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Protein (nutrient)

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Protein milkshakes, made from protein powder (center) and milk (left), are a common bodybuilding supplement.

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 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 contain 17 kilojoules (4 Calories) per gram as opposed to lipids which contain 37.8 kilojoules (9 Calories) and alcohols which contain 29.4 kilojoules (7 Calories). Note that 1 Calorie = 1 kilocalorie = 4.184 kilojoules. These numbers are averages, as each protein is slightly different (range roughly 3.5-4.5). The liver, and to a much lesser extent the kidneys, can convert amino acids used by cells in protein biosynthesis into glucose by a process known as gluconeogenesis. The amino acids leucine and lysine are exceptions.

The indispensable amino acids are leucine, isoleucine, valine, lysine, threonine, tryptophan, methionine, phenylalanine and histidine. Histidine is considered to be an indispensable amino acid because of the detrimental effects on haemoglobin concentrations that have been observed when individuals are fed histidine-free diets.

Most animal sources and certain vegetable sources have the complete complement of all the essential amino acids in adequate proportions. However, it is not necessary to consume a single food source that contains all the essential amino acids, as long as all the essential amino acids are eventually present in the diet: see "Complete protein" and "Protein combining".

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 the Protein Efficiency Ratio (PER) method. These methods examine which proteins are most efficiently used by the body.

Egg whites 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 white protein can be retained and used by the body. Corn has a BA of 70 while peanuts have a relatively low BA of 40.

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 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. 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. 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 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. It was suggested that protein intake amount should be measured by using three parameters (which should be viewed together): absolute intake (g/d), intake per body weight (g/body kg/d) and intake as energy percent.

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. U.S recommended daily protein dietary allowance, measured as intake per body weight, is 0.8 g/kg. However, this recommendation is based on structural requirements, but disregards use of protein for energy metabolism. Several studies have concluded that active people and athletes may require elevated protein intake (compared to 0.8 g/kg). Suggested amounts vary between 1.6 g/kg and 1.8 g/kg, while a proposed maximum daily protein intake would be approximately 25% of energy requirements i.e. approximately 2 to 2.5 g/kg. However, many questions still remain to be resolved.

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. 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 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. 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, diarrhoea, 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.

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.

In developed countries

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Protein deficiency is relatively rare in developed countries but some 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. Bob Lanier, a biology professor at Jesuit College Preparatory School of Dallas claims in his Discourse on Minorities in Developed Countries that protein deficiency is more common today than statistics might reveal. Lanier provides a variety of data and connects widespread protein deficiency among low income minority families to explain poor academic performance.

Excess consumption

The body is unable to store excess protein. 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.

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.

Specific proteins are often the cause of 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.

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.

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.

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." 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. 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."

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".

See also

Food portal

• Biological Value
• Essential amino acid
• High-protein diet
• Low-protein diet

References

Related links

• Malnutrition in Third World Countries