What is Protein?

Protein is an essential nutrient in the human diet and is one of the three major macronutrients. In our bodies proteins have various important functions. They are the most important building blocks and are involved in almost all of the metabolic processes that occur.

Protein is Made of Amino Acids

Proteins are effectively chains or strings of molecules called ‘amino acids’, of which there are 20. These amino acids are building blocks and can be combined together in an infinite number of ways to form different proteins, much like the 26 letters of the alphabet can be combined together into an almost infinite number of words and sentences. Of these 20 amino acids, 11 can be produced or manufactured in the body, which are known as the ‘non-essential amino acids’. The remaining 9 are termed the ‘essential amino acids’ and cannot be made by the body, meaning they have to be consumed in the diet.

Proteins Have Many Functions

Proteins are responsible for a variety of functions in the body, because depending on how they are built and assembled, they can have different effects on the molecules around them. Examples of proteins in the body are:

Digestive enzymes, responsible for breaking down food and nutrients
Haemoglobin, the molecule that carries oxygen in the blood as part of red blood cells
Muscles, which are made of millions of proteins which are able to lengthen and shorten, leading to muscle contraction
Enzymes in the muscles that generate energy for aerobic exercise
Hair, skin and bone, which all contain large amounts of protein

Muscle is the biggest source of protein in the body, which is why when most people think of protein, they often think of meat, which is animal muscle. Clearly, protein is needed for a vast array of functions in the body and eating too little protein can affect these processes which are needed to stay healthy.

Protein in the Diet

Because protein has a variety of functions in all living organisms, it can be found in a surprising variety of foods, and not just the animal products that are commonly associated with protein. Meat is a major source of protein, as muscle tissue contains many proteins that control muscle contraction and relaxation. Eggs contain protein, which in a fertilized egg will contribute to the building of proteins (such as muscle and bone) in the baby chick. Similarly, milk contains protein for calves to use in their growth and development. Many plants contain considerable amounts of protein, most often in the beans or seeds, which need protein for the new plants that grow from them. Although many foods contain protein, not all proteins are created equal. Some proteins contain all essential amino acids and other contain only some. The ones that contain more amino acids are generally referred to as higher quality proteins (see article "What is the Best Protein?").

Protein and Amino Acid Metabolism

When a protein-containing food is eaten, the digestive enzymes in the stomach, along with the acid there break down the protein into smaller chains of amino acids (known as peptides). These are broken down further into the constituent amino acids, which are then absorbed in the small intestine. They can then enter the blood and travel to the liver, where they can be processed. Energy can be derived from them, much like carbohydrates, and per gram of dietary protein, 4 kcal of energy can be obtained. Amino acids are also transported in the blood to tissues where they are needed, including muscle tissue or any other tissue or organ that needs to undergo growth, repair or development.

What are Branched Chain Amino Acids (BCAAs)?

What is the Best Protein?

References

Dolan E, Sale C. Protein and bone health across the lifespan. Proc Nutr Soc. 78(1):45-55, 2019
Herman E, Larkins B. Protein Storage Bodies and Vacuoles. Plant Cell. 11:601-13, 1999
Phillips SM, Tang JE, Moore DR. The role of milk- and soy-based protein in support of muscle protein synthesis and muscle protein accretion in young and elderly persons. J Am Coll Nutr. 28(4):343-54, 2009
Lonnie M, Hooker E, Brunstrom J, Corfe B, Green M, Watson A, Williams E, Stevenson E, Penson S, Johnstone A. Protein for Life: Review of Optimal Protein

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