the bloodstream, and the swelling or edema disappears. Proteins help in the exchange of nutrients between cells and the fluids between the cells. When one has too little protein, the fluid balance of the body is upset, resulting in the tissues holding abnormal amounts of liquid and becoming swollen.

Proteins form hormones and enzymes. Many chemical substances called hormones are proteins. Hormones control such processes as growth, development and reproduction. The thyroid hormone regulates your body's metabolic rate. Insulin hormone regulates the concentration of blood glucose and its transportation into cells, necessary for the brain and nervous system to function.

Almost all enzymes are proteins. Enzymes are proteins. They speed up chemical reactions within every cell. Without enzymes, the cells could not function.

Proteins help form antibodies to fight infection. Other proteins in the body help us fight diseases. Antibodies are proteins in the blood that help protect the body from disease. They are giant protein molecules that circulate in the blood and present a defense against viruses, bacteria and other foreign agents. When your body is invaded by a virus, it enters the cells and multiplies there. If viruses were left free to multiply and be harmful to your body, they would soon overwhelm it with the disease they cause, whether it is a virus that causes flu, measles, smallpox or the common cold.

Once the body has manufactured antibodies against a particular disease agent, such as flu, the cells never forget how to produce them. The next time that virus invades the body, the antibodies will respond even more quickly. This is the way the body acquires immunity against the diseases it is exposed to.

Blood clotting - Blood is a liquid but can turn solid within seconds when you get a cut. When you cut yourself, a fast chain of events leads to the production of fibrin, a stringy, insoluble mass of protein fibers that plugs the cut and stops the leak. Later, more slowly, a scar forms to heal the cut.

Vision - The cells in the retina of the eye contain light-sensitive pigments made up of protein. The protein responds to light by changing its shape, thus beginning the nerve impulses that carry the sense of sight to the higher centers of the brain.

Energy - Proteins can supply your body with energy, but your body prefers to use energy from carbohydrates and fats and save protein for its important functions as discussed above. About 10% of body energy comes from proteins. Most cells more readily use carbohydrates and fats for energy. When you eat more protein than you need, it's broken down and stored as body fat, not as a reserve supply of protein. Be sure to get the calories you need to meet your energy needs in order that your body won't have to use protein for its source of energy.

Proteins are Made of Building Blocks Called Amino Acids

Proteins in food and in your body are made up of 20 different amino acids. The 20 common amino acids that are found in our diets are assembled into the thousands of different proteins needed by the body. Amino acids form the building blocks of proteins. How the amino acids are put together or arranged depends on what kind of protein is made. In just one cell in your body, 10,000 different proteins may exist. Each protein would have a different arrangement of amino acids. All amino acids contain carbon, hydrogen, oxygen and nitrogen. Sometimes they also contain sulfur. A group of amino acids held together by linkages form proteins.

Proteins in food that you eat are broken down inside your body into amino acids. When you eat a protein food, the protein is separated into many clumps of amino acids. The clumps are then separated further into single amino acids, which are absorbed from the intestine and carried by the blood to the liver. As soon as they leave the liver and are carried by the blood to different tissues, they are reassembled into the special combinations that make the proteins to replace cell material that has worn out, to add to tissue which needs to grow, or to make some enzyme or hormone or other active compound.

If any amino acids are left over, they cannot be stored in the body for use at a later time. Instead, they are returned to the liver and stripped of their amino groups. The nitrogen leaves the body mainly as urea through the urine, but the carbon, hydrogen and oxygen fragments that are left can be used to provide energy. If the energy is not needed immediately, the fragments can be converted to fat and stored for use at a later time.