PROTEINS:
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Protein is a necessary component of our diets. Proteins are large, complex molecules that resemble tangled beads strings. Each "bead" on the string is one of a class of smaller molecules known as amino acids. Amino acids are made up of carbon, oxygen, hydrogen, nitrogen, and sulfur in some cases.
The body creates over 50,000 different proteins from the amino acids found in protein. These proteins are the building blocks of our skin, hair, nails, cell membranes, muscles, and connective tissue. Collagen, the main protein in our skin, acts as a barrier against foreign substances. Proteins in cell membranes control which substances can enter and exit cells. Our muscles, which account for approximately 65 percent of the total body weight,Protein is responsible for our bodies' shape and strength. Proteins found in connective tissues such as tendons, ligaments, and cartilage allow our skeletons to function, form internal organs, and hold the organs in place. Proteins in the blood transport oxygen to all cells while also removing carbon dioxide and other wastes. The proteins in muscle, connective tissue, and blood constitute the majority of protein in the body. Other proteins, known as enzymes, speed up metabolic processes, and yet more proteins and amino acids are hormones and neurochemicals, which deliver signals throughout the body and regulate all metabolic processes.
During growth periods, our bodies must manufacture and store a large amount of protein. As a result, the need for protein in our diets increases during growth. Even when we are not growing, each of the body's unique proteins has a finite lifespan and must be replaced on a regular basis. As a result, the need for protein is never satisfied.
Protein in the diet and protein in the body:
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The thousands of proteins that comprise our bodies are assembled on demand from approximately 20 different amino acids. What exactly are amino acids, and where do they come from? The protein from the meat we ate last night does not enter our muscles directly. Proteins in our foods are first digested into small "peptides." Some of these peptides are digested further to form their constituent amino acids. Only amino acids and small peptides are absorbed into the bloodstream by the small intestine. They are then transported to the liver, muscles, brain, and other organs, where they are used to synthesize new proteins or converted to other amino acids required by those organs.
Of the 20 amino acids that comprise all proteins, 9 are considered "essential" because they cannot be produced by our bodies and must be obtained from the foods we consume. Some of the remaining 11 are critical for infants and people suffering from certain diseases (see sidebar: Amino Acid Classification, page 25). The remaining amino acids are considered "necessary" because our bodies can produce adequate amounts if necessary. Nonetheless, they can be easily met by eating a well-balanced diet rich in a variety of foods.
The majority of foods contain protein. Some foods contain more protein than others. "Complete" proteins contain all of the essential amino acids in the amounts required to synthesize our body's proteins.Lean meats and poultry, fish, low-fat dairy products, and eggs are the best sources of complete protein.
The grains and cereals group of foods, which form the foundation of the Food Guide Pyramid, are high in protein, but because they frequently lack one or more essential amino acids, they are referred to as "incomplete" proteins. Corn proteins, for example, are low in the essential amino acids lysine and tryptophan, whereas wheat proteins are low in lysine. Legumes, on the other hand, have a high lysine content but a low methionine content. Soybeans have the most complete protein profile of any legume.Is this to say that you must eat meat, eggs, and dairy products (foods derived from animals) to get all of the amino acids you require? No, not at all. You are more likely to get all of the amino acids you need and in the correct amounts if you eat a variety of foods, including grains and legumes. Vegans (vegetarians who eat no foods of animal origin) and people of various cultures get adequate amounts and types of protein by eating various combinations of plant proteins such as beans, corn, rice, and other cereal grains. Although it was once thought that combining these foods at the same meal was necessary, nutrition experts now agree that they can be eaten at different times throughout the day.
When we eat grains and legumes instead of animal products (which are a more common source of protein in our diets), we gain additional health benefits. Whole grains and legumes are high in vitamins, minerals, fiber, and other nutrients that promote good health. If that isn't enough of a reason to switch, grains and legumes lack the high levels of saturated fat found in animal foods, which, as you'll see below, are linked to a variety of diseases.Contrary to popular belief, simply eating more dietary protein than recommended will not result in bigger muscles. Excess protein is not stored in our bodies. If we consume more protein than our bodies require to replenish the amino acids we used during the day, the extra amino acids are converted to and stored as fat. Protein, like carbohydrates, provides approximately 4 calories of energy per gram. Because our protein requirements are primarily determined by our body size, our protein requirements increase during periods of rapid growth. As a result, protein recommendations vary by age and are slightly higher for pregnant and breastfeeding women than for other adults. The recommended allowances ensure that nearly all healthy people get enough protein.
Despite this, many Americans consume twice as much, often in the form of meat and dairy products high in saturated fat, which increases the risk of coronary artery disease and some forms of cancer. What if we don't get enough protein? Few Americans are at risk of consuming insufficient protein. Protein deficiency can occur in people on severely restricted diets, those who are unable to eat, and those whose needs are increased due to illness or trauma. To replenish depleted pools of essential amino acids needed to make critical proteins such as enzymes and hormones, a protein-deficient person's body begins to rob protein from muscle by digesting that protein to its constituent amino acids. Because muscle is required for a variety of vital functions (for example, diaphragm muscles for breathing and heart muscles for blood pumping), large amounts of muscle protein loss can be fatal. Fortunately, the vast majority of people, including those who engage in regular, strenuous endurance exercise, can easily meet their protein needs by following a balanced diet based on the Food Guide Pyramid.
Protein's Bottom Line:
Adequate protein is essential for growth, metabolism, and health, but eating more protein than we require will not result in larger muscles. Excess protein, on the other hand, is converted to fat. Animal-derived foods are high in protein, but they may also be high in total and saturated fat. Protein is best found in lean meats and dairy products, fish, legumes, and grains.
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