What Is a Protein?

A Key Building-Block of Life

A protein is a large, complex molecule that is a key building block of life. We all know that it is an important part of our diets, but how many of us know how protein actually works in our bodies, and why we need it?

Protein foods

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What It Is

Protein is vital to the functioning of cells in living organisms. Proteins are required for the structure and regulation of the body’s tissues and organs. They are made up of long chains of amino acids—at least 20 different types of amino acids, in fact.

Nine of the amino acids that people need to for making protein—histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine—must come from food.

How It Works

There are as many as 100,000 unique types of proteins within a typical human cell. The functioning of each depends on its shape.

A protein begins in the cell as a long chain of about 300 building blocks (on average) known as amino acids. There are more than 20 different types of amino acids, and how they are ordered determines how the protein chain will fold upon itself and take shape.

Once a protein has its shape, it can bind to other selected molecules in order to carry out its function. That function could be signaling, storing, transporting, providing structure, fighting foreign invaders, acting as a catalyst, or another function.

Types of Protein and Their Uses

Proteins come in a variety of types, according to function. These include the following:


These are components of the immune system that help to protect the body from foreign particles, such as viruses and bacteria. Proteins recognize and bind themselves to foreign substances in order to neutralize them and help protect the body. An example is of an antibody is immunoglobulin G (IgG).


Enzymes carry out almost all of the chemical reactions happen in cells, and also help new molecules to form by reading the genetic information stored in DNA. An enzyme increases the speed at which a chemical reaction happens.

An example of an enzyme is phenylalanine hydroxylase. This enzyme catalyzes the breakdown of the amino acid phenylalanine. Infants born unable to make this enzyme have toxic effects from the incomplete metabolism of phenylalanine.


Also known as signaling proteins, they allow communication between cells. They include some types of hormones. These proteins transmit signals to coordinate biological processes between cells, tissues, and organs. An example of a messenger protein is somatotropin, also known as growth hormone.


Structural proteins allow cells to maintain their shape and organization. On a larger level, they provide the structural elements of connective tissues such as bone and cartilage and help muscles to function. An example of a structural protein is actin.

Transport and Storage

Transport and storage proteins attach themselves to atoms and small molecules, storing or carrying them within cells and throughout the body. An example is ferritin, which stores iron for use by blood cells and other body tissues.

How Much You Need

Since protein is integral to the function of every cell in your body, it’s important to get enough of the macronutrient in your diet—from healthy sources. Getting your protein from diverse sources, including ones that are plant-based, will give you the healthiest balance.

The daily nutritional goals set by the U.S. Department of Agriculture (USDA) are 56 grams of protein for men ages 19 and over, and 46 grams of protein for women.

The protein food group includes meat, poultry, seafood, legumes (beans, peas, and soy products), eggs, nuts, and seeds. The USDA recommends opting for leaner, less fatty meat and poultry items, as well as consuming at least 8 ounces (oz) of cooked seafood per week if you are not a vegetarian.

How to Get Enough in Your Diet

You likely won’t have problems getting enough protein. People in the U.S. are actually consuming far more protein than is needed each day, according to an analysis published in Public Health Nutrition.

The study found that men ages 20 and above take in 234 grams (8.3 ounces) of protein food (including meat, dairy, fish, seafood, eggs, nuts, legumes, and soy) per day, 72% being from meat; while women take in 148 grams of protein food daily, with 70% from meat.

For comparison’s sake, one cooked T-bone steak that weighs 219 grams (7.7 oz) will contain 59 grams of protein—plus 515 calories and 29 grams of fat, according to the USDA.

So not only have you maxed out your daily allotment of protein, but if are a male or female between the ages of 31 and 50 then according to the agency, you also will have eaten 19-29 percent of your daily recommended calorie intake and possibly your entire amount of allotted fat.

A healthier dinner portion of protein would be a 113-gram (4-oz) fillet of fish that is baked or broiled with oil. This yields 25 grams of protein, 188 calories, and 9 grams of fat.

Protein Deficiency

Protein deficiencies are rare for people in wealthier countries, such as the United States. Even vegetarians and vegans typically take in more than the daily recommended amount of protein, according to a review of scientific literature published in the journal Nutrients.

However, a form of malnutrition called kwashiorkor can develop in places experiencing famine, natural disasters, or other disruptions to the food supply. Caused by not having enough protein in the diet, kwashiorkor’s symptoms include:

  • An enlarged, protruding belly
  • Decreased muscle mass
  • Diarrhea
  • Failure to gain weight and grow in children
  • Fatigue
  • Fading skin color
  • Changes to hair color or texture
  • Increased and more severe infections
  • Irritability
  • Swelling ankles and feet

With early nutritional treatment, children with kwashiorkor can recover fully. However, permanent physical and mental impairments can take place if treatment is. If treatment comes too late, shock, coma, and death can result.

A Word From Verywell

As protein is contained in every cell of the body, it’s important to know how it works and how to get enough of it in your diet. However, it’s best to focus more on focus on taking in healthy sources of protein rather than consuming a lot of it.

12 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Genetics Home Reference. What are proteins and what do they do?

  2. Harvard T.H. Chan School of Public Health. Protein.

  3. Geiler-Samerotte K. Protein folding: The good, the bad, and the ugly.

  4. Alberts B, Johnson A, Lewis J, et al. Molecular Biology of the Cell. 4th edition. New York: Garland Science. Protein Function.

  5. Colorado State University VIVO Pathophysiology. Growth hormone (somatotropin).

  6. University of Rochester Medical Center. Ferritin (blood).

  7. U.S. Department of Agriculture. Dietary Guidelines for Americans 2015-2020. Eighth edition.

  8. U.S. Department of Agriculture. All about the protein foods group.

  9. Fehrenbach KS, Righter AC, Santo RE. A critical examination of the available data sources for estimating meat and protein consumption in the USAPublic Health Nutr. 2016;19(8):1358-1367. doi:10.1017/S1368980015003055

  10. U.S. Department of Agriculture. Beef steak, broiled or baked, lean and fat eaten.

  11. Mariotti F, Gardner CD. Dietary protein and amino acids in vegetarian diets—a reviewNutrients. 2019;11(11):2661. doi:10.3390/nu11112661

  12. MedlinePlus. Kwashiorkor.

By Sheryl Huggins Salomon
Sheryl Huggins Salomon is a veteran editor and health journalist specializing in coverage of metabolic health, skin conditions, and BIPOC health trends.