Protein functions in body
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]Protein function in exercise
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 when carbohydrates and lipid resources are low.[4]
Sources
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, 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, firm 4 oz | 11 |
| 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 |
Protein quality
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 theProtein Efficiency Ratio (PER) method. These methods examine which proteins are most efficiently used by the body.
Eggs have been determined to have the standard biological value of 100 (though some sources may have higher biological values), which means that most of the absorbed nitrogen from egg protein can be retained and used by the body. Corn has a BA of 70 while peanuts have a relatively low BA of 40.[8]
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. The amino acids and their derivatives into which dietary protein is degraded are then absorbed 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[9] and between individual milk proteins, beta-lactoglobulin and casein.[10] 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.[11] 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.
Dietary requirements
Considerable debate has taken place regarding issues surrounding protein intake requirements.[12][13] How much protein needed in a person's daily diet is determined in large part by overall energy intake, by 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][14] 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.[15]
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.[16] The American and Canadian guidelines recommend a daily protein dietary allowance, measured as intake per kilogram body weight, is 0.8 g/kg.[12] However, this recommendation is based on structural requirements, but disregards use of protein for energy metabolism.[12] This requirement is for a normal sedentary person.[14]
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.[12][13][14] Suggested amounts vary between 1.6 g/kg and 1.8 g/kg,[13] while a proposed maximum daily protein intake would be approximately 25% of energy requirements i.e. approximately 2 to 2.5 g/kg.[12] However, many questions still remain to be resolved.[13]
Aerobic exercise protein needs
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.[17] 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.[17] 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]
Anaerobic exercise protein needs
Research also indicates that individuals performing strength-training activity require more protein intake than sedentary individuals.[17][1][5] 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][18] This protein intake can easily be achieved because most athletes consume very high energy intake.[18] Often anaerobic exercise athletes assume that this level of protein intake is very high and tend to over consume.[18][4] Research shows that many athletes do already consume intakes above the recommended intake even without the use of protein supplements.[4]
Excess consumption
When a high dietary protein intake is consumed, there is an increase in urea excretion, which suggests that amino acid oxidation is increased.[14] High levels of protein intake increases the activity of branched-chain ketoacid dehydrogenase.[14] As a result, oxidation is facilitated and the amino group of the amino acid is excreted to the liver.[14] This process suggests that excess protein consumption results in protein oxidation and that the protein is excreted.[14] The body is unable to store excess protein.[14][19] 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 a kidney disease occurs, a decrease in protein will often be prescribed.[20] Furthermore, as noted protein provides the body with 4 Calories per gram, when there is excess protein intake the body will utilize as much of it for energy.[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 the body.[2] Finally, as food protein intake is high or low, the body tries to keep protein levels at an equilibrium.[3] This concept is known as the “labile protein reserve” that serves as a short-term protein store to be used for emergencies or daily variations in protein intake.[3] It 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 unbalance from the oxidation of sulfur amino acids.[3] Also whether if bone resorption contributes to bone loss and osteoporosis is inconclusive.[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 probability of forming kidney stones by 250 percent.[3]
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 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.[21]
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 thefood 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.[22]
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."[23] 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.[24] 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."[25]
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 chemicalmelamine was added to the milk or glutens to increase the measured "protein".[26][27]
Food allergies
Specific proteins found in certain food items are often the cause of allergies and allergic reactions.[28] 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.
Deficiency in developing countries
Protein deficiency is a serious 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.[29] 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.[30]
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. Protein-energy malnutrition (PEM) affects 500 million people and kills 10 million annually. In severe cases white blood cell numbers decline and the ability of leukocytes to fight infection decreases.[29]
Future directions in research
More research is underway to better understand whether or not there is a standardized protein amount needed per day.[31] Because of some difficulties in scientific techniques used to measure the increase in daily protein needs from a sedentary person to an athlete, the increase in protein needed is therefore not universal.[3] Finally, the effects of increased protein intake, both animal and plant, should be furthered tested with respect to increasing osteoporosis risk.[3]
