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Protein: The Body’s Building Block

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Protein is essential to maintaining a healthy diet. Protein serves numerous functions in the body, most of which are structural. We will discuss what protein does for our bodies and how to ensure we are getting adequate amounts in our diets.

Protein is a large molecule, referred to as a polypeptide, a compound containing 10 – 100 amino acid molecules.

Protein is needed for growth maintenance, and repair of cells, including muscle cells, and for the production of enzymes, hormones, and deoxyribonucleic acid (DNA) expression.

 When considering the nutritional standpoint of food we must look at amino acid subunits. Amino acids serve a vital role in the body.

Amino Acids

Amino acids are the building blocks of protein. There are about 22 amino acids that are considered biologically important, however, more may exist in nature and in the body. Amino acids are classified nutritionally base on their dietary need by the body.

Indispensable Amino Acids – amino acids that the body cannot synthesize and requires from dietary intake.

Dispensable Amino Acids – amino acids that the body can make in sufficient quantities as needed.

Importance of Protein

Proteins are used for the maintenance, growth, and repair of cells, and the production of hormones, DNA, and enzymes.

Too Much Protein: causes an increase in blood levels of ammonia and uric acid. The excess protein is converted to fat.

Too Little Protein: reduces lean body mass, weakens bones, reduces body function.

Animal Protein Sources

  • Beef
  • Chicken
  • Seafood
  • Turkey
  • Pork

Vegan Protein Sources

  • Complete Protein plant sources:
  • Quinoa
  •  Buckwheat
  •  Soy
  • Nuts
  •  Seeds
  •  Avocados
  • Sweet Potatoes
  • Chickpeas
  •  Lentils
  • Soy

Different Ways of Rating The Quality of Protein

Complete vs. Incomplete Proteins

Complete Proteins: complete proteins are proteins that contain all nine essential amino acids.

Mostly found in animal sources.

Complete Protein Animal Sources

  • Beef
  • Pork
  • Poultry
  • Fish
  • Bison
  • Buffalo
  • Venison
  • Shellfish
  • Eggs
  • Dairy Products

Complete Protein Vegan Sources

  • Quinoa
  • Soybeans
  • Tempeh
  • Tofu
  • Miso
  • Edamame

Incomplete Proteins: incomplete proteins are proteins that are missing one or more essential amino acids.

  • Mostly found in plant sources

Incomplete protein sources;

  • Grains
  • Legumes
  • Nuts
  • Fruits
  • Most vegetables

Protein Efficiency Ratio

Arguably the most criticized method PER (protein efficiency ratio) uses animal testing (mostly on rats) to determine the quality of proteins for humans.  PER refers to the amount of weight gained versus the amount of protein ingested.

Net Protein Utilization

Net protein utilization (NPU) is another way of determining the quality of proteins by measuring a person’s intake of nitrogen from amino acids, and comparing that amount to the amount of nitrogen that the person excretes, and determining how much of the protein is needed to balance out the two amounts.

If the protein’s NPU is low, more of it is needed to achieve nitrogen balance.

The more nitrogen that is absorbed from a protein, the more digestible that protein is.

Biological Value

The methods used to determine protein’s biological value (BV) are not yet standardized, however, it is still the method that most scientists prefer.

The formula for calculating BV is as follows;

The BV does not indicate whether the amino acids will be used for muscle growth or enzyme synthesis. BV measurements can also vary for the same protein depending on what animal species is being tested. Therefore, unless the BV is specifically for humans it may not prove advantageous.

Amino Acid Score

With biological testing to evaluate the protein quality of different foods being limited, a movement began to develop a protein quality rating system based upon amino acid content in relation to humans to find the ideal protein. This movement evolved into the development of the Protein Digestibility Corrected Amino Acid Score (PDCAAS) in the 1980s. The PDCAAS is fabricated on creating a reference standard of the indispensable amino acid composition that would prove most ideal for humans. This method considers the indispensable amino acid digestibility, composition of protein, and the ability of the source to supply the indispensable amino acids in adequate amounts necessitated by humans.

Importance of Nitrogen Balance

Nitrogen balance refers to the condition in which the amount of dietary nitrogen is taken in is equal to the amount of nitrogen excreted. A positive nitrogen balance indicated a possible net growth in body tissue. A negative balance indicates inadequate protein intake and possibly cannibalizing muscle tissue.

Cannibalization – the breakdown of muscle tissue by the body for the purpose of obtaining amino acids for other metabolic purposes and may include other body tissues.

Determining nitrogen balance can be difficult since nitrogen from broken-down amino acids can be excreted in urine and feces and some are lost as sweat, all these excretions must be collected and analyzed.

Digestion of Protein

Protein digestion starts in the mouth. After food is chewed and broken down it is swallowed and passed down the esophagus, it enters the stomach. While in the stomach enzymes such as pepsin help to break down the protein molecules. The food then passes into the intestines where free form di-peptide and tri-peptide amino acids are absorbed immediately. As the food travels through the intestines, enzymes continue to digest any remaining polypeptides. Once the free form polypeptides enter the bloodstream they are transported to the liver where they are either converted into other amino acids, are further broken down and used for energy or excreted, are used to make other proteins, or may be placed into circulation and continue through the rest of the body.


Consuming an adequate amount of protein is essential to maintaining a healthy diet. Although all protein sources aren’t created equally you can still ensure you receive all the vital nutrients nececessarry through mixing different sources in your diet. Animal sourced proteins tend to be complete and therefore, provide all essential amino acids that the bodies needs, while plant sourced proteins tend to be incomplete and therefore, lacking in one or more of these essential amino acids. Amino acids are the building blocks of proteins. Proteins are utilized for the maintenance, growth, and repair of cells, and the production of hormones, DNA, and enzymes.

Here is an article that explains how to calculate your protein intake needs.

Cited Sources:

The significance of protein in food intake and body weight regulation – PubMed (

Women and Protein – An Essential Guide | ISSA (

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