Protein (nutrient): Difference between revisions

{{Short description|Nutrient for the human body}}

{{アミノ酸・プロテイン}}

{{About|protein as a nutrient|protein as a class of molecule|Protein|other uses|Bodybuilding supplement}}

There are nine essential amino acids which humans must obtain from their diet in order to prevent [[protein–energy malnutrition|protein-energy malnutrition]] and resulting death. They are [[phenylalanine]], [[valine]], [[threonine]], [[tryptophan]], [[methionine]], [[leucine]], [[isoleucine]], [[lysine]], and [[histidine]]. There has been debate as to whether there are 8 or 9 essential amino acids. The consensus seems to lean towards 9 since histidine is not synthesized in adults. There are five amino acids which humans are able to synthesize in the body. These five are [[alanine]], [[aspartic acid]], [[asparagine]], [[glutamic acid]] and [[serine]]. There are six conditionally essential amino acids whose synthesis can be limited under special pathophysiological conditions, such as prematurity in the infant or individuals in severe catabolic distress. These six are [[arginine]], [[cysteine]], [[glycine]], [[glutamine]], [[proline]] and [[tyrosine]]. Dietary sources of protein include  [[grain]]s, [[legume]]s, seeds,  

Protein is a nutrient needed by the human body for growth and maintenance. Aside from water, proteins are the most abundant kind of molecules in the body. Protein can be found in all cells of the body and is the major structural component of all cells in the body, especially [[muscle]]. This also includes body organs, hair and skin. Proteins are also used in membranes, such as [[glycoprotein]]s. When broken down into amino acids, they are used as precursors to [[nucleic acid]], co-enzymes, hormones, immune response, cellular repair, and other molecules essential for life. Additionally, protein is needed to form blood cells.

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[[Bodybuilding supplement#Protein|Protein powders]] – such as [[casein]], [[whey]], [[Egg (food)|egg]], [[rice]], [[soy]] and [[cricket flour#Nutritional information|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. 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.

The limitations of the Kjeldahl method were at the heart of the [[Chinese protein adulteration#Protein testing|Chinese protein export contamination]] in 2007 and the [[2008 China milk scandal]] in which the industrial chemical [[melamine]] was [[2008 Chinese milk scandal#Melamine|added to the milk]] or glutens to increase the measured "protein".

{{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. 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.

Absorption of the amino acids and their [[Derivative (chemistry)|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 and between individual milk proteins, [[beta-lactoglobulin]] and casein. 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]]. 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 (biology)|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.

[[File:"Eat More Cottage Cheese...You'll Need Less Meat...A Postal Card Will Bring Recipes...Cottage Cheese or Meat^ Ask... - NARA - 512542.jpg|thumb|An education campaign launched by the [[United States Department of Agriculture]] about 100 years ago, on [[cottage cheese]] as a lower-cost protein substitute for meat.]]

[[File:Average Protein Supply By Region And Origin.svg|thumb|Average protein supply by region and origin]]

Considerable debate has taken place regarding issues surrounding protein intake requirements. 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.

According to US & Canadian [[Dietary Reference Intake]] guidelines, women aged 19–70 need to consume 46 grams of protein per day while men aged 19–70 need to consume 56 grams of protein per day to minimize risk of deficiency. 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. However, this recommendation is based on structural requirements but disregards use of protein for [[energy metabolism]]. This requirement is for a normal sedentary person. 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.

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. 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, 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 [[Calorie restriction|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.

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. 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. 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.

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.

===Special populations=== <!--T:33-->

{{Main|Food allergy}}

A food allergy is an abnormal [[immune system|immune response]] to proteins in food. The signs and symptoms may range from mild to severe. They may include [[pruritus|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]], [[egg as food|eggs]], [[wheat]], [[shellfish]], [[fish]], [[peanut]]s, [[tree nuts]] and [[soybean|soy]].

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. Moreover, people with this disease while using a low protein diet (0.6 g/kg/d - 0.8 g/kg/d) may develop [[Metabolism#Amino acids and proteins|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.

{{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.

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. 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&nbsp;mg), [[saturated fat]]s (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.

[[File:Starved girl.jpg|thumb|212x212px|A child in Nigeria during the [[Biafra War]] with [[kwashiorkor]] – one of the [[Protein-energy malnutrition|three protein energy malnutrition ailments]] affecting over 10 million children in developing countries.]]{{Main|Protein-energy malnutrition}}

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]]