Protein is the major functional and structural component of every cell in the body. Proteins are composed of compounds of carbon, hydrogen, oxygen, and nitrogen, which are arranged as strands of amino acids. Nine of these amino acids cannot be synthesized to meet the body's needs and must be obtained from the diet. Food proteins, known as dietary proteins, include meat, eggs, seafood, and milk. The quality of a source of dietary protein depends on its ability to provide the nitrogen and amino acid requirements that are necessary for the body's growth, maintenance, and repair. Protein is a macronutrient, a dietary component that provides energy. Other macronutrients include fats and carbohydrates.
Protein is an essential component for every type of cell in the body, including muscles, bones, organs, tendons, and ligaments. Proteins are vital to functions such as cellular regeneration and repair, tissue maintenance and regulation, hormone and enzyme production, fluid balance, and energy provision. Protein is also needed to form new enzymes, antibodies, hormones, blood-clotting factors, and blood-transport proteins. The body is constantly undergoing renewal and repair of tissues. The amount of protein needed to build new tissue or maintain structure and function depends on the rate of renewal or the stage of growth and development. For example, the intestinal tract is renewed every couple of days, whereas blood cells have a life span of 60 to 120 days. An infant uses as much as one-third of the dietary protein for the purpose of building new connective and muscle tissues. Protein is often consumed in large amounts by weightlifters and bodybuilders to promote muscle growth and repair.
Amino acids are the basic components of hormones, which are essential chemical signaling messengers of the body. Hormones are secreted into the bloodstream by endocrine glands such as the thyroid gland, adrenal glands, pancreas, and other ductless glands. Hormones regulate bodily functions and processes. For example, the hormone insulin secreted by the pancreas works to lower the blood glucose level after meals. Insulin is made up of 48 amino acids.
Enzymes, which play an essential kinetic (rate-controlling) role in biological reactions, are composed of large protein molecules. Enzymes facilitate the rate of reactions by acting as catalysts and lowering the activation energy barrier between the reactants and the products of the reactions. All chemical reactions that occur during the digestion of food and the metabolic processes in tissues require enzymes, making enzymes (and thus proteins) vital to the overall functions of the body.
Recommended dietary allowance (g/day)*
Children 0-6 mos.
Children 7-12 mos.
Children 1-3 yrs.
Children 4-8 yrs.
Children 9-13 yrs.
Boys 14-18 yrs.
Girls 14-18 yrs.
Men 19≥ yrs.
Women 19≥ yrs.
Amounts based on average body weighte.g., for adults, 0.8 g protein per kg body weight.
AI = Adequate intake
g = gram
SOURCE: Institute of Medicine. “Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids.” Washington, DC: National Academies Press, 2005. https://doi.org/10.17226/10490 (accesed April 1, 2018).
Serving as the basic structural molecule of all the tissues in the body, protein makes up nearly 17% of the total body weight. Amino acids form the fundamental building blocks of protein. Long chains of amino acids, called polypeptides, make up large complexes of protein. The arrangement of amino acids along the chain determines the structure and chemical properties of the protein.
There are 20 different amino acids used by the human body. Of the 20 amino acids, 11 are considered nonessential, meaning that the body is able to adequately synthesize them. The other 9 are essential, meaning that the body is unable to adequately synthesize them, and so they must be supplied through the diet. The essential amino acids are leucine, isoleucine, valine, lysine, threonine, tryptophan, methionine, phenylalanine and histidine. The nonessential amino acids are arginine, alanine, asparagine, aspartic acid, cysteine, glutamine, glutamic acid, glycine, proline, serine, and tyrosine. Foods that have protein contain both nonessential and essential amino acids, the latter of which the body can use to synthesize some of the nonessential amino acids. A healthful diet should consist of a sufficient and balanced supply of both essential and nonessential amino acids in order to ensure high levels of protein production.
Protein digestion begins when the food reaches the stomach and stimulates the release of hydrochloric acid (HCl). The acidic environment helps the protein digestion process by catalyzing the breakdown of peptide bonds and by helping to convert the gastric enzyme pepsinogen to pepsin (the active form). Pepsin attacks the peptide bonds that link amino acids together, breaking the long chain into shorter segments of amino acids known as dipeptides and tripeptides. These protein fragments are then further broken down in the duodenum of the small intestines by the enzymes trypsin and chymotrypsin and by secretions from the pancreas. The final breakdown of proteins into amino acids is done by enzymes in the small intestines.
Once proteins are broken down into their component amino acids, the cells of the small intestine actively absorb them. The amino acids are then released into the bloodstream, where they are rapidly taken up by cells. In the cells, they can be combined to form new proteins for growth and maintenance and other vital cell functions. For the most part, the body does not store protein, as the metabolism of amino acids occurs within a few hours. A waste product of amino acid metabolism is urea, which is composed of excess nitrogen and is excreted in the urine.
Amino acids are also metabolized in the liver into forms that are used as building blocks of protein in tissues. The body may use the amino acids for either anabolic or catabolic reactions. Anabolism refers to the chemical process through which digested and absorbed products are used to effectively build or repair bodily tissues, or to restore vital substances broken down through metabolism. Catabolism, on the other hand, is the process that results in the release of energy through the breakdown of nutrients, stored materials, and cellular substances. Anabolic and catabolic reactions work hand-in-hand, and the energy produced in catabolic processes is used to fuel essential anabolic processes.
An internationally recognized method of measuring the nutritive value of proteins is the Protein Digestibility Corrected Amino Acid Score (PDCAAS), which was developed in 1991 by the World Health Organization (WHO) and the Food and Agriculture Organization (FAO) of the United Nations. The PDCAAS is a measurement of the protein value in food and is calculated by determining the amino acid score of the protein and multiplying it by the true digestibility of the protein, or the proportion of food protein absorbed by the body. According to the WHO and FAO report Protein and Amino Acid Requirements in Human Nutrition, values for the true digestibility of various dietary proteins include:
Multiplying the amino acid score by the digestibility results in a value based on the amount of essential amino acids per unit of protein that is available after the food is digested. A top score of 1.0 indicates that 100% of the essential amino acids are available after digestion. A score higher than 1.0 or 100% is rounded down to 100% because the body does not absorb the additional protein. The additional protein is converted into fat or is removed from the body as waste.
PDCAAS VALUES. Most animal proteins, such as eggs, cheese, milk, meat, and fish, are given a PDCAAS value of 1, which is equal to 100%. The article “Protein—Which Is Best?,” published in the Journal of Sports Medicine, lists some specific PDCAAS values:
Animal proteins contain all of the essential amino acids, so vegetarians need to pay attention to the protein in their diets, according to the U.S. National Institutes of Health (NIH). Eggs are a source of complete protein, but plant-based proteins may have low amounts of some amino acids. As a result, vegetarians may need to obtain protein from different sources to obtain all of the essential amino acids. Vegetarian combinations that provide complete protein include rice or corn tortillas and beans or peanut butter and bread. The Academy of Nutrition and Dietetics advises that vegetarian athletes increase their protein intake by 10% above the recommendations for other endurance and strength athletes.
Protein intake is especially related to muscular development for older adults and strength-training athletes. Muscle tissue is metabolically active and burns calories, even when the person is at rest. However, people need to regularly maintain their muscles or they risk losing muscle function. Also known as resistance training, toning helps keep muscles from losing their elasticity. Muscle building, which is also known as bodybuilding, is the process of creating newer, stronger muscles. Toning and bodybuilding may involve similar equipment, but bodybuilding generally requires heavier weights and longer durations than resistance training.
Some athletes take protein supplements, products made from protein sources such as whey, casein, egg, or soy. Dietary organizations such as the Academy of Nutrition and Dietetics, Dietitians of Canada, and the American College of Sports Medicine recommend food as the primary source of all nutrients, including protein. Dietary supplements are not regulated by the U.S. Food and Drug Administration in the same manner as foods. Athletes interested in supplementation should first consult with a physician and a registered dietitian (RD) or a qualified sports dietitian.
Protein requirements for adults are based on body weight. The daily safe intake level for adults is generally 0.8 grams of protein per kilogram of body weight (2.2 lb.). The U.S. dietary reference intakes (DRIs) recommend that approximately 10%–35% of an adult's daily calories come from protein. The recommended range for children ages 1–3 is 5%–20%, and the range for children and adolescents ages 4–18 is 10%–30%. Inadequate protein intake is rare in the United States.
Additional protein is required when women are pregnant or breastfeeding. WHO and FAO recommend that pregnant women increase protein by 1 g per day during the first trimester, 9 g per day during the second, and 31 g per day during the third. For breastfeeding women, protein intake should be increased by 19 g per day during the first six months of lactation and 12.5 g per day for the remainder of lactation.
People following a general exercise regimen or even a short-term strenuous regimen do not need to consume additional protein. The recommended protein intake increases for endurance athletes such as marathoners and strength-training athletes like weightlifters or bodybuilders. There is no recommendation from the U.S. government for protein intake for endurance and strength athletes, according to a joint position paper issued in 2009 by the Academy of Nutrition and Dietetics (then known as the American Dietetic Association), Dietitians of Canada, and the American College of Sports Medicine (ACSM), titled “Nutrition and Athletic Performance.”
The organizations acknowledge that consuming protein in excess of the recommended intake “to maintain optimum physical performance is commonly done in [the] practice.” Recommendations for endurance and strength athletes range from 1.2–1.7 g of protein per kg of body weight (0.5–0.8 g of protein per lb.). According to the organizations, that intake “can generally be met through diet alone, without the use of protein or amino acid supplements.”
Some people may have an allergy or intolerance to one or more sources of dietary protein. Common food allergens include milk, eggs, fish, shellfish, tree nuts, wheat, peanuts, and soy. Proteins in these foods trigger an abnormal immune response in people allergic to the food. Food intolerances are different from allergies and are caused by the inability of the body to digest or metabolize a component in food. For example, lactose intolerance is caused by a deficiency of the enzyme lactase to break down the sugar lactose, found in milk and milk products. Dietary supplements containing whey or other dairy proteins should not be taken by people with allergies to these foods.
People interested in starting a strength-training program should check with their primary physician that it is okay for them to exercise.
An intake of protein that is less than that required by the body can result in protein deficiency, which means the body is breaking down tissues faster than they are being replaced. Without adequate protein, the body cannot maintain or build new tissues. Insufficient amounts or quality of protein can result in serious medical conditions. Complications include a weakened immune system, decreased blood plasma, and the development of medical conditions such as anemia or edema. Protein malnutrition in infants is called kwashiorkor, and it poses a major health problem in developing countries in Africa, Central and South America, and certain parts of Asia. An infant with kwashiorkor experiences poor muscle and tissue development, loss of appetite, mottled skin, patchy hair, diarrhea, edema, and, eventually, death. Similar symptoms are present in adults with protein deficiency. Treatment or prevention of this condition lies in adequate consumption of protein-rich foods.
See also Bodybuilding diet ; Breastfeeding ; Cancer ; Whole grains .
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Academy of Nutrition and Dietetics, 120 South Riverside Plz., Ste. 2000, Chicago, IL, 60606-6995, (312) 899-0040, (800) 877-1600, email@example.com, http://www.eatright.org .
American College of Sports Medicine, 401 West Michigan St., Indianapolis, IN, 46202-3233, (317) 637-9200, Fax: (317) 634-7817, http://www.acsm.org .
British Nutrition Foundation, High Holborn House, 52-54 High Holborn, London, United Kingdom, WC1V 6RQ, +44 20 7404 6504, Fax: +44 20 7404 6747, firstname.lastname@example.org, http://www.nutrition.org.uk .
Food and Nutrition Information Center, National Agricultural Library, 10301 Baltimore Ave., Rm. 105, Beltsville, MD, 20705, (301) 504-5414, Fax: (301) 504-6409, email@example.com, http://fnic.nal.usda.gov .
Institute of Medicine, National Academy of Sciences, 500 Fifth St. NW, Washington, DC, 20001, (202) 334-2352, firstname.lastname@example.org, http://www.iom.edu .
Revised by Liz Swain,