Protein (nutrient)
Amino acids are the building blocks of protein. They appear in every cell and serve as precursors to nucleic acids, co-enzymes, hormones, immune responses, repair mechanisms, and other molecules essential for life. Proteins account for more mass than any other type of molecule after water. This nutrient is present in hair, skin, blood, bone, and especially abundant in muscle tissue. Cellular messengers like hormones and transport molecules such as enzymes and antibodies are constructed from proteins. Cell membrane components including glycoproteins, G proteins, and ion channels also rely on these structures. The specific types of amino acids and their sequence determine the unique three-dimensional structure and function of each protein. Amino acids obtained through protein catabolism enable the biosynthesis of non-protein molecules like nucleotides, certain neurotransmitters, and heme.
Humans must obtain nine essential amino acids from their diet to prevent protein-energy malnutrition and resulting death. These include phenylalanine, valine, threonine, tryptophan, methionine, leucine, isoleucine, lysine, and histidine. There has been debate regarding whether there are eight or nine essential amino acids. Rosane Oliveira wrote in February 2016 that the consensus leans toward nine since histidine is not synthesized in adults. The human body can synthesize five amino acids: alanine, aspartic acid, asparagine, glutamic acid, and serine. Six conditionally essential amino acids exist whose synthesis can be limited under special pathophysiological conditions. Examples include arginine, cysteine, glycine, glutamine, proline, and tyrosine during prematurity in infants or severe catabolic distress. Dietary sources of protein include grains, legumes, nuts, seeds, meats, dairy products, fish, and eggs.
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. 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. Protein powders such as casein, whey, egg, rice, soy, and cricket flour are processed and manufactured sources of protein. People eating a balanced diet do not need protein supplements. Game meat serves as an affordable protein source in some countries. 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 grown in bioreactor vats.
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 since 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 crude protein content. 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. Nitrogen-based protein measurement cannot distinguish between true protein and non-protein nitrogen. NPN occurs in significant amounts in milk, edible insects, and fish. Accidental contamination and intentional adulteration of protein meals with NPN sources have been known to occur in the food industry for decades. The limitations of the Kjeldahl method were at the heart of the Chinese protein export contamination in 2007 and the 2008 China milk scandal where melamine was added to increase measured protein.
The most important aspect and defining characteristic of protein from a nutritional standpoint is its amino acid composition. Multiple systems rate proteins by their usefulness based on relative percentage of amino acids and digestibility. These include biological value, net protein utilization, and PDCAAS which stands for Protein Digestibility Corrected Amino Acids Score. The PDCAAS rating 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 FAO/WHO in 1993 as the preferred best method to determine protein quality. In 2013, the FAO proposed changing to Digestible Indispensable Amino Acid Score. True protein analysis measures only the proteins in milk whereas crude protein measures all sources of nitrogen including nonprotein nitrogen like urea. Measuring peptide bonds in grains has been put into practice in several countries including Canada, the UK, Australia, Russia, and Argentina using near-infrared reflectance technology.
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 hydrochloric acid. It continues with trypsin and chymotrypsin in the small intestine. Before 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 rates of individual amino acids depend on the protein source. For example, digestibilities differ between soy and milk proteins and between beta-lactoglobulin and casein. About 50% of ingested milk protein is absorbed between the stomach and jejunum. Ninety percent is absorbed by the time digested food reaches the ileum. Newborns of mammals can absorb intact proteins at the small intestine enabling passive immunity transfer of immunoglobulins from mother to newborn via milk.
According to US and Canadian Dietary Reference Intake guidelines, women ages 19, 70 need to consume 46 grams of protein per day while men ages 19, 70 need 56 grams daily to minimize risk of deficiencies. These Recommended Dietary Allowances were calculated based on 0.8 grams protein per kilogram body weight. Average body weights used were 57 kg for women and 70 kg for men. In the United States, average protein consumption is higher than the RDA. Results from NHANES 2013, 2014 showed average protein consumption for women ages 20 and older was 69.8 grams and for men 98.3 grams per day. Active people and athletes may require elevated protein intake due to increased muscle mass and sweat losses. Suggested amounts vary from 1.2 to 1.4 g/kg for endurance exercise to as much as 1.6-1.8 g/kg for strength exercise. A proposed maximum daily protein intake would be approximately 25% of energy requirements or about 2 to 2.5 g/kg.
Protein deficiency and malnutrition can lead to intellectual disability and kwashiorkor. Symptoms of kwashiorkor include apathy, diarrhea, inactivity, failure to grow, flaky skin, fatty liver, and edema of the belly and legs. This edema results from lipoxygenase action on arachidonic acid forming leukotrienes. PEM accounts for 6 million deaths annually worldwide. In industrialized nations, PEM is predominantly seen in hospitals, associated with disease, or found in the elderly. While there is no conclusive evidence that a high protein diet causes chronic kidney disease, consensus suggests people with this disease should decrease consumption. Individuals with phenylketonuria must keep phenylalanine intake extremely low to prevent mental disability. Excessive protein intake may increase calcium excretion in urine leading to higher risk of kidney stone formation. High protein diets have been shown to lead to an additional 1.21 kg of weight loss over three months versus baseline protein diet. The 2015, 2020 Dietary Guidelines for Americans recommend reducing overall intake of protein foods while monitoring sodium and saturated fats.
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Common questions
What are the essential amino acids humans must obtain from their diet to prevent protein-energy malnutrition?
Humans must obtain nine essential amino acids from their diet including phenylalanine, valine, threonine, tryptophan, methionine, leucine, isoleucine, lysine, and histidine. These specific amino acids cannot be synthesized by adults and are required to prevent death from protein-energy malnutrition.
How much protein do women and men ages 19 to 70 need daily according to US Dietary Reference Intake guidelines?
Women ages 19 to 70 need to consume 46 grams of protein per day while men in that age range require 56 grams daily. These Recommended Dietary Allowances were calculated based on 0.8 grams of protein per kilogram body weight using average weights of 57 kg for women and 70 kg for men.
Which methods determine total nitrogen content to calculate crude protein in food samples?
The Kjeldahl method and the Dumas method determine the total nitrogen in a sample to calculate crude protein content. The Kjeldahl test is typically used because it is the method the AOAC International has adopted for this purpose.
What percentage of dietary protein comes from plant foods worldwide compared to animal-derived foods in North America?
Plant protein foods contribute over 60% of the per capita supply of protein on a worldwide basis. In contrast animal-derived foods contribute about 70% of protein sources in North America.
When did the FAO propose changing the PDCAAS rating to Digestible Indispensable Amino Acid Score?
The FAO proposed changing to Digestible Indispensable Amino Acid Score in 2013 as an update to the Protein Digestibility Corrected Amino Acids Score. The PDCAAS rating was previously adopted by the US Food and Drug Administration and the FAO/WHO in 1993 as the preferred best method to determine protein quality.