Proteins are present in every cell in the body. Protein is used to build, repair and maintain most tissues in the body. Proteins are also used to make enzymes, hormones and other substances the body can use.

Proteins are composed of chains of amino acids. There are generally a few dozen to several hundred amino acids in a single protein. The human body requires 22 different amino acids. Nine of these, called essential amino acids, must be supplied by the diet. The body can make the remaining 13 amino acids (nonessential amino acids) from other nutrients (e.g., fats, carbohydrates, other amino acids).

Animal sources and soy are considered complete proteins because they provide all nine essential amino acids. Plant sources other than soy are considered incomplete proteins because they lack one or more of the essential amino acids. Incomplete proteins lacking in one amino acid may be eaten with another incomplete protein that is missing a different amino acid. Incomplete proteins that provide all nine essential amino acids when eaten together are called complementary proteins.

The amount of protein needed may vary from one person to another. However, most people can get their recommended dietary allowance of protein by eating a 3-ounce serving of meat or meat substitute twice every day. The majority of Americans get more than enough protein.

Protein deficiency may lead to a condition known as protein-energy malnutrition (PEM). PEM causes severe weight loss, wasting and, in children, poor growth.

Consuming protein beyond amounts the body requires provides no additional health benefit. For example, eating more protein will not cause additional muscle development. Any excess protein in the diet is removed from the body through urine and stool, or is converted into fat. Excessive amounts of protein have been associated with a number of health problems, including heart disease and osteoporosis.

Protein is the nutrient responsible for helping to build, repair and maintain most of the tissues in the body, including the muscles, internal organs, bones, skin and blood. There are estimated to be tens of thousands of different proteins in the human body. Proteins are present in every cell in the body.

Proteins are composed of much smaller substances called amino acids that join together in chains called peptides. Two amino acids joined together form a dipeptide. Three bonded amino acids form a tripeptide. When four or more amino acids are joined into a chain, they form a polypeptide. Proteins are complex molecules made up of strings of many joined amino acids (typically a few dozen to several hundred) that are folded over onto themselves. The shapes formed by these strings allow proteins to do their work.

The formation and breakdown of proteins and amino acids is a constant process. When the body digests protein in food, certain enzymes (e.g., protease enzymes) break the protein down into amino acids. In addition, proteins are constantly being both made and broken down within the body, a process known as protein turnover. Amino acids from foods join with amino acids produced within the body to form an amino acid pool. Any of the amino acids from this pool may be used in protein turnover or to create other compounds or energy the body can use.

Protein has numerous functions in the body. These functions include:

• Growth and maintenance. Proteins make up a portion of all cells in the body and help support the growth and repair of body tissue.
  
  
• Transportation. Proteins also act as transport cells, delivering nutrients and other substances to the cells throughout the body. For example, hemoglobin, the part of red blood cells that carries oxygen, is composed of protein.
  
  
• Fluid balance. Proteins are partially responsible for maintaining fluid balance within the body. Proteins also act as buffers to help maintain the pH balance of body fluids.
  
  
• Creation of necessary chemicals. Proteins play a vital part in creating other substances used by the body. These include:
  
  
• • Enzymes. About half of all dietary protein is used to make enzymes. Enzymes are used to stimulate chemical reactions in the body, such as the breaking down and building up of various substances. Enzyme activity in the body may include the digestion and absorption of food and the creation of new cells, tissues, chemicals and other substances.
    
    
  • Hormones. Some hormones used to regulate body processes are created from proteins.
    
    
  • Antibodies. Some antibodies used to protect the body against disease are created from proteins.

Proteins may be used for a variety of other functions in the body, including vision, blood clotting and to make neurotransmitters (substances that carry messages between nerves). If there is an abundance of available protein or if the body is not receiving enough fats and carbohydrates, proteins can also be broken down for energy. One gram of protein provides the body with 4 calories. If these calories are not needed, the protein can be converted into fat for later use.

Proteins are essential for many body processes. However, the amount of protein needed may vary from one person to another. Large bodies require more protein to maintain and repair body tissues. Growing bodies (e.g., children, pregnant women) and those recovering from injuries need more protein to help build new tissues. Women who are breastfeeding need more protein to produce milk.

The average adult can figure out about how much protein he or she needs every day by calculating the Dietary Reference Intake (DRI). An adult’s protein DRI is equal to 0.8 grams of protein per each kilogram (2.2 pounds) of body weight. The recommended dietary allowance of protein for most adult males is 56 grams per day. For women, it is 46 grams per day. During childhood, protein requirements may increase. Adolescents require 0.85 grams per kilogram of their weight each day, toddlers require 1.1 grams per kilogram, and infants 1.5 grams of protein per kilogram of their weight every day. Pregnant and breastfeeding women generally need an additional 25 grams of protein every day.

For most people, the protein DRI can be achieved by eating a 3-ounce serving of meat or meat substitute twice every day. The majority of Americans get more than enough protein. Even endurance athletes, who need more protein than non-athletes, typically get plenty of protein in their diet.

During digestion, different sources of protein are broken down into different types of amino acids. The human body requires 22 different amino acids. Nine of these, called essential amino acids, cannot be produced by the body and must be supplied by food or dietary supplements. The body can make the remaining 13 amino acids (nonessential amino acids) from other nutrients (e.g., fats, carbohydrates, other amino acids). Proteins may be considered complete or incomplete. Complete proteins provide all nine essential amino acids. Incomplete proteins are lacking in one or more of the essential amino acids.

A nonessential amino acid may become essential if the body lacks the substances required to make it or if a medical condition inhibits its production. When this occurs, the typically nonessential amino acid becomes conditionally essential. For example, the genetic condition phenylketonuria inhibits the production of tyrosine, making it a conditionally essential amino acid in people with this condition. Other amino acids which are considered conditionally essential are arginine, cysteine and glycine.

Three essential amino acids are known as branched-chain amino acids. These three, isoleucine, leucine and valine, are important substances in the formation, strengthening and repair of muscle tissues. Other essential amino acids include:

• Histidine. Considered an essential amino acid, but mostly for children. Important for growth and healing, as well as the creation of histamine (a substance that reduces a person’s sensitivity to allergens).
  
  
• Lysine. Important in new tissue formation, calcium absorption and for producing antibodies, enzymes and hormones.
  
  
• Methionine. Helps to maintain the liver and assist in nerve and immune functions. The breakdown of methionine produces another amino acid called homocysteine. Homocysteine may play a role in the development of atherosclerosis, the narrowing of blood vessels that is a major risk factor for heart disease, heart attack and stroke.
  
  
• Phenylalanine. An important amino acid to the human brain, helping to regulate mood. It is needed for growth of children and for protein metabolism in children and adults. It is normally converted to tyrosine in the human body.
  
  
• Threonine. Important in the formation of many body tissues. Also plays a role in immune function.
  
  
• Tryptophan. An important amino acid to the human brain, affecting many functions of mood and perception.

Nonessential amino acids also have a number of different functions in the human body, including growth, the production of new tissues, metabolism and healing.

Protein deficiency leads to a condition known as protein-energy malnutrition (PEM). This condition is rare in developed nations, occurring most frequently in areas of the world where food is scarce and hunger and starvation are common, such as developing countries. PEM causes severe weight loss, wasting and, in children, poor growth. Children with PEM may be particularly thin for their height or short for their age. There are two forms of PEM:

• Kwashiorkor. Kwashiorkor is a protein deficiency where there may still be adequate caloric intake. It generally occurs following a recent and sudden period of starvation. The body may still have enough fat to supply it with energy, but there is no intake of protein to resupply it with amino acids. The body does not store extra protein as it does with fat, so the levels of proteins and substances made from proteins (e.g., enzymes, hormones) in the body are greatly reduced. Protein carriers are also in short supply, so fat cannot be properly transported out of the liver. The liver becomes large and fatty, causing a noticeable bulge in the abdomen.
  
  Insufficient amounts of the protein albumin (responsible for maintaining fluid balance) are responsible for some of the distinguishing features of kwashiorkor. When albumin levels are not sufficient, fluids pool in various areas of the body, causing the swollen face and limbs that are characteristic of kwashiorkor.
  
  
• Marasmus. Marasmus includes a deficiency in both caloric intake and protein, characterized by muscle wasting following a prolonged period of starvation. The body is breaking down its own muscle tissue for energy. Eventually, even the muscles of the heart may be sacrificed. People with marasmus are often very still and quiet, using no more energy than absolutely necessary. Children with the condition generally do not even cry out for food.

Signs of less severe protein deficiency include anemia and hair loss. This condition is also rare in the United States because the typical American diet provides more protein than is needed. However, protein deficiency can occur, especially among the very poor and people with eating disorders (e.g., anorexia nervosa, bulimia nervosa).

Consuming protein beyond amounts the body requires (e.g., 46 to 56 grams per day for most adults) provides no additional health benefits. Any excess protein in the diet is removed from the body through urine and stool or is converted into fat. Eating more protein will not increase muscle development.

Excessive amounts of protein have been associated with a number of health problems, including:

• Heart disease. Homocysteine is produced during the processing of the amino acid methionine. Homocysteine may play a role in the development of atherosclerosis, the narrowing of blood vessels that is a major risk factor for heart disease, heart attack and stroke. Furthermore, many protein sources are also high in fats, especially saturated fats. This can also increase the risk of developing atherosclerosis.
  

  

• Osteoporosis. The body’s breakdown of proteins may release potentially harmful byproducts, and the body uses calcium and other substances to neutralize these byproducts. However, calcium used for this purpose is no longer available to increase bone density. In addition, if there is not enough calcium in the diet, the mineral may be pulled directly from the bone during protein digestion. This weakens the bone and increases the risk of osteoporosis.

Protein may need to be restricted in patients with certain medical conditions, such as liver or kidney disease. Processing large amounts of proteins and amino acids can become stressful for a diseased liver or kidneys. If these organs do not efficiently process these substances, a byproduct called urea may build up in the kidneys, blood or elsewhere. This can lead to kidney stones, uremic poisoning and possibly gout. Because of this, patients with liver or kidney disease often need to limit their intake of protein. A physician and dietitian can suggest ways to get adequate protein without stressing an impaired liver or kidneys.

Recent studies have also linked excessively high protein consumption to a greater risk of developing certain types of cancer.

Some popular diets recommend consumption of high amounts of protein and low amounts of carbohydrates. Studies have shown that these diets may provide greater immediate weight loss than diets low in fat and high in carbohydrates. For example, one recent study found that consuming higher levels of dietary protein may trigger production of a hormone called peptide YY. People will higher levels of this hormone may feel more full, causing them to eat less and to lose weight.

However, other studies have found that many people who initially lose weight on high-protein diets eventually gain back the pounds. Highly restrictive diets like these may be difficult for many people to maintain, resulting in a rebound of weight after the diet has been deserted.

In addition to the health concerns regarding high protein intake, there are also concerns regarding a low intake of carbohydrates, especially since many important nutrients come from carbohydrate-rich foods.

Animal sources and soy are considered complete proteins because they provide all nine essential amino acids. These are considered higher quality proteins. Plant sources other than soy are considered incomplete proteins because there are lacking in one or more of the essential amino acids. They are considered lower quality proteins.

Incomplete proteins lacking in one amino acid can be eaten with another incomplete protein that is missing a different amino acid. This way, all nine essential amino acids are consumed. For example, eating legumes (which contain the essential amino acid lysine, but not methionine) together with grains (which contain methionine, but not lysine) provides all essential amino acids. When consumed together, these food combinations are called complementary proteins.

A measurement called the protein digestibility-corrected amino acid score (PDCAAS) can be used to identify high-quality (complete) proteins. It compares the amino acids contained within a particular food with the amino acids required by the human body. It also adjusts  for ease of digestion because proteins from animal sources tend to be more easily digested than those from plant sources.

However, foods considered sources of complete proteins are not necessarily healthier than those considered to be incomplete proteins. This is because complete proteins are often accompanied by high amounts of saturated fat. The complete nutritional package of a food must be considered. For example, lean cuts of meat provide complete protein with much less fat than other cuts of meat.

Soy is the only plant source of protein that provides all nine essential amino acids. This makes soy an important food choice for vegetarians, especially vegans, who do not consume any animal products. Soybeans can be processed into a number of different food products (e.g., tofu, soy milk, soy flour, tempeh), many of which take on the flavor or texture of the foods they are served with. Soy may also have additional, specific health benefits.

Amino acids are also available in supplement form. However, these are not used by the body as easily as those that come from dietary protein. Amino acid supplements can also lead to an imbalance of amino acid levels in the body. Patients are urged to speak with their physician or dietitian before taking amino acid supplements.

According to the U.S. Department of Agriculture (USDA), protein levels of some common foods include:

Preparing questions in advance can help patients have more meaningful discussions with health professionals regarding their conditions. Patients may wish to ask their physician or registered dietitian the following questions related to proteins:

1. How much protein do I need to eat every day?
   
   
2. Does my diet provide me with all the essential amino acids I need?
   
   
3. Do I have a medical condition that may make a nonessential amino acid conditionally essential?
   
   
4. What complementary proteins go well with my favorite foods?
   
   
5. How might I reduce the amount of saturated fat in my protein choices?
   
   
6. How might I reduce my protein intake without removing my favorite foods from my diet?
   
   
7. Might I benefit from an amino acid supplement?
   
   
8. Does the protein content of my diet put me at risk for heart disease or osteoporosis?
   
   
9. Might any of my medical conditions benefit from reducing my protein intake?
   
   
10. How much protein is in a serving of my favorite foods?