protein (nutrient) protein (nutrient) proteins are essential nutrients for the human body. they are one of the building blocks of body tissue and can also serve as a fuel source. as a fuel, proteins provide as much energy density as carbohydrates: 4 kcal (17 kj) per gram; in contrast, lipids provide 9 kcal (37 kj) per gram. the most important aspect and defining characteristic of protein from a nutritional standpoint is its amino acid composition. this article is about protein as a nutrient. for protein as a class of molecule, see protein. for other uses, see bodybuilding supplement. amino acids are the building blocks of protein. amino acids are necessary nutrients. present in every cell, they are also precursors to nucleic acids, co-enzymes, hormones, immune response, repair and other molecules essential for life. proteins are polymer chains made of amino acids linked together by peptide bonds. during human digestion, proteins are broken down in the stomach to smaller polypeptide chains via hydrochloric acid and protease actions. this is crucial for the absorption of the essential amino acids that cannot be biosynthesized by the body. there are nine essential amino acids which humans must obtain from their diet in order to prevent 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 grains, legumes, nuts, seeds, beans, meats, dairy products, fish, eggs, edible insects, and seaweeds. protein functions in human body 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 glycoproteins. 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. sources some sources of animal-based protein nutritional value and environmental impact of animal products, compared to agriculture overall categories contribution of farmed animal product calories 18 proteins 37 land use 83 greenhouse gases 58 water pollution 57 air pollution 56 freshwater withdrawals 33 protein occurs in a wide range of food. on a worldwide basis, plant protein foods contribute over 60% of the per capita supply of protein. in north america, animal-derived foods contribute about 70% of protein sources. insects are a source of protein in many parts of the world. 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. meat, dairy, eggs, soy, 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. some research highlights game meat as a protein source. vegan sources of proteins include legumes, nuts, seeds and fruits. vegan foods with protein concentrations greater than 7% include 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. plant sources of protein. people eating a balanced diet do not need protein supplements. the table below presents food groups as protein sources. food sourcelysinethreoninetryptophansulfur-containingamino acids legumes64381225 cereals and whole grains31321237 nuts and seeds45361746 fruits45291127 animal85441238 colour key: protein source with highest density of respective amino acid. protein source with lowest density of respective amino acid. protein milkshakes, made from protein powder (center) and milk (left), are a common bodybuilding supplement protein powders – such as casein, whey, egg, rice, soy and cricket flour– are processed and manufactured sources of protein. testing in foods 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. on food labels the protein is given 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. 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. 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." 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. 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." 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. 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". 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". protein quality 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. digestion most proteins are decomposed to single amino acids by digestion 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 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. newborn 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. dietary requirements 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. 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. dietary recommendations 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. active people 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, while a proposed maximum daily