Protein (nutrient)
Proteins are essential nutrients for the human body. They are one of the constituents of body tissue and also serve as a fuel source. As fuel, proteins have the same energy density as carbohydrates: 17 kJ per gram. The defining characteristic of protein from a nutritional standpoint is its amino acid composition.
Proteins are polymer chains made of amino acids linked by peptide bonds. During human digestion, proteins are broken down in the stomach into 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 that humans must obtain from their diet 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 eight or nine essential amino acids. The consensus seems to lean toward nine since histidine is not synthesized in adults. There are five amino acids that the human body can synthesize: 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: arginine, cysteine, glycine, glutamine, proline and tyrosine. Dietary sources of protein include grains, legumes, nuts, seeds, meats, dairy products, fish, and eggs.
Functions
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 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.
Animal-based protein
, 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 buckwheat, oats, rye, millet, maize, rice, wheat, sorghum, amaranth, and quinoa. Game meat is an affordable protein source in some countries.Non-animal-based protein
Plant sources of proteins include legumes, nuts, seeds, grains, and some vegetables and fruits. Plant 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.
Testing in foods
Nitrogen-based crude 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. 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 as the amount of 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.Nitrogen-based protein measurement cannot distinguish between true protein and non-protein nitrogen. NPN occurs in significant amounts in milk, edible insects, and fish. In addition, accidental contamination and intentional adulteration of protein meals with NPN 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.
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".
True protein
In at least one segment of the food industry, the dairy industry, some countries 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 technology, a type of infrared spectroscopy is used.The more traditional approach to true protein analysis is amino acid analysis. Data from such analysis has additional nutritional meaning, as humans and other animals have specific requirements for essential amino acids. The Food and Agriculture Organization of the United Nations 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 or similar method... is considered acceptable." Using standard methods for amino acid analysis, the true protein content can be reported as the sum of the anhydrous masses of all 18 amino acids analyzed. AA analysis can be performed using standard methods including ISO 13903 and AOAC 988.15.
In the context of dairy products, NPN can also be calculated by precipitating away all protein and measuring the nitrogen content in the remaining fraction.
Ruminant metabolizable protein
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".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 which was developed by the FDA as a modification of the Protein efficiency ratio method. The PDCAAS rating was adopted by the US Food and Drug Administration and the Food and Agricultural Organization of the United Nations/World Health Organization 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 is a measure of the proportion of absorbed protein from a food which becomes incorporated into the proteins of the organism's body.