When you don’t consume enough calories to meet your needs, your body uses stored fat and carbohydrate to provide most of the additional energy it needs. But some protein is also broken down to provide energy and to make glucose. Since our bodies do not store protein, functional body proteins, such as those that make up enzymes, transport proteins, and muscles, must be broken down to yield amino acids that can then be used as fuel or to make glucose. This provides cells with a constant fuel supply but also robs the body of the functions of these proteins. In order for your body to use amino acids for energy, the nitrogen-constraining amino group must be removed from the amino acids by a process called deamination. The removed nitrogen is converted into the waste product urea; urea is then taken out of the blood by the kidneys and excreted in the urine. The compounds remaining after the amino group is removed can be used to produce ATP or to make glucose.

Amino acids are also used for energy when the amount of protein consumed in the diet is greater than that needed to make body proteins and other molecules. This occurs in most of us every day because our typical diets contain about twice as much protein as we need. The body first uses amino acids from the diet to make body proteins. Then, sense extra amino acids can’t be stored, they are used for energy. If your diet doesn’t provide enough carbohydrate, amino acids can be used to synthesize glucose. If your diet includes more calories than you need, amino acids can be converted into fatty acids and can contribute to weight again.

Then you think of the protein in your body, you probably think of muscle, but muscle contains only a few of the many types of proteins in your body. Like the protein in foods, protein in the body is not single substance. There may be as many as 100.000 different proteins in the human body, each with a unique function. Some have important structural=2 0roles and others help regulate body processes.

Proteins provide structure to the body at many levels. The most visible example is the protein in muscles. The large chest and biceps of a weight lifter or the massive thighs of a speed skater are due to the extra muscle proteins that these athletes have built by exercising those muscles. Skin and hair are also composed largely of protein. The most abundant protein in the body is collagen; it holds cells together and forms the protein framework of bones and teeth. Protein also provides structure to individual calls; it is found in membranes, fluids, and organelles.

When the diet is deficient in protein, protein-based structures break down. The muscles become smaller, the skin loses its elasticity, and the hair becomes thin and can easily be pulled out by the roots. These outward signs of dietary protein deficiency have become marketing strategies for cosmetic companies. Shampoo and hand lotion manufacturers add protein to their products, suggesting that protein applied to the hair or skin will improve its structure. However, the proteins that make up hair and skin and other body structures can be made only in side the body. So a healthy diet will do more for hair and skin quality than expensive protein shampoo or lotions.

The chemical reactions that break down molecules to produce energy and build molecules needed by the body require the help of enzymes. Enzymes are proteins that speed up metabolic reactions but are not used up or destroyed in the process. Without enzymes, metabolic reactions would occur too slowly to support life. Each of the reactions involved in the production of ATP and the synthesis and breakdown of carbohydrates, lipids, and proteins requires a specific enzyme whit a specific structure. If the structure of the enzyme molecule is changed, it can no longer function in the reaction it is designed to accelerate.

Enzymes that function in the body are made by the body and therefore do not need to be consumed in the diet. The enzymes that we do consume in foods are broken down during digestion and are from the gastrointestinal trac t as amino acids. Purified enzymes sold as dietary supplements are also broken down in the gut. These may provide some function before they are broken down. For example, people with lactose intolerance often take lactase, the enzyme that breaks down milk sugar. Lactose supplements break down lactose that is consumed while the lactose is in the gut. Eventually, these enzymes are also digested and absorbed as amino acids.

Transport proteins act shuttles. In the blood they carry substances from one organ to another. For example, hemoglobin, the protein that gives red blood cells their red color, picks up oxygen in the lungs and transports it to other organs of the body. The proteins in lipoproteins are needed to transport lipids from the intestines and liver to body cells. Some vitamins, such as vitamin A, require specific proteins to be transported in the blood. When protein is deficient, the vitamin A transport protein is not available and tissues become deficient in vitamin A even if it is consumed in the diet. For this reason, a protein deficiency can cause a vitamin A deficiency.

Some transport proteins shuttle substances across cell membranes to get them into and out of individual cells. For instance, proteins are needed to move glucose across the cell membrane. Transport proteins in the intestinal mucosa are needed to absorb amino acids from the intestinal lumen.

Proteins are an important part of the body’s defense mechanisms. Skin, which is made up primarily of protein, is the first barrier against infection and injury. Foreign particles such as dirt or bacteria that are on the skin cannot enter the body and can be washed away. If the skin is broken and blood vessels are injured, blood-clotting proteins help prevent too much blood from being lost. If a foreign particle such as a virus or bacterium enters the body, the immune system fights it off by synthesizing proteins called antibodies. Each antibody has a unique structure that allows it to attach to a specific invader. When an antibody binds to an invading substance, the production of more of the same type of antibody is stimulated, and other parts of the immune system are signaled to help destroy the invader. The next time the type of invading. The next time the same type of invading bacterium or virus enters the body, the immune system is already prim ed to produce specific antibodies to fight off the infection. This is also how immunizations against diseases, such as measles, work. A small amount of dead or inactivated virus is injected into the body; the injected material does not cause disease, but it does stimulate the immune system to produce antibodies to the virus. When the body comes in contact whit the live virus, a large-scale immune attack is mounted and the infection is prevented. When the immune system malfunctions as a result of protein deficiency or other causes, such as HIV infection, the ability to protect the body from infection is compromised.

Some proteins give cells and organism the ability to move, contract, and change shape. When you climb a flight of stairs, walk across the room, or run around the block, you are relying on the muscle proteins actin and myosin. These two proteins slide past each other to shorten the muscle and cause contraction. A similar process causes contraction in the muscle and in the muscles of the digestive tract, blood vessels, and body glands. Actin and myosin can cause contraction in non-muscle cells. This contraction helps individual cells, such as white blood cells of the immune system, change shape and move. The energy for cont raction comes from ATP, which is derived primarily from the metabolism of carbohydrate and fat.

Hormones are chemical messengers that are secreted into the blood by one tissue or organ and act on target cells in other parts of the body. Hormones made from cholesterol are steroid hormones; those made of amino acids are classified as protein or peptide hormones. Insulin and glucagons are protein hormones involved in maintaining a steady level of blood glucose.

The distribution of body cells, in the bloodstream, and in the spaces between cells is important for homeostasis. Fluid moves back and forth across membranes to maintain appropriate concentrations of particles and fluids insides and outside cells and tissue. Proteins help regulate this fluid balance in two ways. First, protein transporters located in the cell membranes pump particles from one side of a membrane to another. Fluid then follows to keep the concentration of particles equal. Second, large protein molecules present in the blood keep fluid in the blood, both by=2 0preventing it from being forced into tissues and by attracting fluid in tissues back into blood vessels.

The chemical reactions of metabolism require a specific level of acidity, or pH, to function properly. In the gastrointestinal tract, acid levels vary widely. The digestive enzyme pepsin works best in the acid environment of the stomach, whereas the pancreatic enzymes operate best in the more neutral environment of the small intestine. Inside the body, pH must be maintained at a relatively neutral level in order to allow metabolic reactions to proceed normally. If the pH changes, these reactions slow or stop. For instance, during strenuous exercise, lactic acid builds up in the muscles, making them more acidic. The increased acidity makes the metabolic reactions needed to fuel activity less efficient and contributes to fatigue. Proteins both within cells and in the blood help prevent large changes in acidity. For instance, the protein hemoglobin in red blood cells helps neutralize acid produced when carbon dioxide, a waste product of cells neutralize acid produced when carbon dioxide, a waste product of cellular respiration, reacts with water.

We need to eat protein to stay healthy. If we don’t eat enough, body proteins that are broken down each day cant replaced. The consequences of too little protein can be dramatic and devastating. But is there such a thing as too much protein? We plan our meals around meat, buy high-protein bars, and drink protein-rich shakes and smoothies.

"Nutrition everyday choices" by Crosvenor / Smolin