The Functions of Protein

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Why is protein so important to all our science teacher friend s activities? How does his body use the bowl of whole-grain cereal topped with seeds, nuts, and milk that he frequently eats for breakfast? What happens when you eat breakfast like that? What would happen if you didn’t get enough protein? If you got too much?


Let’s look at exactly what happens from the moment you start munching such a protein-rich breakfast.


Chewing is the first step in digestion. Your teeth grind up the solids and mix them with saliva. Your saliva contains an enzyme called ptyalin, which begins to digest the starches with which the proteins are mixed.

Swallowing the food carries it into the stomach, where the protein begins to be digested. The enzyme pepsin, together with gastric juices containing hydrochloric acid, breaks down the protein into its amino acids the form in which the body uses protein. (You see why it doesn’t matter to the body whether the proteins come from a combination of cereal, seeds, and milk, or from a complete source alone like eggs. Once in the stomach, all protein foods just get broken down into their amino acids, anyway.) This particular breakfast also contains iron, calcium, vitamins, and minerals.

The process of breaking down the proteins into constituent amino acids continues in your small intestine. There pancreatic and intestinal proteases carry out the bulk of this process. It is completed in the lining of the intestine.

How fast is the protein absorbed? This depends, partly, on what else is in your stomach and digestive tract. If you’ve been eating mostly vegetable protein foods, it won’t take long the fiber in these foods speeds digestion by cleaning out the digestive tract for efficient functioning. It also depends on the amino acids in the protein itself. Apparently, the various amino acids compete with each other for absorption. If you eat them in the right proportion, the egg white ratio, they are all absorbed together. If not, the ones of which there extra compete to prevent the absorption of those that are lacking, and efficiency is impaired. This may explain why the correct balance of amino acids is so important. Since we are talking about a breakfast of well-balanced, properly combined protein sources, high in fiber, about 92 percent of the protein in it would probably be absorbed.

The capillaries in the walls of your intestines absorb the amino acids from your morning meal and carry them into the bloodstream. Via the portal vein, they are transported to the liver, which distributes them to the many sites where they are needed.

Where are they needed? All over! Protein is one of the most important nutrients. Your muscles are not the only part of the body that needs protein. Every single cell in your body contains protein: hair, nerves, skin, blood, sperm, or eggs. So do almost all the nonliving substances the body produces -enzymes, hormones, blood plasma, even your saliva. In fact, bile, urine, and sweat are the only bodily substances that (in a state of normal health) contain no protein at all.


About half of the solid substance in our bodies is protein. It gives the cell walls and structures inside the cells their shape without which they could not survive.

Although it is strong, protein is also unstable. Your cells undergo a constant cycle of breakdown (catabolism) and rebuilding (anabolism). About half the protein that makes up the liver, for example, is broken down every ten days. The amino acids in the cells have to be constantly replaced, and it is the newly digested amino acids from your food that serves this purpose.

Ideally, your diet provides just enough amino acids to replace the proteins that are broken down. This balance is called a “dynamic equilibrium”: breakdown and reconstruction proceed at the same pace. If you eat too much protein, your body will still maintain its dynamic equilibrium. But the process may be speeded up: amino acids in your cells may be replaced at a faster rate than normal. There is some evidence that this may make you age faster than otherwise. Some excess protein may also be burned as energy or eliminated as waste. In any event, if you eat more protein than you need, your body will have to dispose of extra urea the nitrogen-containing waste product of protein metabolism. Urea is formed in the liver and excreted via the kidneys. This extra work for your liver and kidneys can be stressful it may make you feel fatigued, or as mentioned earlier, it can cause more severe problems, especially if you don’t drink lots of water to help the kidneys filter urea out of the bloodstream.

Not only older people, but infants, too, are especially vulnerable. Infants may become severely dehydrated if their bodies use up too much water flushing out excess urea. You may not feed a little baby meat. But don’t overfeed your child with cow’s milk either it has a lot more protein than human mother’s milk. Giving an infant solid food too young can also dangerously speed up protein metabolism remember, cereals contain protein, too. Dehydration due to excess protein intake can be deadly to an infant.

If you get too little protein, all the cells of your body will suffer (beginning with the least important ones your hair and nails). Protein deficiency, however, is very uncommon in the United States. If anything, we usually eat too much of it compared with other foods. In underdeveloped nations, kwashiorkor, the disease associated with protein deficiency, is more common. Kwashiorkor results in low blood levels of the essential amino acids, among other biochemicals, and often occurs between ages one and four. It causes stunted growth, malnutrition, and in severe cases, death. People whose diets are chronically low in protein may suffer nervous disorders, damage to eyes, teeth, and hearing, and baldness.


In an emergency, if you are not getting enough carbohydrates, your most important source of energy, your body may switch over and oxidize protein as fuel instead. When protein is broken down in this way, it is capable of providing the same caloric value as carbohydrates to fuel the cellular functions that maintain life.

However, to use protein as a fuel is to mismanage your internal energy policy. A meal of half a fried chicken with no fruit, salad, or vegetables forces the body to use protein from the chicken for energy. This requires more energy than breaking down carbohydrates and wastes protein that may be needed for cell repair and physical growth. Disposing of the extra nitrogen from the protein can cause fatigue or worse. This is one reason why high-protein, low-carbohydrate-reducing diets are not advisable. If you want to eat a healthy diet, it is best for 65 to 70 percent of your food to be complex carbohydrates for energy, 20 to 25 percent protein for the raw materials of your tissues, and 10 percent fat.

This balance among fat, carbohydrate, and protein intake is especially important for pregnant women and children. Children are growing all the time: they need to use all the protein they eat for optimal cell growth and repair. If they burn their protein for energy, growth may be partially stunted and resistance to some diseases lowered. Similarly, pregnant women need their protein to satisfy the needs of the fast-developing fetus, whose nourishment comes solely from the mother’s food. Therefore, both protein and carbohydrates must be eaten in good supply during these periods protein for growth, carbohydrates for energy. Eggs, milk, and other nonmeat protein sources should be combined with fresh fruits and vegetables to ensure an adequate supply of both nutrients. (However, for the best food combining system, fruits should be eaten first.) The proper order for maximum digestion, absorption, utilization, and elimination is as follows. First, eat the foods most quickly digested, such as simple sugars found in fruits, juices, and honey. Then the complex carbohydrates, such as bread, whole grains, cereals, pasta, beans, legumes, and soups. (Vegetables raw or steamed can combine easily with these complex carbohydrates.) Last, the longer-digesting proteins and fatty foods, including nuts, cheese, and animal proteins. If you follow this order, you can save yourself a lot of unnecessary indigestion.


Protein helps to control many of the individual processes that must be carried out throughout the body to maintain life. For example, in osmosis, the mechanism by which vital fluids pass through cell walls, protein helps ensure an even balance on both sides. Proteins can adapt themselves to be either base or acid; therefore, they help to protect important body fluids such as blood and gastric juices from pH changes. The biochemistry of our cells is regulated by enzymes. These are made up of proteins, which thus aid thousands of physiological processes. Crucial regulatory hormones such as insulin and adrenaline are mainly protein; so is hemoglobin, the part of the red blood cells that transports oxygen and carbon dioxide. Without hemoglobin, you would suffocate. Without protein, many of these physiological functions lapse into chaos. We cannot afford to live without it.


Your bodily structures, both hard and soft, contain protein. Your toenails, fingernails, bones, and teeth contain protein; so do your hair and blood vessels. The proteins in these structures are held together by bonds that vary according to whether they are hard or soft tissue. The soft group, for example, gets its elasticity from protein. When your hairdresser uses heat to give you a new hairstyle, he or she is breaking down the protein bonds (specifically disulfide bonds) and reforming them in the new style. In muscle, some protein takes the form of the contractile fibers that enable you to move. A huge number of amino acids are found in collagen, the adhesive material that holds individual cells together. Without collagen, you would literally fall apart. These amino acids account for about a third of your body’s total supply.

It is obvious how essential protein is to live.

It has been found by Dr. Kilmer McCully of Harvard Medical School that the real culprit in heart disease is protein and not cholesterol. It seems that homocysteine produced as a result of methionine metabolism is converted to the nontoxic compound cystathionine in the presence of vitamin B6. If there is no B6 present homocysteine will damage the artery wall linings which results in arteriosclerosis and finally heart disease. Therefore, the need for vitamin Be is much greater when you consume a diet largely of animal fat and protein.


Protein food is broken up by your teeth, but pepsin and hydrochloric acid in the stomach begin to break it into its component amino acids. Proteases in the small intestine nearly finish this job, which is completed in the lining of the small intestine prior to absorption.

It is believed that competition for absorption explains why you need that proportion of amino acids in your food we call complete.

Once absorbed into the bloodstream, the amino acids of protein are carried to every single cell, where they are used to rebuild the structure of the cell. As a structural material, it makes up about half the solid substance in our bodies. Some of these cells produce new protein products, including enzymes, hormones, blood plasma, and saliva.

Protein is unstable: it is constantly breaking down (catabolism) and must be replaced. The structure of your body is always being rebuilt (anabolism). There is a “dynamic equilibrium” balancing the pace of breakdown and reconstruction. If you eat too much protein, that can speed up your metabolism. (The increased height of second-generation immigrants to the United States is probably caused by increased protein intake. But greater height has been correlated with shorter life spans.)

Protein from your food is only oxidized as fuel in an emergency when your body is not getting enough carbohydrate from your food; under starvation conditions, the body only touches structural protein after consuming stores of starch and fat.

But using protein as a fuel is mismanaging energy. It takes more energy to use protein as fuel than carbohydrate, and this wastes protein that might be used for cell repair or physical growth. For pregnant women and children, a diet ratio of 65 to 70 percent carbohydrate for energy, 20 to 25 percent protein for anabolism, and 10 percent fat, is especially important.

Excess protein may also be eliminated as waste. Its waste product, urea, formed in the liver and excreted by the kidneys, can be stressful to these organs and should be flushed out by drinking plenty of water. For this reason, special care should be taken not to give infants and older people too much protein.

As a regulator, protein is a component of enzymes, hormones, and numerous complex biochemical processes and protective mechanisms. Protein in various body structures may make them elastic (like muscle), or rigid (like bone), bound together tightly (as in nails), or easily bound and unbound (as in hair). Protein in the form of collagen literally holds your body together.


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