Protein is an essential part of our diets. Proteins are large, complex molecules resembling tangled strings of beads. Each of the "beads" on the string is one of a group of smaller molecules called amino acids. Amino acids are composed of carbon, oxygen, hydrogen, and nitrogen, and some contain sulfur.

Using the amino acids from the protein you eat, the body makes more than 50,000 different proteins. These proteins are the main structural elements of our skin, hair, nails, cell membranes, muscles, and connective tissue. Collagen, the main protein in our skin, provides a barrier to the invasion of foreign substances. Proteins in cell membranes determine what substances can enter and exit cells. Our muscles, which contain some 65 percent of the body’s total protein, give our bodies their shape and strength. Proteins in connective tissues such as tendons, ligaments, and cartilage enable our skeletons to function, form internal organs, and hold the organs in place. Proteins in the blood carry oxygen to all cells and remove carbon dioxide and other waste products. The proteins in muscle, connective tissue, and blood make up most of the protein in the body. Other proteins called enzymes accelerate metabolic processes, and still other proteins and amino acids are hormones and neurochemicals, the substances that deliver signals throughout the body and regulate all metabolic processes.

During periods of growth, our bodies must manufacture and store large amounts of protein. Therefore, the requirement for protein in our diets is higher during growth. But even when we are not growing, each of the unique proteins in the body has a finite lifespan and must be replaced continuously. So the need for protein never ends.

Dietary Protein and Body Protein

The thousands of proteins that make up our bodies are assembled on demand from some 20 different amino acids. What are these amino acids, and where do they come from? The protein from the meat we ate last night is not directly incorporated into our muscles. The proteins in the foods we eat are digested first into small "peptides." Some of these peptides are further digested into their constituent amino acids. Only amino acids and small peptides are actually absorbed by the small intestine into the bloodstream. They are then delivered to the liver, muscles, brain, and other organs, where they are used to make new proteins or converted to other amino acids needed by those organs.

Of the 20 amino acids that make up all proteins, 9 are considered "essential" because they cannot be made in our bodies and must be obtained from the foods we eat. Of the remaining 11, some are essential for infants and persons with certain diseases (see sidebar: Amino Acid Classification, page 25). The rest of the amino acids are considered "nonessential," because our bodies can make them in adequate amounts, if necessary. Nevertheless, they are easily supplied by eating a well-balanced diet that includes a variety of foods.

Most foods contain protein. Some foods are better sources of protein than others. "Complete" proteins are those that contain all the essential amino acids in amounts needed to synthesize our body’s proteins. The best sources of complete protein are lean meats and poultry, fish, low-fat dairy products, and eggs (see Part II, High-Protein Foods, page 291, and Dairy Foods, page 345).

The grains and cereals group of foods, which form the base of the Food Guide Pyramid, are excellent sources of protein, but because these proteins often lack one or more essential amino acids, they are called "incomplete" proteins. For example, the proteins in corn are low in the essential amino acids lysine and tryptophan, and wheat is low in lysine. In contrast, legumes tend to be rich in lysine but a bit low in methionine. Among the legumes, soybeans contain the most complete protein.

Does this mean you must eat meat, eggs, and dairy products (foods of animal origin) to get all the amino acids you need? Not at all. By eating a variety of different foods, including grains and legumes, you are likely to get all the amino acids you need and in the correct amounts. People of many cultures and vegans (vegetarians who eat no foods of animal origin) get adequate amounts and types of protein by eating various combinations of plant proteins including beans, corn, rice, and other cereal grains. Although it was once thought necessary to combine these foods at the same meal, nutrition experts now agree that they can be eaten at various times throughout the day.

When we eat grains and legumes, rather than foods of animal origin (a more frequent source of protein in our diets), we gain additional health benefits. Whole-grain foods and legumes are rich in vitamins, minerals, fiber, and other substances that optimize health. If that does not seem like reason enough to make the trade, grains and legumes lack the high levels of saturated fat present in foods of animal origin, which, as you will learn below, are linked to many diseases.

Contrary to popular belief, simply eating more dietary protein, in excess of recommended amounts, will not result in bigger muscles. Our bodies do not store excess protein. If we eat more protein than our bodies need to replenish the amino acids we have used during the day, the excess amino acids are converted to, and stored as, fat. Dietary protein, like carbohydrates, supplies about 4 calories of energy per gram. Because our requirements for protein mainly depend on our body’s size, our need for protein increases during times of rapid growth. Therefore, the recommendations for protein are age-dependent and are slightly higher for pregnant and breastfeeding women than for other adults.The recommended allowances ensure an adequate protein intake by nearly all healthy people. Nevertheless, many Americans typically consume twice this amount, often in the form of meat and dairy products that are high in saturated fat, which increases the risk for coronary artery disease and some forms of cancer.

Amino Acid Classification

Of the 20 amino acids that make up all proteins, only 9 are considered "essential" in our diets because they cannot be made by our bodies and must be obtained from the foods we eat.

Essential in Our Diets

Essential in Our Diets Under Some Circumstances

Nonessential in Our Diets









Aspartic acid



Glutamic acid
















*Cysteine can be synthesized from the essential amino acid methionine.

fTyrosine can be synthesized from phenylalanine when supplied in adequate amounts.

What if we eat too little protein? Few Americans are at risk of eating too little protein. However, individuals on severely restricted diets, those who are unable to eat, and those whose needs are increased because of illness or trauma may experience protein deficiency. To replenish the pools of essential amino acids that have been depleted, in order to make critical proteins such as enzymes and hormones, the body of a protein-deficient person begins to rob protein from muscle by digesting that protein to its constituent amino acids. Because muscle is needed for various vital functions (for example, diaphragm muscles for breathing and heart muscles for pumping our blood), the loss of large amounts of muscle protein can be fatal. Fortunately, the vast majority of people, even those who engage in regular, rigorous endurance exercise, can easily meet their need for protein by eating a balanced diet based on the Food Guide Pyramid.

The Bottom Line on Protein

Adequate protein is critical for growth, metabolism, and health, but eating more protein than we need will not build bigger muscles. Conversely, excess protein is converted to fat. Foods of animal origin are high in protein but may also be high in total and saturated fat. Lean meats and dairy products, fish, legumes, and grains are the best sources of protein.

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