Proteins are polymer chains made of amino acids linked together by peptide bonds. Proteins and carbohydrates contain 4 kcal per gram as opposed to lipids which contain 9 kcal per gram.
Amino acids can be divided into either essential amino acids or non-essential amino acids. The essential amino acids, which must be obtained from food sources, are leucine, isoleucine, valine, lysine, threonine, tryptophan, methionine, phenylalanine and histidine. On the other hand, non-essential amino acids can be made by the body from other amino acids. The non-essential amino acids are arginine, alanine, asparagine, aspartic acid, cysteine, glutamine, glutamic acid, glycine, proline, serine, and tyrosine.
In nutrition, proteins are broken down in the stomach during digestion by enzymes known as proteases into smaller polypeptides to provide amino acids for the body, including the essential amino acids that cannot be biosynthesized by the body itself. Thus, protein from one's diet should provide both essential and non-essential amino acids for protein synthesis.[1]
Most animal and certain vegetable proteins are considered complete proteins with a full complement of essential amino acids in adequate proportions. People who avoid animal products may practice protein combining to get the essential amino acids in their diet. .
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Protein is a nutrient needed by the human body for growth and maintenance.[2] Aside from water, protein is the most abundant molecule in the body.[2] Protein is found in all cells of the body and is the major structural component of all cells in the body, especially muscle.[2][3] This also includes body organs, hair and skin.[2] Proteins also are utilized in membranes, such as glycoproteins.[3] When broken down into amino acids, they are used as precursors to nucleic acid and vitamins.[3] Hormones and enzymes are also formed from amino acids in which they help regulate metabolism, support the immune system and other body functions.[4] Finally, protein is needed to form blood cells.[2]
Proteins are one of the key nutrients for success in terms of sports.[4] They play a major role in the response to exercise.[4] Amino acids, the building blocks of proteins, are used for building new tissue, including muscle, as well as repairing damaged tissues.[4] Proteins, however, only provide a small source of fuel for the exercising muscles, being used as fuel typically only when carbohydrates and lipid resources are low.[4]
There are many different sources of protein ranging from whole protein foods (such as milk, meat, fish, egg, and vegetables) to a variety of protein powders (such as casein, whey, egg, rice and soy).[5] Protein powders are processed and manufactured sources of protein.[5] Protein powders may provide an additional source of protein for exercising muscles.[5] The type of protein is important in terms of its influence on protein metabolic response and possibly on the muscle's exercise performance.[5] The different physical and/or chemical properties within the various types of protein may affect the rate of protein digestion.[5] As a result, the amino acid availability and the accumulation of tissue protein is altered because of the various protein metabolic responses.[5]
| Food | Amount of
protein (grams) |
|---|---|
| Spirulina 1 cup | 64 |
| Tempeh 1 cup | 41 |
| Dried Parsley 1 cup | 31 |
| Lentils, cooked 1 cup | 18 |
| Black Beans, cooked 1 cup | 15 |
| Tofu, 3.4 oz | 20 |
| Quinoa, cooked 1 cup | 9 |
| Peanut Butter 2 tbsp | 8 |
| Almonds 1/4 cup | 8 |
| Sun-dried Tomato 1 cup | 8 |
| Brown rice, cooked 1 cup | 5 |
| Broccoli, cooked 1 cup | 4 |
| Potato 1 med. | 4 |
| Lambsquarters 1 cup | 4 |
This table compares the different plant foods with the amount of protein in grams per serving[6][7] [8]
Different proteins have different levels of biological availability (BA) to the human body. Many methods have been introduced to measure protein utilization and retention rates in humans. They include biological value, net protein utilization, and PDCAAS (Protein Digestibility Corrected Amino Acids Score) which was developed by the FDA as an improvement over the Protein Efficiency Ratio (PER) method. These methods examine which proteins are most efficiently used by the body. The PDCAAS rating is a fairly recent evaluation method; it 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.[9]
Most proteins are decomposed to single amino acids in digestion.
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 intestine. Before the absorption in the small intestine, most proteins are already reduced to single amino acid or peptides of several amino acids. Most of peptides longer than 4 amino acids are not absorbed. Absorption into the intestinal absorptive cells is not the end. There most of 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[10] and between individual milk proteins, beta-lactoglobulin and casein.[11] 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.[12] 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 from milk.
Considerable debate has taken place regarding issues surrounding protein intake requirements.[13][14] 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, individual's energy and carbohydrate intake, as well as the presence of illness or injury.[1][5][15] 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 breast-feeding in order to nourish a baby, or when the body needs to recover from malnutrition or trauma or after an operation.[16]
If enough energy is not taken in through diet, as in the process of starvation, the body will use protein from the muscle mass to meet its energy needs, leading to muscle wasting over time. If the individual does not consume adequate protein in nutrition, then muscle will also waste as more vital cellular processes (e.g. respiration enzymes, blood cells) recycle muscle protein for their own requirements.
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 avoid a deficiency.[17] The American and Canadian guidelines recommend a daily protein dietary allowance, measured as intake per kilogram body weight, is 0.8 g/kg.[13] However, this recommendation is based on structural requirements, but disregards use of protein for energy metabolism.[13] This requirement is for a normal sedentary person.[15]
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.[13][14][15] Suggested amounts vary between 1.6 g/kg and 1.8 g/kg,[14] while a proposed maximum daily protein intake would be approximately 25% of energy requirements i.e. approximately 2 to 2.5 g/kg.[13] However, many questions still remain to be resolved.[14]
Endurance athletes differ from strength-building athletes in that endurance athletes do not build muscle mass from training. 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.[18] Although the protein requirement for athletes still remains controversial, 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.[18] 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. Some scientists suggest that endurance athletes may increase daily protein intake to a maximum of 1.2–1.4 g per kg body weight.[5]
Research also indicates that individuals performing strength-training activity require more protein than sedentary individuals.[1][5][18] 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.[5][19] Many athletes maintain a high-protein diet as part of their training, and so protein deficiency is less likely among this group than among non-athletes.[19] In fact, some athletes who specialize in anaerobic sports (e.g. weightlifting) assume a very high level of protein intake is necessary, and may over-consume.[4][19] Research indicates that many athletes consume more protein than they need even without the use of protein supplements.[4]
When a high dietary protein intake is consumed, there is an increase in urea excretion, which suggests that amino acid oxidation is increased.[15] High levels of protein intake increase the activity of branched-chain ketoacid dehydrogenase.[15] As a result, oxidation is facilitated, and the amino group of the amino acid is excreted to the liver.[15] This process suggests that excess protein consumption results in protein oxidation and that the protein is excreted.[15] The body is unable to store excess protein.[15][20] Protein is digested into amino acids, which enter the bloodstream. Excess amino acids are converted to other usable molecules by the liver in a process called deamination. Deamination converts nitrogen from the amino acid into ammonia, which is converted by the liver into urea in the urea cycle. Excretion of urea is performed by the kidneys. These organs can normally cope with any extra workload, but, if kidney disease occurs, a decrease in protein will often be prescribed.[21] Furthermore, as noted, protein provides the body with 4 calories per gram, and when there is excess protein intake, the body will utilize as much of it for energy as possible.[2] After that stage, the body will produce fat from the excess protein, turning it into fat cells.[2] On the other hand, if people do not eat enough calories, body protein and protein from the food will be utilized for energy.[2] This is not ideal as the main function of protein is to maintain muscle mass.[2] Finally, 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", which serves as a short-term protein store to be used for emergencies or daily variations in protein intake.[3] However, that reserve is not utilized as longer-term storage for future needs.[3]
Many researchers have also found that excessive intake of protein increases calcium excretion in urine.[3] It has been thought that this occurs to maintain the pH imbalance from the oxidation of sulfur amino acids.[3] Also, it is inconclusive whether bone resorption contributes to bone loss and osteoporosis.[3] However, it is also found that a regular intake of calcium would be able to stabilize this loss.[3]
Another issue arising from overconsumption of protein is a higher risk of kidney stone formation from calcium in the renal circulatory system.[3] It has been found that high animal protein intake in healthy individuals increases the probability of forming kidney stones by 250 percent.[3]
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 fibre 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.[22]
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.[23]
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."[24] 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.[25] 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."[26]
The limitations of the Kjeldahl method were at the heart of the Chinese protein export contamination in 2007 and the 2008 Chinese Milk Scandal in which the industrial chemical melamine was added to the milk or glutens to increase the measured "protein".[27][28]
Specific proteins found in certain food items are often the cause of allergies and allergic reactions.[29] This is because the structure of each form of protein is slightly different; some may trigger a response from the immune system while others remain harmless. Many people are allergic to casein, the protein in milk; gluten, the protein in wheat and other grains; the particular proteins found in peanuts; or those in shellfish or other seafoods. Food allergies should not be confused with food intolerance.
Protein deficiency is a common cause of ill health and death in developing countries. Protein deficiency plays a part in the disease kwashiorkor. Famine, overpopulation and other factors can increase rates of malnutrition and protein deficiency. Protein deficiency can lead to reduced intelligence or mental retardation (see nutrition disorder).
In countries that suffer from widespread protein deficiency, food is generally full of plant fibers, which makes adequate energy and protein consumption very difficult.[30] Protein deficiency is generally caused by lack of total food energy, making it an issue of not getting food in total. 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 normal functioning of proteins in fluid balance and lipoprotein transport.[31]
Moringa trees are known to overcome protein deficiency in developing countries as the leaves and other parts of the tree contain comparably to soy bean high amount of crude proteins and amino acids.
Dr. Latham, director of the Program in International Nutrition at Cornell University claims that malnutrition is a frequent cause of death and disease in third world countries. In 1979, protein-energy malnutrition (PEM) was estimated to affect 500 million people and kill 10 million annually. In severe cases white blood cell numbers decline and the ability of leukocytes to fight infection decreases.[30]