:Protein (nutrient)

After water, proteins account for more mass in an organism than any other type of molecule. Protein is present in every cell, and it is a structural component of every body tissue and organ, including hair, skin, blood, and bone. Protein is especially abundant in muscle. Cellular messengers (hormones) and transport molecules are constructed from proteins, including enzymes and antibodies, as are cell membrane components, such as glycoproteins, G proteins, and ion channels. The types of amino acids and their sequence determine the unique 3-dimensional structure and function of a protein.

Amino acids obtained through protein catabolism also enable the biosynthesis of non-protein molecules that are essential for life, such as nucleotides, certain neurotransmitters, and heme.{{cite journal | last = Hermann | first = Janice R. | title = Protein and the Body | journal = Oklahoma Cooperative Extension Service, Division of Agricultural Sciences and Natural Resources, Oklahoma State University | date = April 2021 | pages = T–3163–1 – T–3163–4 | url = http://pods.dasnr.okstate.edu/docushare/dsweb/Get/Document-2473/T-3163web.pdf }}{{cite web |last=Chandel |first=Navdeep S. |title=Amino Acid Metabolism |url=https://cshperspectives.cshlp.org/content/13/4/a040584.full |website=Cold Spring Harbor Perspectives in Biology |access-date=March 31, 2025 }}

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{{Bar chart|title= Nutritional value and environmental impact of animal products, compared to agriculture overallDamian Carrington, [https://www.theguardian.com/environment/2018/may/31/avoiding-meat-and-dairy-is-single-biggest-way-to-reduce-your-impact-on-earth "Avoiding meat and dairy is 'single biggest way' to reduce your impact on Earth "], The Guardian, 31 May 2018 (page visited on 19 August 2018).|float= right|label_type= Categories|data_type= Contribution of farmed animal product [%]|bar_width= 20|width_units= em|data_max= 100|label1= Calories|data1= 18|label2= Proteins|data2= 37|label3= Land use|data3= 83|label4= Greenhouse gases|data4= 58|label5= Water pollution|data5= 57|label6= Air pollution|data6= 56|label7= Freshwater withdrawals|data7= 33}}

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Protein occurs in a wide range of food.{{cite book | last = Steinke | first = Waggle | name-list-style = vanc |title= New protein foods in human health: nutrition, prevention and therapy|publisher= CRC Press|year= 1992|pages=91–100|isbn= 978-0-8493-6904-9|display-authors= etal}}{{cite journal | vauthors = Young VR, Pellett PL | title = Plant proteins in relation to human protein and amino acid nutrition | journal = The American Journal of Clinical Nutrition | volume = 59 | issue = 5 Suppl | pages = 1203S–1212S | date = May 1994 | pmid = 8172124 | doi = 10.1093/ajcn/59.5.1203s | url = http://www.ajcn.org/content/59/5/1203S.full.pdf }} On a worldwide basis, plant protein foods contribute over 60% of the per capita supply of protein.{{cn|date=March 2025}} In North America, animal-derived foods contribute about 70% of protein sources. Insects are a source of protein in many parts of the world.{{cite journal |title= Opportunities and hurdles of edible insects for food and feed|last= Dobermann |first= D.|journal= Nutrition Bulletin|volume= 42|issue= 4|pages= 293–308|date= November 2017|publisher= John Wiley & Sons, Inc.|doi= 10.1111/nbu.12291|doi-access= free}} In parts of Africa, up to 50% of dietary protein derives from insects. It is estimated that more than 2 billion people eat insects daily.{{cite web |url= http://scienceinpoland.pap.pl/en/news/news%2C28495%2Cexpert-more-2-billion-people-worldwide-eat-insects-every-day.html |title= More than 2 billion people worldwide eat insects every day|last= Pap |first= Fundacja|website= ScienceInPoland.pap.pl | publisher= Republic of Poland Ministry of Science and Higher Education |access-date= 26 February 2020}}

Meat, dairy, eggs, soybeans, fish, whole grains, and cereals are sources of protein. Examples of food staples and cereal sources of protein, each with a concentration greater than 7%, are (in no particular order) buckwheat, oats, rye, millet, maize (corn), rice, wheat, sorghum, amaranth, and quinoa. Game meat is an affordable protein source in some countries.{{cite book | last1 = Jahnke | first1 = Hans E. | chapter = Appendix 3D: The value of game meat as a source of cheap protein | title = Conservation and Utilization of Wildlife in Uganda: A Study in Environmental Economics | url = https://books.google.com/books?id=ev3aAAAAMAAJ | series = IFO Forschungsberichte der Abteilung Entwicklungsländer | volume = 54 | publisher = Brill | date = 1974 | isbn = 9783803901095 | access-date = 5 March 2020}}

Plant sources of proteins include legumes, nuts, seeds, grains, and some vegetables and fruits. Plant foods with protein concentrations greater than 7% include (but are not limited to) soybeans, lentils, kidney beans, white beans, mung beans, chickpeas, cowpeas, lima beans, pigeon peas, lupines, wing beans, almonds, Brazil nuts, cashews, pecans, walnuts, cotton seeds, pumpkin seeds, hemp seeds, sesame seeds, and sunflower seeds.

Photovoltaic-driven microbial protein production uses electricity from solar panels and carbon dioxide from the air to create fuel for microbes, which are grown in bioreactor vats and then processed into dry protein powders. The process makes highly efficient use of land, water and fertiliser.{{Cite journal |last1=Leger |first1=Dorian |last2=Matassa |first2=Silvio |last3=Noor |first3=Elad |last4=Shepon |first4=Alon |last5=Milo |first5=Ron |last6=Bar-Even |first6=Arren |date=2021-06-29 |title=Photovoltaic-driven microbial protein production can use land and sunlight more efficiently than conventional crops |journal=Proceedings of the National Academy of Sciences |volume=118 |issue=26 |doi=10.1073/pnas.2015025118 |issn=0027-8424 |pmid=34155098|pmc=8255800 |bibcode=2021PNAS..11815025L |doi-access=free }}{{Cite web |last=Carrington |first=Damian |date=2021-06-21 |title=Microbes and solar power 'could produce 10 times more food than plants' |url=http://www.theguardian.com/environment/2021/jun/21/microbes-and-solar-power-could-produce-10-times-more-food-than-plants |access-date=2022-02-27 |website=The Guardian}}

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People eating a balanced diet do not need protein supplements.{{cite book|title= Amino acid content of foods and biological data on proteins (FAO nutritional studies number 24)|publisher= Food and Agriculture Organization|year= 1985|isbn=978-92-5-001102-8|url= http://www.fao.org/DOCREP/005/AC854T/AC854T00.htm}}

The table below presents food groups as protein sources.

Colour key:

:{{Color box|lightgreen}} Protein source with highest density of respective amino acid.

:{{Color box|yellow}} Protein source with lowest density of respective amino acid.

File:Protein shake.jpg ]]

Protein powders – such as casein, whey, egg, rice, soy and cricket flour– are processed and manufactured sources of protein.

The classic assays for protein concentration in food are the Kjeldahl method and the Dumas method. These tests determine the total nitrogen in a sample. The only major component of most food which contains nitrogen is protein (fat, carbohydrate and dietary fiber do not contain nitrogen). If the amount of nitrogen is multiplied by a factor depending on the kinds of protein expected in the food the total protein can be determined. This value is known as the "crude protein" content. The use of correct conversion factors is heavily debated, specifically with the introduction of more plant-derived protein products.{{cite journal |last1=Mariotti |first1=François |title=Converting Nitrogen into Protein—Beyond 6.25 and Jones' Factors |journal=Critical Reviews in Food Science and Nutrition |date=2008 |volume=48 |issue=2 |pages=177–184 |doi=10.1080/10408390701279749 |pmid=18274971 |s2cid=205688934 |url=https://www.tandfonline.com/doi/full/10.1080/10408390701279749 |access-date=10 August 2023}} However, on food labels the protein is calculated by the nitrogen multiplied by 6.25, because the average nitrogen content of proteins is about 16%. The Kjeldahl test is typically used because it is the method the AOAC International has adopted and is therefore used by many food standards agencies around the world, though the Dumas method is also approved by some standards organizations.{{cite web

|url=http://www-unix.oit.umass.edu/~mcclemen/581Proteins.html

|author= D. Julian McClements

|title=Analysis of Proteins

|publisher=University of Massachusetts Amherst

|access-date=27 April 2007}}

Accidental contamination and intentional adulteration of protein meals with non-protein nitrogen sources that inflate crude protein content measurements have been known to occur in the food industry for decades. To ensure food quality, purchasers of protein meals routinely conduct quality control tests designed to detect the most common non-protein nitrogen contaminants, such as urea and ammonium nitrate.{{cite news

|url=https://www.usatoday.com/money/industries/food/2007-04-25-melamine-usat_N.htm

|first=Elizabeth

|last=Weise

|title=Food tests promise tough task for FDA

|work=USA Today

|date=24 April 2007

|access-date=29 April 2007}}

In at least one segment of the food industry, the dairy industry, some countries (at least the U.S., Australia, France and Hungary) have adopted "true protein" measurement, as opposed to crude protein measurement, as the standard for payment and testing: "True protein is a measure of only the proteins in milk, whereas crude protein is a measure of all sources of nitrogen and includes nonprotein nitrogen, such as urea, which has no food value to humans. ... Current milk-testing equipment measures peptide bonds, a direct measure of true protein."{{cite web | url = http://aipl.arsusda.gov/reference/trueprot.htm | vauthors = VanRaden PM, Powell RL | title = Genetic evaluations for true protein | publisher = United States Department of Agriculture | access-date = 27 April 2007 }} Measuring peptide bonds in grains has also been put into practice in several countries including Canada, the UK, Australia, Russia and Argentina where near-infrared reflectance (NIR) technology, a type of infrared spectroscopy is used.{{cite news

|url=http://www.sciam.com/article.cfm?chanID=sa006&colID=5&articleID=ACB480D7-E7F2-99DF-386D411734605ECC

|first=Alison

|last=Snyder

|title=Protein Pretense: Cheating the standard protein tests is easy, but industry hesitates on alternatives

|publisher=Scientific American

|date=August 2007

|access-date=9 November 2007

|archive-url=https://web.archive.org/web/20071107120935/http://www.sciam.com/article.cfm?articleID=ACB480D7-E7F2-99DF-386D411734605ECC&chanID=sa006&colID=5

|archive-date=7 November 2007

|url-status=dead

}} The Food and Agriculture Organization of the United Nations (FAO) recommends that only amino acid analysis be used to determine protein in, inter alia, foods used as the sole source of nourishment, such as infant formula, but also provides: "When data on amino acids analyses are not available, determination of protein based on total N content by Kjeldahl (AOAC, 2000) or similar method ... is considered acceptable."{{cite web

|url=http://www.fao.org/DOCREP/006/Y5022E/y5022e03.htm

|title=Food energy – methods of analysis and conversion factors

|publisher=FAO

|access-date=9 November 2007}}

The testing method for protein in beef cattle feed has grown into a science over the post-war years. The standard text in the United States, Nutrient Requirements of Beef Cattle, has been through eight editions over at least seventy years.[https://www.nap.edu/catalog/19014/nutrient-requirements-of-beef-cattle-eighth-revised-edition nap.edu: "Nutrient Requirements of Beef Cattle Eighth Revised Edition (2016)"] The 1996 sixth edition substituted for the fifth edition's crude protein the concept of "metabolizeable protein", which was defined around the year 2000 as "the true protein absorbed by the intestine, supplied by microbial protein and undegraded intake protein".[https://www.uaex.edu/publications/PDF/MP391.pdf uaex.edu: "Beef cattle nutrition series - Part 3: Nutrient Requirement Tables"], University of Arkansas Division of Agriculture publication MP391

The limitations of the Kjeldahl method were at the heart of the Chinese protein export contamination in 2007 and the 2008 China milk scandal in which the industrial chemical melamine was added to the milk or glutens to increase the measured "protein".{{cite news|first=Stephen |last=Chen |title=Melamine – an industry staple |pages=Page A2 |work=South China Morning Post |date=18 September 2008}}{{cite journal |last1=Moore |first1=Jeffrey C. |last2=DeVries |first2=Jonathan W. |last3=Lipp |first3=Markus |last4=Griffiths |first4=James C. |last5=Abernethy |first5=Darrell R. | name-list-style = vanc |title=Total Protein Methods and Their Potential Utility to Reduce the Risk of Food Protein Adulteration |journal=Comprehensive Reviews in Food Science and Food Safety |date=17 August 2010 |volume=9 |issue=4 |pages=330–357 |doi=10.1111/j.1541-4337.2010.00114.x|pmid=33467839 |doi-access=free }}

{{see further|Protein quality|Amino acid score}}

The most important aspect and defining characteristic of protein from a nutritional standpoint is its amino acid composition. There are multiple systems which rate proteins by their usefulness to an organism based on their relative percentage of amino acids and, in some systems, the digestibility of the protein source. They include biological value, net protein utilization, and PDCAAS (Protein Digestibility Corrected Amino Acids Score) which was developed by the FDA as a modification of the Protein efficiency ratio (PER) method. The PDCAAS rating was adopted by the US Food and Drug Administration (FDA) and the Food and Agricultural Organization of the United Nations/World Health Organization (FAO/WHO) in 1993 as "the preferred 'best'" method to determine protein quality. These organizations have suggested that other methods for evaluating the quality of protein are inferior.[http://www.fao.org/docrep/U5900t/u5900t07.htm Boutrif, E., Food Quality and Consumer Protection Group, Food Policy and Nutrition Division, FAO, Rome: "Recent Developments in Protein Quality Evaluation" Food, Nutrition and Agriculture, Issue 2/3, 1991] In 2013 FAO proposed changing to Digestible Indispensable Amino Acid Score.

Most proteins are decomposed to single amino acids by digestion in the gastro-intestinal tract.[http://pharmaxchange.info/press/2013/07/digestion-of-dietary-proteins-in-the-gastro-intestinal-tract-gi-tract/ Digestion of Dietary Proteins in the Gastro-Intestinal Tract]

Digestion typically begins in the stomach when pepsinogen is converted to pepsin by the action of hydrochloric acid, and continued by trypsin and chymotrypsin in the small intestine.

Before the absorption in the small intestine, most proteins are already reduced to single amino acid or peptides of several amino acids. Most peptides longer than four amino acids are not absorbed. Absorption into the intestinal absorptive cells is not the end. There, most of the peptides are broken into single amino acids.

Absorption of the amino acids and their derivatives into which dietary protein is degraded is done by the gastrointestinal tract. The absorption rates of individual amino acids are highly dependent on the protein source; for example, the digestibilities of many amino acids in humans, the difference between soy and milk proteins{{cite journal | vauthors = Gaudichon C, Bos C, Morens C, Petzke KJ, Mariotti F, Everwand J, Benamouzig R, Daré S, Tomé D, Metges CC | title = Ileal losses of nitrogen and amino acids in humans and their importance to the assessment of amino acid requirements | journal = Gastroenterology | volume = 123 | issue = 1 | pages = 50–9 | date = July 2002 | pmid = 12105833 | doi = 10.1053/gast.2002.34233 | doi-access = free }} and between individual milk proteins, beta-lactoglobulin and casein.{{cite journal | vauthors = Mahé S, Roos N, Benamouzig R, Davin L, Luengo C, Gagnon L, Gaussergès N, Rautureau J, Tomé D | title = Gastrojejunal kinetics and the digestion of [15N]beta-lactoglobulin and casein in humans: the influence of the nature and quantity of the protein | journal = The American Journal of Clinical Nutrition | volume = 63 | issue = 4 | pages = 546–52 | date = April 1996 | pmid = 8599318 | doi = 10.1093/ajcn/63.4.546 | url = http://www.ajcn.org/cgi/content/abstract/63/4/546 | doi-access = free }} For milk proteins, about 50% of the ingested protein is absorbed between the stomach and the jejunum and 90% is absorbed by the time the digested food reaches the ileum.{{cite journal | vauthors = Mahé S, Marteau P, Huneau JF, Thuillier F, Tomé D | title = Intestinal nitrogen and electrolyte movements following fermented milk ingestion in man | journal = The British Journal of Nutrition | volume = 71 | issue = 2 | pages = 169–80 | date = February 1994 | pmid = 8142329 | doi = 10.1079/BJN19940124 | doi-access = free }} Biological value (BV) is a measure of the proportion of absorbed protein from a food which becomes incorporated into the proteins of the organism's body.

Newborns of mammals are exceptional in protein digestion and assimilation in that they can absorb intact proteins at the small intestine. This enables passive immunity, i.e., transfer of immunoglobulins from the mother to the newborn, via milk.{{cite journal | vauthors = Hurley WL, Theil PK | title = Perspectives on immunoglobulins in colostrum and milk | journal = Nutrients | volume = 3 | issue = 4 | pages = 442–74 | date = April 2011 | pmid = 22254105 | pmc = 3257684 | doi = 10.3390/nu3040442 | doi-access = free }}

File:"Eat More Cottage Cheese...You'll Need Less Meat...A Postal Card Will Bring Recipes...Cottage Cheese or Meat^ Ask... - NARA - 512542.jpg about 100 years ago, on cottage cheese as a lower-cost protein substitute for meat.]]

File:Average Protein Supply By Region And Origin.svg

Considerable debate has taken place regarding issues surrounding protein intake requirements.{{cite journal | vauthors = Bilsborough S, Mann N | title = A review of issues of dietary protein intake in humans | journal = International Journal of Sport Nutrition and Exercise Metabolism | volume = 16 | issue = 2 | pages = 129–52 | date = April 2006 | pmid = 16779921 | doi = 10.1123/ijsnem.16.2.129 | s2cid = 10339366 | doi-access = free }}{{cite journal | vauthors = Lemon PW | title = Beyond the zone: protein needs of active individuals | journal = Journal of the American College of Nutrition | volume = 19 | issue = 5 Suppl | pages = 513S–521S | date = October 2000 | pmid = 11023001 | doi = 10.1080/07315724.2000.10718974 | s2cid = 14586881 }} The amount of protein required in a person's diet is determined in large part by overall energy intake, the body's need for nitrogen and essential amino acids, body weight and composition, rate of growth in the individual, physical activity level, the individual's energy and carbohydrate intake, and the presence of illness or injury. Physical activity and exertion as well as enhanced muscular mass increase the need for protein. Requirements are also greater during childhood for growth and development, during pregnancy, or when breastfeeding in order to nourish a baby or when the body needs to recover from malnutrition or trauma or after an operation.{{cite web

|title=Protein and amino acid requirements in human nutrition

|url=http://whqlibdoc.who.int/trs/WHO_TRS_935_eng.pdf

|vauthors=WHO, FAO, UNU

|publisher=WHO Press

|year=2007

|access-date=8 July 2008

}}

According to US & Canadian Dietary Reference Intake guidelines, women ages 19–70 need to consume 46 grams of protein per day while men ages 19–70 need to consume 56 grams of protein per day to minimize risk of deficiencies. These Recommended Dietary Allowances (RDAs) were calculated based on 0.8 grams protein per kilogram body weight and average body weights of 57 kg (126 pounds) and 70 kg (154 pounds), respectively.{{citation| title = Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein and Amino Acids| publisher = Institute of Medicine. National Academy Press| year = 2005| doi = 10.17226/10490| isbn = 978-0-309-08525-0| url = https://www.nap.edu/read/10490/chapter/12}} However, this recommendation is based on structural requirements but disregards use of protein for energy metabolism. This requirement is for a normal sedentary person.{{cite journal | vauthors = Tarnopolsky MA, Atkinson SA, MacDougall JD, Chesley A, Phillips S, Schwarcz HP | title = Evaluation of protein requirements for trained strength athletes | journal = Journal of Applied Physiology | volume = 73 | issue = 5 | pages = 1986–95 | date = November 1992 | pmid = 1474076 | doi = 10.1152/jappl.1992.73.5.1986 | s2cid = 46188182 }} In the United States, average protein consumption is higher than the RDA. According to results of the National Health and Nutrition Examination Survey (NHANES 2013–2014), average protein consumption for women ages 20 and older was 69.8 grams and for men 98.3 grams/day.{{citation| title = What We Eat in America, NHANES 2013-2014| publisher = U.S. Department of Agriculture, Agricultural Research Service | year = 2016| url = https://www.ars.usda.gov/ARSUserFiles/80400530/pdf/1314/Table_1_NIN_GEN_13.pdf}} According to research from Harvard University, the National Academy of Medicine{{Cite web |date=2012-09-18 |title=Protein |url=https://nutritionsource.hsph.harvard.edu/what-should-you-eat/protein/#how-much-need |access-date=2024-10-09 |website=The Nutrition Source |language=en-us}} suggests that adults should consume at least 0.8 grams of protein per kilogram of body weight daily, which is roughly equivalent to a little more than 7 grams for every 20 pounds of body weight. This recommendation is widely accepted by health professionals as a guideline for maintaining muscle mass, supporting metabolic functions, and promoting overall health.

Several studies have concluded that active people and athletes may require elevated protein intake (compared to 0.8 g/kg) due to increase in muscle mass and sweat losses, as well as need for body repair and energy source. Indeed, it has been shown that protein contribute around 3% of the total energy expenditure during exercise.Clauss, M., & Jensen, J. (2025). Effect of exercise intensity, duration, and volume on protein oxidation during endurance exercise in humans: A systematic review with meta-analysis. Scandinavian Journal of Medicine & Science in Sports, 35(4): e70038. https://doi.org/10.1111/sms.70038 Suggested amounts vary from 1.2 to 1.4 g/kg for those doing endurance exercise to as much as 1.6-1.8 g/kg for strength exercise and up to 2.0 g/kg/day for older people,Baum, Jamie I., Il-Young Kim, and Robert R. Wolfe. "Protein consumption and the elderly: what is the optimal level of intake?." Nutrients 8.6 (2016): 359. while a proposed maximum daily protein intake would be approximately 25% of energy requirements i.e. approximately 2 to 2.5 g/kg. However, many questions still remain to be resolved.

In addition, some have suggested that athletes using restricted-calorie diets for weight loss should further increase their protein consumption, possibly to 1.8–2.0 g/kg, in order to avoid loss of lean muscle mass.{{cite journal | vauthors = Phillips SM, Van Loon LJ | title = Dietary protein for athletes: from requirements to optimum adaptation | journal = Journal of Sports Sciences | volume = 29 Suppl 1 | issue = sup1 | pages = S29-38 | date = 2011-01-01 | pmid = 22150425 | doi = 10.1080/02640414.2011.619204 | s2cid = 33218998 }}

Endurance athletes differ from strength-building athletes in that endurance athletes do not build as much muscle mass from training as strength-building athletes do.{{citation needed|date=October 2013}} Research suggests that individuals performing endurance activity require more protein intake than sedentary individuals so that muscles broken down during endurance workouts can be repaired.{{cite journal | vauthors = Phillips SM | title = Dietary protein for athletes: from requirements to metabolic advantage | journal = Applied Physiology, Nutrition, and Metabolism| volume = 31 | issue = 6 | pages = 647–54 | date = December 2006 | pmid = 17213878 | doi = 10.1139/H06-035 }} Although the protein requirement for athletes still remains controversial (for instance see Lamont, Nutrition Research Reviews, pages 142 - 149, 2012), research does show that endurance athletes can benefit from increasing protein intake because the type of exercise endurance athletes participate in still alters the protein metabolism pathway. The overall protein requirement increases because of amino acid oxidation in endurance-trained athletes. Endurance athletes who exercise over a long period (2–5 hours per training session) use protein as a source of 5–10% of their total energy expended. Therefore, a slight increase in protein intake may be beneficial to endurance athletes by replacing the protein lost in energy expenditure and protein lost in repairing muscles. One review concluded that endurance athletes may increase daily protein intake to a maximum of 1.2–1.4 g per kg body weight.{{cite journal | vauthors = Lemon PW | title = Do athletes need more dietary protein and amino acids? | journal = International Journal of Sport Nutrition | volume = 5 Suppl | pages = S39-61 | date = June 1995 | pmid = 7550257 | doi=10.1123/ijsn.5.s1.s39}}

Research also indicates that individuals performing strength training activity require more protein than sedentary individuals. Strength-training athletes may increase their daily protein intake to a maximum of 1.4–1.8 g per kg body weight to enhance muscle protein synthesis, or to make up for the loss of amino acid oxidation during exercise. Many athletes maintain a high-protein diet as part of their training. In fact, some athletes who specialize in anaerobic sports (e.g., weightlifting) believe a very high level of protein intake is necessary, and so consume high protein meals and also protein supplements.[https://hub.olympic.org/athlete365/wp-content/uploads/2016/01/1378_IOC_NutritionAthleteHandbook_1e.pdf Nutrition for Athletes, International Olympic Committee] {{Webarchive|url=https://web.archive.org/web/20180122235001/https://hub.olympic.org/athlete365/wp-content/uploads/2016/01/1378_IOC_NutritionAthleteHandbook_1e.pdf |date=22 January 2018 }} Nutrition Working Group of the Medical and Scientific Commission of the International Olympic Committee, Revised and Updated in June 2016.

{{Main|Food allergy}}

A food allergy is an abnormal immune response to proteins in food. The signs and symptoms may range from mild to severe. They may include itchiness, swelling of the tongue, vomiting, diarrhea, hives, trouble breathing, or low blood pressure. These symptoms typically occurs within minutes to one hour after exposure. When the symptoms are severe, it is known as anaphylaxis. The following eight foods are responsible for about 90% of allergic reactions: cow's milk, eggs, wheat, shellfish, fish, peanuts, tree nuts and soy.{{cite web|author =National Institute of Allergy and Infectious Diseases|title=Food Allergy An Overview|url=https://www.niaid.nih.gov/topics/foodAllergy/Documents/foodallergy.pdf|date=July 2012|url-status=dead|archive-url=https://web.archive.org/web/20160305145206/http://www.niaid.nih.gov/topics/foodallergy/documents/foodallergy.pdf|archive-date=2016-03-05}}

While there is no conclusive evidence that a high protein diet can cause chronic kidney disease, there is a consensus that people with this disease should decrease consumption of protein. According to one 2009 review updated in 2018, people with chronic kidney disease who reduce protein consumption have less likelihood of progressing to end stage kidney disease.{{Cite journal|last1=Hahn|first1=Deirdre|last2=Hodson|first2=Elisabeth M.|last3=Fouque|first3=Denis|date=4 October 2018|title=Low protein diets for non-diabetic adults with chronic kidney disease|journal=The Cochrane Database of Systematic Reviews|volume=10|issue=10 |pages=CD001892|doi=10.1002/14651858.CD001892.pub4|issn=1469-493X|pmc=6517211|pmid=30284724}}{{cite journal | vauthors = Schwingshackl L, Hoffmann G | title = Comparison of high vs. normal/low protein diets on renal function in subjects without chronic kidney disease: a systematic review and meta-analysis | journal = PLOS ONE| volume = 9 | issue = 5 | pages = e97656 | date = 2014 | pmid = 24852037 | pmc = 4031217 | doi = 10.1371/journal.pone.0097656 | bibcode = 2014PLoSO...997656S | doi-access = free }} Moreover, people with this disease while using a low protein diet (0.6 g/kg/d - 0.8 g/kg/d) may develop metabolic compensations that preserve kidney function, although in some people, malnutrition may occur.

Individuals with phenylketonuria (PKU) must keep their intake of phenylalanine{{snd}}an essential amino acid{{snd}}extremely low to prevent a mental disability and other metabolic complications. Phenylalanine is a component of the artificial sweetener aspartame, so people with PKU need to avoid low calorie beverages and foods with this ingredient.{{cite web|title=phenylketonuria|url=https://ghr.nlm.nih.gov/condition/phenylketonuria|website=Genetics Home Reference|access-date=12 September 2016|date=8 September 2016|url-status=live|archive-url=https://web.archive.org/web/20160727233812/https://ghr.nlm.nih.gov/condition/phenylketonuria|archive-date=27 July 2016}}

{{See also|Protein poisoning}}

The U.S. and Canadian Dietary Reference Intake review for protein concluded that there was not sufficient evidence to establish a Tolerable upper intake level, i.e., an upper limit for how much protein can be safely consumed.

When amino acids are in excess of needs, the liver takes up the amino acids and deaminates them, a process converting the nitrogen from the amino acids into ammonia, further processed in the liver into urea via the urea cycle. Excretion of urea occurs via the kidneys. Other parts of the amino acid molecules can be converted into glucose and used for fuel.{{cite journal | vauthors = Ten Have GA, Engelen MP, Luiking YC, Deutz NE | title = Absorption kinetics of amino acids, peptides, and intact proteins | journal = International Journal of Sport Nutrition and Exercise Metabolism | volume = 17 Suppl | pages = S23-36 | date = August 2007 | pmid = 18577772 | doi=10.1123/ijsnem.17.s1.s23}}{{cite book | first1 = Sareen S. | last1 = Gropper | first2 = Jack L. | last2 = Smith | first3 = James L. | last3 = Groff | name-list-style = vanc |title=Advanced nutrition and human metabolism |publisher=Wadsworth Cengage Learning |location=Belmont, CA |year=2009 |isbn=978-0-495-11657-8 }} When food protein intake is periodically high or low, the body tries to keep protein levels at an equilibrium by using the "labile protein reserve" to compensate for daily variations in protein intake. However, unlike body fat as a reserve for future caloric needs, there is no protein storage for future needs.

Excessive protein intake may increase calcium excretion in urine, occurring to compensate for the pH imbalance from oxidation of sulfur amino acids. This may lead to a higher risk of kidney stone formation from calcium in the renal circulatory system. One meta-analysis reported no adverse effects of higher protein intakes on bone density.{{cite journal | vauthors = Shams-White MM, Chung M, Du M, Fu Z, Insogna KL, Karlsen MC, LeBoff MS, Shapses SA, Sackey J, Wallace TC, Weaver CM | title = Dietary protein and bone health: a systematic review and meta-analysis from the National Osteoporosis Foundation | journal = The American Journal of Clinical Nutrition | volume = 105 | issue = 6 | pages = 1528–1543 | date = June 2017 | pmid = 28404575 | doi = 10.3945/ajcn.116.145110 | doi-access = free }} Another meta-analysis reported a small decrease in systolic and diastolic blood pressure with diets higher in protein, with no differences between animal and plant protein.{{cite journal | vauthors = Rebholz CM, Friedman EE, Powers LJ, Arroyave WD, He J, Kelly TN | title = Dietary protein intake and blood pressure: a meta-analysis of randomized controlled trials | journal = American Journal of Epidemiology | volume = 176 | pages = S27-43 | date = October 2012 | issue = Suppl 7 | pmid = 23035142 | doi = 10.1093/aje/kws245 | doi-access = free }}

High protein diets have been shown to lead to an additional 1.21 kg of weight loss over a period of 3 months versus a baseline protein diet in a meta-analysis.{{cite journal | vauthors = Santesso N, Akl EA, Bianchi M, Mente A, Mustafa R, Heels-Ansdell D, Schünemann HJ | title = Effects of higher- versus lower-protein diets on health outcomes: a systematic review and meta-analysis | journal = European Journal of Clinical Nutrition | volume = 66 | issue = 7 | pages = 780–8 | date = July 2012 | pmid = 22510792 | pmc = 3392894 | doi = 10.1038/ejcn.2012.37 }} Benefits of decreased body mass index as well as HDL cholesterol were more strongly observed in studies with only a slight increase in protein intake rather where high protein intake was classified as 45% of total energy intake. Detrimental effects to cardiovascular activity were not observed in short-term diets of 6 months or less. There is little consensus on the potentially detrimental effects to healthy individuals of a long-term high protein diet, leading to caution advisories about using high protein intake as a form of weight loss.{{cite journal | vauthors = Ko GJ, Obi Y, Tortorici AR, Kalantar-Zadeh K | title = Dietary protein intake and chronic kidney disease | journal = Current Opinion in Clinical Nutrition and Metabolic Care | volume = 20 | issue = 1 | pages = 77–85 | date = January 2017 | pmid = 27801685 | pmc = 5962279 | doi = 10.1097/MCO.0000000000000342 }}

The 2015–2020 Dietary Guidelines for Americans (DGA) recommends that men and teenage boys increase their consumption of fruits, vegetables and other under-consumed foods, and that a means of accomplishing this would be to reduce overall intake of protein foods.{{cite web|url=http://health.gov/dietaryguidelines/2015/guidelines/chapter-2/a-closer-look-at-current-intakes-and-recommended-shifts/|title=2015 – 2020 Dietary Guidelines for Americans: Shifts Needed To Align With Healthy Eating Patterns: A Closer Look at Current Intakes and Recommended Shifts: Protein Foods|date=December 2015|publisher=U.S. Department of Health and Human Services and U.S. Department of Agriculture.|access-date=9 January 2016|edition=8|quote=Some individuals, especially teen boys and adult men, also need to reduce overall intake of protein foods by decreasing intakes of meats, poultry, and eggs and increasing amounts of vegetables or other under-consumed food groups|archive-url=https://web.archive.org/web/20160109170222/http://health.gov/dietaryguidelines/2015/guidelines/chapter-2/a-closer-look-at-current-intakes-and-recommended-shifts/|archive-date=9 January 2016|url-status=dead}} The 2015 - 2020 DGA report does not set a recommended limit for the intake of red and processed meat. While the report acknowledges research showing that lower intake of red and processed meat is correlated with reduced risk of cardiovascular diseases in adults, it also notes the value of nutrients provided from these meats. The recommendation is not to limit intake of meats or protein, but rather to monitor and keep within daily limits the sodium (< 2300 mg), saturated fats (less than 10% of total calories per day), and added sugars (less than 10% of total calories per day) that may be increased as a result of consumption of certain meats and proteins. While the 2015 DGA report does advise for a reduced level of consumption of red and processed meats, the 2015-2020 DGA key recommendations recommend that a variety of protein foods be consumed, including both vegetarian and non-vegetarian sources of protein.{{cite journal|last=Agata|first=Dabrowska|date=2016-02-02|title=Dietary Guidelines for Americans: Frequently Asked Questions|url=https://digital.library.unt.edu/ark:/67531/metadc824573/?q=protein%20diet|journal=Digital Library|language=English}}

File:Starved girl.jpg with kwashiorkor – one of the three protein energy malnutrition ailments affecting over 10 million children in developing countries.{{cite web|url=http://emedicine.medscape.com/article/984496-overview|title=Marasmus and Kwashiorkor|date=May 2009|publisher=Medscape Reference}}]]{{Main|Protein-energy malnutrition}}

Protein deficiency and malnutrition (PEM) can lead to variety of ailments including Intellectual disability and kwashiorkor.{{cite web|title=Human nutrition in the developing world|first=Michael C.|last=Latham|publisher=Food and Agriculture Organization of the United Nations|year=1997|url=http://www.fao.org/DOCREP/W0073e/w0073e00.htm}} Symptoms of kwashiorkor include apathy, diarrhea, inactivity, failure to grow, flaky skin, fatty liver, and edema of the belly and legs. This edema is explained by the action of lipoxygenase on arachidonic acid to form leukotrienes and the normal functioning of proteins in fluid balance and lipoprotein transport.{{cite book | first1 = Jeffery | last1 = Schwartz | last2 = Bryant | first2 = Carol A. | last3 = DeWalt | first3 = Kathleen Musante | first4 = Anita | last4 = Courtney | name-list-style = vanc |title=The cultural feast: an introduction to food and society |publisher=Thomson/Wadsworth |location=Belmont, California |year=2003|pages=282, 283 |isbn=978-0-534-52582-8}}

PEM is fairly common worldwide in both children and adults and accounts for 6 million deaths annually. In the industrialized world, PEM is predominantly seen in hospitals, is associated with disease, or is often found in the elderly.

{{Portal|Food}}

• Azotorrhea
• Biological value
• Bodybuilding supplement
• Leaf protein concentrate
• Low-protein diet
• Ninja diet
• Protein bar
• Protein toxicity
• Single-cell protein
• List of proteins in the human body

{{Reflist}}

Category:Nutrients

Category:Proteins as nutrients