Protein in nutrition
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Proteins are broken down through digestion that begins in the stomach. Proteins are broken down by enzymes known as proteases into smaller polypeptides to provide amino acids for the organism, including the essential amino acids that the organism cannot biosynthesize itself. Aside from their role in protein synthesis, amino acids are also important nutritional sources of nitrogen.
Proteins, like carbohydrates, contain 4 kilocalories per gram as opposed to lipids which contain 9 kilocalories and alcohols which contain 7 kilocalories. Proteins can be converted into carbohydrates through a process called gluconeogenesis.
Sources of proteins
Dietary sources of protein include meats, eggs, grains, legumes, and dairy products such as milk and cheese. Animal sources of proteins have the complete complement of all twenty amino acids. Vegetable sources are deficient in amino acids and their proteins are said to be incomplete. For example, most legumes typically lack four, including the essential amino acid methionine, while grains lack two, three, or four, including the essential amino acid lysine. See complete protein.
Different proteins have different levels of biological availability 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 or NPU, 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. In general they conclude that animal complete proteins that contain all the essential amino acids such as milk, eggs, meat, and the complete vegetable protein soy are of most value to the body.
According to the recently updated 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. The difference is due to the fact that men's bodies generally have more muscle mass than those of women.
Because the body is continually breaking down protein from tissues, even adults who do not fall into the above categories need to include adequate protein in their diet every day. If enough energy is not taken in through diet, the body will use protein from the muscle mass to meet its energy needs, leading to muscle wasting over time.
Other recommendations suggest 1 gram of protein per kilogram of bodyweight while some extreme sources suggest that higher intakes of 1–2 grams of protein per kilogram of bodyweight are desirable.
How much protein needed in a person's daily diet is determined in large part by overall energy intake, as well as by the body's need for nitrogen and essential amino acids. Physical activity and exertion as well as enhanced muscular mass increase your need for protein. Requirements are also greater during childhood for growth and development, during pregnancy or when breast-feeding in order to nourish your baby, or when the body needs to recover from malnutrition or trauma or after an operation.
Protein deficiency in the third world
Protein deficiency is a serious cause of ill health and death in the third world. Protein deficiency plays a part in the disease kwashiorkor. War, famine, overpopulation and other factors can increase rates of malnutrition and protein deficiency. Protein deficiency can lead to reduced intelligence or Mental retardation, see, Deficiency in Proteins, fats, carbohydrates.
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. 
Protein deficiency in developed countries
Protein deficiency is rare in developed countries but small numbers of people have difficulty getting sufficient protein due to poverty. Protein deficiency can also occur in developed countries in people who are dieting or crash dieting to lose weight, or in older adults, who may have a poor diet. Convalescent people recovering from surgery, trauma, or illness may become protein deficient if they do not increase their intake to support their increased needs. A deficiency can also occur if the protein a person eats is incomplete and fails to supply all the essential amino acids.
Excess protein consumption
Because the body is unable to store it, excess protein is broken down and converted into sugars or fatty acids. The liver removes nitrogen from the amino acids, so that they can be burned as fuel, and the nitrogen is incorporated into urea, the substance that is excreted 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.
Excessive protein intake may also cause the body to lose calcium, which could lead to bone loss in the long-term. However, many protein powders, for instance, come supplemented with various amounts of calcium per serving size so as to counteract the calcium-loss effect.
Some suspect excessive protein intake is linked to several problems:
- Overreaction within the immune system
- Liver dysfunction due to increased toxic residues
- Loss of bone density, frailty of bones is due to calcium and glutamine being leached from bone and muscle tissue to balance increased acid intake from diet (blood pH is maintained at around 7.4). This effect is not present if intake of alkaline minerals (from fruits and vegetables, cereals are acidic as are proteins, fats are neutral) is high. In such cases, protein intake is anabolic to bone.
Many researchers think excessive intake of protein forces increased calcium excretion. If there is to be excessive intake of protein, it is thought that a regular intake of calcium would be able to stabilize, or even increase the uptake of calcium by the small intestine, which would be more beneficial in older women.
Proteins are often progenitors in allergies and allergic reactions to certain foods. 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 perfectly safe. 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. It is extremely unusual for the same person to adversely react to more than two different types of proteins, due to the diversity between protein or amino acid types..
Testing for protein in foods
The classic assay for protein concentration in food is the Kjeldahl method. This test determines 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. 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 used because it is the method the AOAC International has adopted and is therefore used by many food standards agencies around the world.
Digestion of protein
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 differ 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.
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