Proteins are nutrients. Along with carbohydrates and fat, your body needs protein, a nutrient made up of essential and nonessential amino acids, for good health. Your body manufactures 13 nonessential amino acids, which aren't available from food. For the body to process protein properly, the foods that you eat must contain the nine essential amino acids that are available only from dietary sources.
Most of us recognize the term protein in a nutritional context as referring to a class of foods that includes meats, dairy products, eggs, and other items. Certainly, proteins are an important part of nutrition, and obtaining complete proteins in one's diet is essential to the proper functioning of the body. But the significance of proteins extends far beyond the dining table. Vast molecules built from enormous chains of amino acids, proteins are essential building blocks for living systems—hence their name, drawn from the Greek proteios, or "holding first place." Proteins are integral to the formation of DNA, a molecule that contains genetic codes for inheritance, and of hormones. Most of the dry weight of the human body and the bodies of other animals is made of protein, as is a vast range of things with which we come into contact on a daily basis. In addition, a special type of protein called an enzyme has still more applications.
How does your body use protein from food?
Protein helps to maintain and replace the tissues in your body, and it's found in almost every living cell and fluid. Your muscles, organs and many of your hormones are made up of protein, and it is also used in the manufacture of hemoglobin, the red blood cells that carry oxygen to your body. Protein is also used to manufacture antibodies that fight infection and disease and is integral to your body's blood clotting ability. Both children and adults need plenty of protein to grow and develop.
Good Sources of Protein
Good low- or nonfat sources of protein include:
* Beef, poultry, pork and lamb
* Fish and shellfish
* Dairy products, including cottage cheese, cheese, yogurt and milk
* Eggs, egg whites or egg substitutes
* Dry beans, peas, oats and legumes
* Tofu and soy products
* Nuts and seeds
Proteins are considered either complete proteins (which supply enough essential amino acids) or incomplete proteins (which lack adequate essential amino acids). Meat, eggs and dairy products are considered complete proteins, but vegetables, beans and other plant products are considered incomplete proteins. However, some incomplete proteins can be combined to create a complete protein - rice and beans, peanut butter and jelly, and corn and beans are examples of complete-protein meals.
How Much Protein Do I Need?
Your protein intake will be dependent upon your age, your medical condition, your activity level and your size. The Food Guide Pyramid recommends that for most adults, two to three servings of protein a day is adequate.
Some common serving sizes of protein include:
* 3 to 4 ounces of cooked lean meat, poultry and fish (a portion about the size of a deck of playing cards)
* 1/2 cup of cooked dry beans, lentils or legumes
* 1 egg or 2 tablespoons of peanut butter, which count as 1 ounce of lean meat
If you eat a diet low in fat, choose low-fat protein portions such as fish, shellfish, beans, egg substitutes and nonfat milk products.
The Role of Protein in Special Diets
Although many good sources of protein are found in meat or animal products, vegetarians can still consume adequate amounts of protein. Vegetarians who eat dairy products and eggs can still choose from a variety of plant and animal protein sources. Vegans who eat only plant sources of food can still rely on tofu, soy products, oats, beans, lentils and peanut butter for protein.
People who eat too much protein may be at risk for high cholesterol or gout, a joint disorder. High-protein diets, such as the Atkins Diet and Protein Power, have also been implicated in kidney problems because of the extra effort the body must expend to process large amounts of protein. High-protein diets may also be high in fat and may lead to heart disease, according to the American Heart Association.
If you are concerned that you aren't getting enough protein in your diet, consult your physician or a registered dietitian for dietary help.
More Protein May Mean Stronger Bones
Both protein and calcium are part of a balanced diet and necessary for good health. Protein helps your body maintain tissue and provides a form of energy, whereas calcium works to maintain strong bones and regulate hormones. However, because it's not known how protein intake affects bone mass, a study recently published in the American Journal of Clinical Nutrition investigated whether taking calcium and vitamin D supplements affected the bone mass and protein intake in 342 senior men and women.
The participants took either a supplement containing calcium and vitamin D or a sugar pill placebo for three years. Halfway through the study, the participants reported their food intake (including protein). The participants' bone mineral density (a test that shows the strength of a person's bones) was measured throughout the study.
The people in the study who took calcium supplements and had higher protein intakes had greater increases in overall bone mineral density than the people taking the placebo. As a result researchers concluded that people who take vitamin D and calcium supplements to increase bone mineral density may benefit from increasing protein intake, too.
Nutrition Fact Sheet: Proteins
The building blocks of human proteins are twenty amino acids that may be consumed from both plant and animal sources. Of these 20 amino acids, 9 are considered to be essential because their carbon skeletons cannot be synthesized by human enzymes . The remaining "nonessential" amino acids can be synthesized endogenously with transfer of amino groups to carbon compounds that are formed as intermediates of glucose (glucogenic amino acids) and lipid (ketogenic amino acids) metabolism.
Protein is the basic structural material of all cells. Biologically active proteins include enzymes, immunoglobulins, hormones, neurotransmitters, nutrient transport and storage compounds, and cell membrane receptors. Plasma proteins (e.g., albumin) contribute to oncotic pressure that directs the flow of fluid and metabolic waste from the intracellular compartment into the capillary venules. These proteins (e.g., hemoglobin) also contribute to plasma buffering capacity and oxygen-carbon dioxide transport (e.g., hemoglobin, myoglobin). Acute phase reactant proteins (e.g., ferritin, prealbumin) secreted by the liver bind minerals such as iron and zinc rendering them unavailable to support microbial proliferation.
Biological value of a dietary protein is determined by the amount and proportion of essential amino acids it provides. If any one of the essential amino acids is not available in sufficient amounts or is present in excessive amounts relative to other essential amino acids, protein synthesis will not be supported. Under these circumstances, labile body proteins such as plasma albumin will be catabolized to provide the limiting amino acid so that protein synthesis may continue.
Protein from animal sources (meat, fish, dairy products, egg white) is considered high biological value protein or a "complete" protein because all nine essential amino acids are present in these proteins. An exception to this rule is collagen-derived gelatin which is lacking
Plant sources of protein (grains, legumes, nuts, and seeds) generally do not contain sufficient amounts of one or more of the essential amino acids. Thus protein synthesis can occur only to the extent that the limiting amino acids are available. These proteins are considered to have intermediate biological value or to be partially complete because, although consumed alone they do not meet the requirements for essential amino acids, they can be combined to provide amounts and proportions of essential amino acids equivalent to high biological proteins from animal sources.
Plants that are entirely lacking in essential amino acids are considered incomplete proteins or sources of low biological value protein. These sources include most fruits and vegetables. A low biological value means that it is difficult or impossible to compensate for insufficient amounts of essential amino acids by combining different sources as with partially complete proteins.
Classification of Amino Acids
Essential Amino Acids Nonessential Amino Acids 1. Histidine 1. Alanine 2. Isoleucine 2. Arginine* 3. Leucine 3. Aspartic acid 4. Lysine 4. Cysteine* 5. Methionine 5. Cystine 6. Phenylalanine 6. Glutamic acid 7. Threonine 7. Glutamine* 8. Tryptophan 8. Glycine 9. Valine 9. Proline 10. Serine 11. Tyrosine
*These amino acids, along with taurine, may be considered conditionally essential in that their requirements are increased during periods of catabolic stress.
If protein needs are not adequately met by dietary sources, an imbalance may develop. This imbalance is reflected by levels of urinary nitrogen which exceed the amounts being consumed from dietary protein. This increase in urinary nitrogen is due to the catabolism of visceral proteins and lean body mass to provide the essential amino acids that
are not available in adequate amounts from dietary sources. Negative nitrogen balance may result from consumption of insufficient quantity of high biological protein, consumption of poor quality dietary protein of any quantity, or consumption of intermediate quality protein sources that are not appropriately mixed because the quantities of essential amino acids consumed will not be sufficient to support demand for synthesis of vital proteins. In addition to appropriate quantity and
quality of protein consumed, sufficient energy must also be consumed to support protein metabolism or negative nitrogen balance will develop regardless of the quality or quantity of protein consumed.
Protein malnutrition or kwashiokor is the clinical consequence of uncorrected negative nitrogen balance. Protein deficiencies rarely occur when energy intake is adequate except in impoverished areas where adequate quality or quantity of protein is not consumed due to high costs of protein sources. The most common cause of protein deficiency
insufficient energy intake, which is exacerbated when demand for both protein and energy is high. Protein-energy malnutrition (PEM) or marasmus may develop clinically from malabsorption syndrome, with excessive protein losses from burns, wound exudates, or fistula drainage, or with losses in urine from renal disease. Risk of PEM is
also increased under conditions of metabolic stress, such as infection, trauma, burns, AIDS and surgery, where high levels of catabolic hormones increase protein catabolism. Clinical features of PEM include weight loss, diarrhea, loss of lean body mass, muscle weakness, depigmented hair and skin, pressure sores, and depressed immune
Dietary protein consumed in excess of requirements is not stored, but is deaminated followed by oxidation of the carbon skeleton through pathways of glucose or fat metabolism, or its storage as glycogen or fat, depending upon the specific amino acid and the energy balance at the time. The nitrogen waste generated is excreted in the urine as either urea or ammonia.
High protein intakes can increase urinary calcium excretion, but the effect on calcium balance is controversial since amino acids also increase the efficiency of intestinal absorption. Other health effects of high protein intakes are less clear including the relationship of long-term high protein intakes to risk of renal disease or of diabetic nephropathy.
The effect of exercise on protein requirements is not as much as commonly believed. Endurance athletes actually have a higher requirement than body-builders due to catabolic losses of lean body mass following aerobic exercise. Nevertheless, this increased requirement can be readily met without supplementation when the high energy intakes required by athletes are consumed. Use of amino acid supplements may actually interferes with synthesis of body protein by creating imbalances. Since amino acids compete for absorption, presentation of large quantities of free amino acids to the intestinal mucosal surface reduces the amount that can be absorbed from the available supply.
Approximately 10-15% of total daily energy intake should be consumed as protein. Protein needs for sedentary adults average about 50 grams. Growth, pregnancy, lactation, and exercise increase protein needs as indicated in the table below.
Protein Requirements Infants (0-6 months) g/lb 1.0 Infants (6-12 months) g/lb 0.72 Children (1-3 years) g/lb 0.55 Children (4-6 years) g/lb 0.50 Children (7-10 years) g/lb 0.45 Adolescence (11-14 years) total g/day 46 Adolescence (15-18 years)total g/day 44-59 Young adults (19-24 years)total g/day 46-58 Pregnancy total g/day 60 Lactation total g/day 65 Sedentary Adult g/lb 0.4 Recreational Activity 0.5-0.75 Competitive Athletics g/lb 0.6-0.9 Muscle Building g/lb 0.7-0.9 Maximum Usable Amount: 1 gram/pound body weight
References: Mahan, L.K. and Escott-Stump, S. Krause's Food,
Nutrition & Diet Therapy, 10th ed., 2000.ð Rosenbloom, Christine.
Sports Nutrition. A Guide for Working Professionals, 3rd ed.,
Dietary Sources of Protein
Meat, poultry and fish are rich sources of high biological value protein. Plant sources of protein (legumes, nuts, and seeds) contribute additional amounts of protein. See the table below for a detailed list of dietary protein sources.
Food Protein (grams) Food Protein(grams) Dairy Meat Substitutes Skim milk, 1 cup 8.3 Tofu, 3 oz 6.9 Whole milk, 1 cup 8.0 Veggie burger, 3 oz 25.7 Ice cream, 1 cup 5.0 Peanut butter, 2 Tbl 8.1 Yogurt, low-fat, 1 cup 10.7 Almonds, 1 oz 5.4 Cottage cheese,1 cup 28.0 Sesame seeds, 1 oz 7.5 American cheese, 1 oz 7.0 Black beans, 1/2 cup 7.5 Egg, 1 large 6.3 Pinto beans, 1/2 cup 7.0 Fish, Meat & Poultry Garbanzo beans, 1/2 cup 7.3 Tuna, 3 oz drained 21.7 Fruits Salmon, 3 oz ckd 16.8 Banana, 1 medium 1.2 Ground beef, 3 oz 25.7 Apple, large 0 Beef, 3 oz ckd 27.0 Orange, large 1.7 Pork chop, 3 oz ckd 24.5 Vegetables Ham, 1 oz 5.9 Corn, ckd, 1/2 cup 2.2 Chicken breast, 3 oz 18.9 Carrots, ckd, 1/2 cup 0.8 Chicken, dark meat, 3 oz 23.6 Green beans, ckd, 1/2 cup 1.0 Turkey breast, 3 oz 25.7 Green peas, ckd, 1/2 cup 4.1 Turkey, dark meat, 3 oz 24.3 Potatoes, white, 1/2 cup 1.2
Science of Everyday Things: Proteins
How It Works
The Complexities of Biochemistry
Protein is a foundational material in the structure of most living things, and as such it is rather like concrete or steel. Just as concrete is a mixture of other ingredients and steel is an alloy of iron and carbon, proteins, too, are made of something more basic: amino acids. These are organic compounds made of carbon, hydrogen, oxygen, nitrogen, and (in some cases) sulfur bonded in characteristic formations.
Amino acids are discussed in more depth within the essay devoted to that topic, though, as noted in that essay, it is impossible to treat such a subject thoroughly without going into an extraordinarily lengthy and technical discussion. Such is the case with many topics in biochemistry, the area of the biological sciences concerned with the chemical substances and processes in organisms: the deeper within the structure of things one goes, and the smaller the items under investigation, the more complex are the properties and interactions.
Amino acids react with each other to form a bond, called a peptide linkage, between the carboxyl group of one amino acid (symbolized as-COOH) and the amino group (-NH2) of a second amino acid. In this way they can make large, chainlike molecules called polymers, which may contain as few as two or as many as 3,000 amino-acid units. If there are more than 10 units in a chain, the chain is called a polypeptide, while a chain with 50 or more amino-acid units is known as a protein.
All the millions of different proteins in living things are formed by the bonding of only 20 amino acids into long polymer chains. Because each amino acid can be used many times along the chain, and because there are no restrictions on the length of the chain, the number of possible combinations for the creation of proteins is truly enormous: about two quadrillion, or 2,000,000,000,000,000. Just as not all sequences of letters make sense, however, not all sequences of amino acids produce functioning proteins. In fact, the number of proteins that have significance in the functioning of nature is closer to about 100,000. This number is considerably smaller than two quadrillion—about 1/2,000,000,000th of that larger number, in fact—but it is still a very large number.
Components Other Than Amino Acids
The specific properties of each kind of protein are largely dependent on the kind and sequence of the amino acids in it, yet many proteins include components other than amino acids. For example, some may have sugar molecules (sugars are discussed in the essay on Carbohydrates) chemically attached. Exactly which types of sugars are attached and where on the protein chain attachment occurs vary with the specific protein. Other proteins may have lipid, or fat, molecules chemically bonded to them. Sugar and lipid molecules always are added when synthesis of the protein's amino-acid chain is complete. Many other types of substance, including metals, also may be associated with proteins; for instance, hemoglobin, a pigment in red blood cells that is responsible for transporting oxygen to the tissues and removing carbon dioxide from them, is a protein that contains an iron atom.
Structures and Synthesis
Protein structures generally are described at four levels: primary, secondary, tertiary, and quaternary. Primary structure is simply the two-dimensional linear sequence of amino acids in the peptide chain. Secondary and tertiary structures both refer to the three-dimensional shape into which a protein chain folds. The distinction between the two is partly historical: secondary structures are those that were first discerned by scientists of the 1950s, using the techniques and knowledge available then, whereas an awareness of tertiary structure emerged only later. Finally, quaternary structure indicates the way in which many protein chains associate with one another. For example, hemoglobin consists of four protein chains (spirals, actually) of two slightly different types, all attached to an iron atom.
Protein synthesis is the process whereby proteins are produced, or synthesized, in living things according to "directions" given by DNA (deoxyribonucleic acid) and carried out by RNA (ribonucleic acid) and other proteins. As suggested earlier, this is an extraordinarily complex process that we do not attempt to discuss here. Following synthesis, proteins fold up into an essentially compact three-dimensional shape, which is their tertiary structure.
The steps involved in folding and the shape that finally results are determined by such chemical properties as hydrogen bonds, electrical attraction between positively and negatively charged side chains, and the interaction between polar and nonpolar molecules. Non Polar molecules are called hydrophobic, or "water-fearing," because they do not mix with water but instead mix with oils and other substances in which the electric charges are more or less evenly distributed on the molecule. Polar molecules, on the other hand, are termed hydrophilic, or "water-loving," and mix with water and water-based substances in which the opposing electric charges occupy separate sides, or ends, of the molecule. Typically, hydrophobic amino-acid side chains tend to be on the interior of a protein, while hydrophilic ones appear on the exterior.
Proteins Are Everywhere
Although it is very difficult to discuss the functions of proteins in simple terms, and it is similarly challenging to explain exactly how they function in everyday life, it is not hard at all to name quite a few areas in which these highly important compounds are applied. As we noted earlier, much of our bodies' dry weight—that is, the weight other than water, which accounts for a large percentage of the total—is protein. Our bones, for instance, are about one-fourth protein, and protein makes up a very high percentage of the material in our organs (including the skin), glands, and bodily fluids.
Humans are certainly not the only organisms composed largely of protein: the entire animal world, including the animals we eat and the microbes that enter our bodies (see Digestion and Parasites and Parasitology) likewise is constituted largely of protein. In addition, a whole host of animal products, including leather and wool, are nearly pure protein. So, too, are other, less widely used animal products, such as hormones for the treatment of certain conditions—for example, insulin, which keeps people with diabetes alive and which usually is harvested from the bodies of mammals.
Proteins allow cells to detect and react to hormones and toxins in their surroundings, and as the chief ingredient in antibodies, which help us resist infection, they play a part in protecting our bodies against foreign invaders. The lack of specific proteins in the brain may be linked to such mysterious, terrifying conditions as Alzheimer and Creutzfeldt-Jakob diseases (discussed in Disease). Found in every cell and tissue and composing the bulk of our bodies' structure, proteins are everywhere, promoting growth and repairing bone, muscles, tissues, blood, and organs.
One particularly important type of protein is an enzyme, discussed in the essay on that topic. Enzymes make possible a host of bodily processes, in part by serving as catalysts, or substances that speed up a chemical reaction without actually participating in, or being consumed by, that reaction. Enzymes enable complex, life-sustaining reactions in the human body—reactions that would be too slow at ordinary body temperatures—and they manage to do so without forcing the body to undergo harmful increases in temperature. They also are involved in fermentation, a process with applications in areas ranging from baking bread to reducing the toxic content of wastewater. (For much more on these subjects, see Enzymes.)
Inside the body, enzymes and other proteins have roles in digesting foods and turning the nutrients in them—including proteins—into energy. They also move molecules around within our cells to serve an array of needs and allow healthful substances, such as oxygen, to pass through cell membranes while keeping harmful ones out. Proteins in the chemical known as chlorophyll facilitate an exceptionally important natural process, photosynthesis, discussed briefly in Carbohydrates.
Proteins, Blood, and Crime
The four blood types (A, B, AB, and O) are differentiated on the basis of the proteins present in each. This is only one of many key roles that proteins play where blood is concerned. If certain proteins are missing, or if the wrong proteins are present, blood will fail to clot properly, and cuts will refuse to heal. For sufferers of the condition known as hemophilia, caused by a lack of the proteins needed for clotting, a simple cut can be fatal.
Similarly, proteins play a critical role in forensic science, or the application of medical and biological knowledge to criminal investigations. Fingerprints are an expression of our DNA, which is linked closely with the operation of proteins in our bodies. The presence of DNA in bodily fluids, such as blood, semen, sweat, and saliva, makes it possible to determine the identity of the individual who perpetrated a crime or of others who were present at the scene. In addition, a chemical known as luminol assists police in the investigation of possible crime scenes. If blood has ever been shed in a particular area, such as on a carpet, no matter how carefully the perpetrators try to conceal or eradicate the stain, it can be detected. The key is luminol, which reacts to hemoglobin in the blood, making it visible to investigators. This chemical, developed during the 1980s, has been used to put many a killer behind bars.
These are just a very few of the many applications of proteins, including a very familiar one, discussed in more depth at the conclusion of this essay: nutrition. Given the importance and complexity of proteins, it might be hard to imagine that they can be produced artificially, but, in fact, such production is taking place at the cutting edge of biochemistry today, in the field of "designer proteins."
Many such designs involve making small changes in already existing proteins: for example, by changing three amino acids in an enzyme often used to improve detergents' cleaning power, commercial biochemists have doubled the enzyme's stability in wash water. Medical applications of designer proteins seem especially promising. For instance, we might one day cure cancer by combining portions of one protein that recognizes cancer with part of another protein that attacks it. One of the challenges facing such a development, however, is the problem of designing a protein that attacks only cancer cells and not healthy ones.
In the long term, scientists hope to design proteins from scratch. This is extremely difficult today and will remain so until researchers better understand the rules that govern tertiary structure. Nevertheless, scientists already have designed a few small proteins whose stability or instability has enhanced our understanding of those rules. Building on these successes, scientists hope that one day they may be able to design proteins to meet a host of medical and industrial needs.
Proteins in the Diet
Proteins are one of the basic nutrients, along with carbohydrates, lipids, vitamins, and minerals (see Nutrients and Nutrition). They can be broken down and used as a source of emergency energy if carbohydrates or fats cannot meet immediate needs. The body does not use protein from food directly: after ingestion, enzymes in the digestive system break protein into smaller peptide chains and eventually into separate amino acids. These smaller constituents then go into the bloodstream, from whence they are transported to the cells. The cells incorporate the amino acids and begin building proteins from them.
Animal and Vegetable Proteins
The protein content in plants is very small, since plants are made largely of cellulose, a type of carbohydrate (see Carbohydrates for more on this subject); this is one reason why herbivorous animals must eat enormous quantities of plants to meet their dietary requirements. Humans, on the other hand, are omnivores (unless they choose to be vegetarians) and are able to assimilate proteins in abundant quantities by eating the bodies of plant-eating animals, such as cows. In contrast to plants, animal bodies (as previously noted) are composed largely of proteins. When people think of protein in the diet, some of the foods that first come to mind are those derived from animals: either meat or such animal products as milk, cheese, butter, and eggs. A secondary group of foods that might appear on the average person's list of proteins include peas, beans, lentils, nuts, and cereal grains.
There is a reason why the "protein team" has a clearly defined "first string" and "second string." The human body is capable of manufacturing 12 of the 20 amino acids it needs, but it must obtain the other eight—known as essential amino acids—from the diet. Most forms of animal protein, except for gelatin (made from animal bones), contain the essential amino acids, but plant proteins do not. Thus, the nonmeat varieties of protein are incomplete, and a vegetarian who does not supplement his or her diet might be in danger of not obtaining all the necessary amino acids.
For a person who eats meat, it would be extremely difficult not to get enough protein. According to the U.S. Food and Drug Administration (FDA), protein should account for 10% of total calories in the diet, and since protein contains 4 calories per 0.035 oz. (1 g), that would be about 1.76 oz. (50 g) in a diet consisting of 2,000 calories a day. A pound (0.454 kg) of steak or pork supplies about twice this much, and though very few people sit down to a meal and eat a pound of meat, it is easy to see how a meat eater would consume enough protein in a day.
For a vegetarian, meeting the protein needs may be a bit more tricky, but it can be done. By combining legumes or beans and grains, it is possible to obtain a complete protein: hence, the longstanding popularity, with meat eaters as well as vegetarians, of such combinations as beans and rice or peas and cornbread. Other excellent vegetarian combos include black beans and corn, for a Latin American touch, or the eastern Asian combination of rice and tofu, protein derived from soybeans.