In this article:
|What You Will Learn|
|What Is Protein?|
|What Proteins Do|
|Amino Acids in Proteins|
What do you think of when you hear the word protein? Meat? Beans? Strong? Life? Many people think of muscle and fitness. Proteins do have something to do with life and vitality because they are a necessary component of every cell. Proteins are necessary for a person to grow and to fight infection and disease.
You are made up of protein. Your body parts are made up of proteins – even the inside parts you can't see. Genes, hormones and enzymes are also proteins.
The Greek word protein means "first place." Sometimes we place a lot of importance on proteins over the other two classes of nutrients that give us energy – carbohydrates and fat. We may have grown up thinking that a meal isn't a meal unless it includes some meat. In planning our meals, we might first think of what meat we will have and then select other foods to go with it. We now know we need to plan our meals in reverse – plan a meal to include complex carbohydrates, such as whole-grain breads and cereals, vegetables, fruit, milk and then maybe add a little meat. A healthy diet is based on foods that come from plants; adding a little animal protein improves the protein quality.
There are many different kinds of proteins. All of the nutrients that give us energy – such as protein, carbohydrates and fat – are made up of carbon, hydrogen and oxygen. But only proteins have nitrogen. This makes the structure and role in health special. Nitrogen is essential for life.
In this lesson, you will learn that protein is a nutrient. You will learn about how it is put together (structure) and what its functions are. You will learn which foods provide protein, including animal and plant foods, and which foods are the best sources of protein for your body. With this information, you will be able to decide which foods will be the best sources of protein for your family.
MyPlate can help you see what foods you and your family should eat each day for good health. Foods that make similar nutritional contributions are grouped into basic food groups. It also tells you how much food is needed each day.
MyPlate recommends getting the most nutrients from plant foods – vegetables, fruits and whole-grain foods, such as breads and cereals. It is also recommended that you consume low-fat or fat-free milk, yogurt or cheese every day. Although animal products make important contributions to our diet, they are the source of saturated fat. Eating the recommended number of calories daily and making lower-fat choices help you avoid getting too much saturated fat in your diet. We need to eat a little meat with a lot of plant foods. Selecting plant foods high in protein is healthful.
Protein is one of the three macronutrients your body needs for survival. Macronutrients are nutrients needed in large amounts. The other two macronutrients are carbohydrates and fat. Proteins supply the same amount of energy as carbohydrates. One gram of protein provides four calories. The body's primary need is for energy. It will ignore the special functions of proteins if it needs energy and no other source is available. But we don't want to have to rely on proteins for energy. Proteins need to be used for other important functions, such as body building, tissue repair and maintenance. Getting the amount of carbohydrates we need is important so the proteins won't be used as a source of energy.
Proteins are part of every living cell. Many different kinds of proteins form vital parts of the body. Examples include muscles, skin, proteins in blood, enzymes and hormones.
If you don't count water, protein is the most plentiful substance in your body. About half of your dry weight is represented by proteins. About one-third of protein is in muscle, and 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. 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.
Growth and maintenance - We need proteins 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, teenage years, pregnancy and when breast-feeding. Our need for protein increases 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 broken 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.
Fluid balance - Proteins regulate body processes to maintain fluid balance. Proteins in the blood are called albumin and globulin, and they 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, also can lead to edema. If a person suffers from protein malnutrition and is 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.
Hormones and enzymes - 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, which is necessary for the brain and nervous system to function.
Almost all enzymes are proteins. They speed chemical reactions within every cell. Without enzymes, the cells could not function.
Immune system - Proteins help form antibodies to fight infection. 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 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 save protein for its important functions as discussed above. About 10 percent of body energy comes from proteins. Most cells more readily use carbohydrates and fats for energy. Be sure to get the calories you need to meet your energy needs so 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 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 a protein.
Proteins in food 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, add to tissue which needs to grow or 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 later. Instead, they are returned to the liver.
The kinds and amounts of amino acids in a protein determine its nutritive value. We get protein from both animal and plant foods. During the Stone Age, our ancestors got most of their protein from plants. Much later, our ancestors began eating meat. Today, most of the proteins we eat come from animal products.
Some amino acids are "essential" (required in the diet) and some are "nonessential" (not a required part of the diet). Essential amino acids cannot be formed in the body, so you need to get them from the diet.
Animal proteins, such as animal muscle (meats), milk and eggs, can supply all of the amino acids in about the same proportions in which they are needed. These are rated as having a high nutritive value. Animal proteins are considered high-quality proteins or complete proteins. They can support body growth and maintenance because they contain all of the essential amino acids in sufficient amounts. Animal-based foods, such as meat, poultry, fish, milk, eggs and cheese are considered complete protein sources.
Plant proteins are usually thought of as "low-quality proteins" or incomplete proteins. They lack one or more essential amino acids. However, appropriate combinations of plant foods can supply sufficient quantities of all the essential amino acids.
Complementary proteins are two or more incomplete protein sources that together provide adequate amounts of all the essential amino acids. For example, rice contains low amounts of certain essential amino acids. These same essential amino acids, however, are found in greater amounts in dry beans. Conversely, dry beans contain lower amounts of other essential amino acids that are not found in sufficient amounts in rice. These two foods together can provide adequate amounts of all the essential amino acids the body needs. If animal-based foods are not an item on your grocery list, then you can get your protein needs met by eating a variety of protein-containing plant foods each day. So when you eat a meal of red beans and rice, you do not need a glass of skim milk to round out the protein you need. You do not need to have ham or other meat with the red beans, either.
In the past, it was thought that complementary proteins needed to be eaten at the same meal for your body to use them together. Studies now show that your body can combine complementary proteins that are eaten within the same day.
As a rule, since Americans regularly eat foods with proteins of high nutritive value, they don't need to be concerned about the adequacy of the amino acids they get. Rather, the concern is with eating too much protein from animal sources, which generally are more expensive and are higher in saturated fats than plant sources. Since meats contribute so much saturated fat to our diet, it is best to select lean cuts of meat.