Proteins: Functions, Sources, Deficiency

What are Proteins?

Proteins are very big molecules and they are made up of small units called amino acids. There are many different amino acids that mix together in different numbers and formations to produce lots of different proteins.

Some amino acids are essential for children and adults and they must be provided ready-made by the protein in food. All the other amino acids that children and adults need do not have to be ready-made and can be put together inside the body from the protein eaten in food.

Protein foods that contain all of the essential amino acids are said to have high biological value (HBV). Protein foods that are missing one or more essential amino acids are said to have low biological value (LBV). If a mixture of these LBV proteins is eaten every day, all the essential amino acids will be provided, so they are good sources of protein.

Foods are made up of different proteins. Each type of protein has a name. Some examples of protein names are:

  • ovalbumin: found in egg white
  • gluten: found in wheat
  • collagen: found in meat
  • caseinogen: found in cheese
  • lactoglobulin: found in milk

Functions of Proteins

Everyone needs to have protein every day, though the amount that is needed will change during our lives as we grow, become adults, and change our lifestyles and activities.

Babies, children, and teenagers need more protein for their body size than adults to allow for their body growth as well as everything else that protein is needed for. Adults, who have stopped growing, need protein to maintain and repair their bodies and grow their hair, and fingernails, and replace body cells (e.g. red blood cells). Pregnant and lactating (breastfeeding) women need extra protein to allow for the development of the baby.

Protein is needed for many jobs in the body so if we do not have enough, the body will begin to suffer. This is what happens if a child does not have enough protein:

  • They stop growing.
  • Their hair becomes very thin.
  • They cannot digest food properly.
  • They have diarrhea.
  • They catch infections easily.
  • Fluid builds up under their skin (this is called oedema).
  • They become very thin and weak.

This is what happens if an adult does not have enough protein:

  • They lose fat and muscle from their body.
  • Their internal organs become weak.
  • Their hair and skin become dry.
  • They get oedema.

On the other hand, if we have too much protein in our diet, it makes the liver and kidneys work harder because they have to process the protein in the body. This may eventually put a strain on them. If we do not use the extra protein for energy, the body will store it as fat.

Learn more about the functions of protein in your body

Deficiency of Proteins

In protein deficiency, when the diet supplies too little protein or lacks a specific essential amino acid relative to the others (a limiting amino acid), the body slows its synthesis of proteins while increasing its breakdown of body tissue protein to liberate the amino acids it needs to build other proteins of critical importance.

When these proteins are not available to perform their roles, many of the body’s life-sustaining activities come to a halt. The most recognizable consequences of protein deficiency include slow growth in children, impaired brain and kidney functions, weakened immune defenses, and impaired nutrient absorption from the digestive tract.

It is very rare to see protein deficiency in developed countries like the UK.

Food Sources of Proteins

These foods are HBV sources of protein:

  • milk
  • meat
  • fish
  • cheese
  • eggs
  • soya beans.

These foods are LBV sources of protein:

  • cereals (rice, wheat millet, oats, quinoa)
  • peas, beans (except soya beans), and lentils
  • nuts and seeds.

Recommended Intakes of Protein

The committee that established the RDA states that a generous daily protein allowance for a healthy adult is 0.8 grams per kilogram (2.2 pounds) of healthy body weight. The RDA covers the need for replacing worn-out tissue, so it increases for larger people; it also covers the needs for building new tissue during growth, so it is slightly higher for infants, children, and pregnant and lactating women.

To calculate your protein RDA:

  • Look up the healthy weight for a person of your height. If your present weight falls within that range, use it for the following calculations. If your present weight falls outside the range, use the midpoint of the healthy weight range as your reference weight.
  • Convert pounds to kilograms, if necessary (pounds divided by 2.2 equals kilograms).
  • Multiply kilograms by 0.8 to get your RDA in grams per day. (Teens 14 to 18 years old, multiply by 0.85.)

In setting the RDA, the committee assumes that the protein eaten will be of good quality, that it will be consumed together with adequate energy from carbohydrates and fat, and that other nutrients in the diet will be adequate. The committee also assumes that the RDA will be applied only to healthy individuals with no unusual alteration of protein metabolism.

Most people assume that Americans eat too much protein. Research demonstrates that a median protein intake for U.S. adult males is about 16 percent of total kcalories, an amount that falls directly within the DRI suggested range of between 10 and 35 percent of kcalories. Women, children, and some elderly people may typically take in less protein—13 to 15 percent. A small percentage of adolescent girls and elderly women consume insufficient protein or barely enough to meet their needs.

Side Effects of Too Much Protein

While many of the world’s people struggle to obtain enough food and enough protein to survive, in developed nations protein is so abundant that problems of protein excess are observed. Overconsumption of protein offers no benefits and may pose health risks for the heart and weakened kidneys.

Heart Disease

Protein itself is not known to contribute to heart disease and mortality, but some of its food sources may do so. Selecting too many animal-derived protein foods, such as fatty red meats, processed meats, and fat-containing milk products, adds a burden of fat calories and saturated fat to the diet and crowds out fruits, vegetables, legumes, and whole grains.

Consequently, it is not surprising that people who eat substantial amounts of high-fat meats—particularly processed meats such as lunch meats and hot dogs—have higher body weights and a greater risk of obesity, heart disease, and diabetes than those who eat less.

Kidney Disease

Excretion of the end products of protein metabolism depends, in part, on an adequate fluid intake and healthy kidneys. A high protein intake increases the work of the kidneys but does not appear to damage healthy kidneys or cause kidney disease.10 In people with chronic kidney disease, however, a high-protein diet may accelerate the kidneys’ decline. One of the most effective ways to slow the progression of kidney disease is to restrict dietary protein.

Protein and Amino Acid Supplements

Why do people take protein or amino acid supplements? Athletes often take them when trying to build muscle. Dieters may take them to spare their bodies’ protein while losing weight. Some women take them to strengthen their fingernails. People take individual amino acid supplements, too—to cure herpes, improve sleep, lose weight, and relieve pain and depression. Do protein and amino acid supplements really do these things? Probably not. Are they safe? Not always.

Protein Supplements

Though protein supplements are popular with athletes, well-fed athletes do not need them. Dietary protein is necessary for building muscle tissue, and consuming protein in conjunction with resistance exercise helps muscles build new proteins. Protein supplements, however, do not improve athletic performance beyond the gains from well-timed meals of ordinary foods.

Weight-loss dieters may benefit from consistently consuming protein-rich foods because protein often satisfies the appetite. Research is ongoing to determine whether sufficient protein content of a meal may help to prolong feelings of fullness or delay the urge to eat.

However, extra protein from powders, pills, or beverages is unlikely to dampen the appetite further, although it contributes unneeded calories—the wrong effect for weight loss. Evidence does not support taking protein supplements for weight loss, and common sense opposes it.

Amino Acid Supplements

Enthusiastic popular reports have led to the widespread use of individual amino acids. One such amino acid is lysine, promoted to prevent or relieve the infections that cause herpes sores on the mouth or genital organs. Lysine does not cure herpes infections. Whether it reduces outbreaks or even whether it is safe is unknown because scientific studies are lacking. Tryptophan supplements are advertised to relieve pain, depression, and insomnia.

Tryptophan plays a role as a precursor for the brain neurotransmitter serotonin, an important regulator of sleep, appetite, mood, and sensory perception. The DRI committee concludes that high doses of tryptophan may induce sleepiness, but they may also cause side effects, such as nausea and skin disorders.

The body is designed to handle whole proteins best. It breaks them into manageable pieces (dipeptides and tripeptides) and then splits these, a few at a time, simultaneously releasing them into the blood. This slow bit-by-bit assimilation is ideal because groups of chemically similar amino acids compete for the carriers that absorb them into the blood. An excess of one amino acid can produce such a demand for a carrier that it limits the absorption of another amino acid, creating a temporary imbalance.

The DRI committee reviewed the available research on amino acids, but with next to no safety research in existence, the committee was unable to set Tolerable Upper Intake Levels for supplemental doses. Until research becomes available, no level of amino acid supplementation can be assumed to be safe for all people.

A known side effect of these products is digestive disturbances: amino acids in concentrated supplements cause excess water to flow into the digestive tract, causing diarrhea. Anyone considering taking amino acid supplements should be cautious not to exceed levels normally found in foods.

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