Protein – The Facts

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Protein plays an integral part in the human body. It is described as “the body’s building blocks” as it is found in all living cells and has both structural and functional properties. Every gram of protein consumed provides the body with 17 kilojoules of energy. This article will look at what a protein is, what are the sources, how it is formed, the digestion of protein and what roles it plays in the human body.

The Composition of Protein

Proteins are a large molecule with a complex structure. Every molecule of protein is a three-dimensional structure made up of Amino Acids bound together with peptides that form a chain and a non-protein portion of haemoglobin containing iron called the Haem. There are 20 different amino acids each having a different function in the body. 

The structure of Amino Acids is made up of two functional groups a single hydrogen atom and a R or variant group bound to a central carbon atom:

  • Amino group – A functional group made up of 1 nitrogen bonded to 2 hydrogen atoms.
  • Carboxylic group – A functional group made up of 1 carbon atom bonded to an oxygen atom and a hydroxyl group (oxygen and hydrogen).
  • Hydrogen 
  • Variant or R group – This group differs depending on the biological action and gives each amino acid its unique characteristic.
  • (Timberlake, 2013, p.675-677)

The bonding of Amino Acids with peptides is through a condensation reaction and form chains of varying lengths

  • Dipeptides – 2 amino acids
  • Tripeptides – 3 amino acids
  • Oligopeptides – 4 to10 amino acids
  • Polypeptides – 50 to 100 amino acids
  • (Whitney et al., 2014, p.183-184)
Protein Absorption and Biological Function

Proteins are absorbed through the breaking down into amino acids for absorption. This is achieved through the use of enzymes so they can be digested. This takes place in the stomach, small intestine and pancreas where these enzymes are released. Once a protein has been masticated in the mouth it travels down the Oesophagus and then digestion begins.

  • Stomach – Hydrochloric Acid (HCI), Pepsin, Rennin (only in children under 2) – HCI denatures structure and reacts with pepsinogen from chief cells in the stomach wall to form pepsin which breaks down into polypeptides and a small amount of free amino acids.
  • Small Intestine and Pancreas – Enteropeptidase, Trypsin, Chymotrypsin, Carboxypeptidases, Elastase & Collagenase, Intestinal tripeptidases, Intestinal dipeptidases, Intestinal aminopeptidase. 
  • Polypeptides are converted to tripeptides, dipeptides and amino acids when received into the small intestine an alkaline environment from the acidic environment of the stomach.
  • The pancreas releases protease enzymes

Trypsinogen → Trypsin
Chymotrypsinogen → Chymotrypsin

  • Absorption takes place through the microvilli of the small intestine with the presence of peptidases 
  • Dipeptidase
  • Carboxypeptidase
  • Aminopeptidase

The amino acids are then transported to the liver where they are used for structure or function.

  • Build new proteins
  • Oxidised for energy
  • Create nitrogen containing compounds.

Structural roles: Create, repair and maintain connective tissue.

Functional roles: Maintaining function of body systems, involved in the creation of hormones, enzymes that act as a catalyst in chemical reactions, transports oxygen throughout the body (haemoglobin), maintain fluid balance.

(Whitney et al., 2014, p.185-188)

Dietary sources and Recommended daily intake (RDI)

Amino acids are classified as Essential (indispensable) and Non-essential (dispensable).

Essential – There are 9 essential amino acids and these cannot be synthesized by the body in the amounts needed and must be consumed in the diet. To ensure an adequate intake of protein is consumed the digestibility and quality must be considered. The use of “The protein digestibility-corrected amino acid score” (PDCAAS) rates protein quality into First class and Second class proteins.

First class – Complete proteins containing essential amino acids in the correct proportions. The best sources are from animal origin such as meat, eggs, poultry, seafood and dairy with the acceptation of Soy, Quinoa and Amaranth.

Second class – These proteins do not contain or do not have sufficient essential amino acids in the correct proportions and must be consumed with a complementing protein for adequate needs. These are mainly proteins sourced from plant foods such as grains, nuts, seeds and vegetables. Lysine, Threonine, Tryptophan and Methionine are common limiting amino acids. Consideration must be made when eating a plant based diet of these insufficiencies. 

Non-essential – These amino acids are produced by the body and are not needed in the diet. Of the 11 non-essential amino acids, 6 are classified as semi-essential and may need to be consumed when the body is under stress due to illness or disease.

(Whitney et al., 2014, p.182,197-198)

Recommended Daily Intake

15% – 25% of daily Kj intake should be sourced from protein.

RDI of protein differs for children and adults. RDI for children is based on age, weight and gender (there is no RDI for infants under 12 months). For adults RDI will depend on age, activity and will increase if a woman is pregnant or lactating.


1-3 years14g per day (1.08g/kg)
4-8 years20g per day (0.91g/kg)
9-13 years40g per day (0.94g/kg)
14-18 years65g per day (0.99g/kg)


1-3 years14g per day (1.08g/kg)
4-8 years20g per day (0.91g/kg)
9-13 years35g per day (0.87g/kg)
14-18 years45g per day (0.77g/kg)


19-70 years64g per day (0.84g/kg)
Over 70 years81g per day (1.07g/kg)


19-70 years46g per day (0.75g/kg)
Over 70 years57g per day (0.94g/kg)

Pregnancy and Lactation

14-18 years58g per day (1.02g/kg)63g per day (1.1g/kg)
19-30 years60g per day (1.00g/kg)67g per day (1.1g/kg)
31-50 years60g per day (1.00g/kg)67g per day (1.1g/kg)

(Australian Government Department of National Health and Medical Research Council, 2014)

Protein Deficiency, Insufficiency and Excess

Protein deficiency is referred to as Protein-energy malnutrition (PEM). The most severe forms of PEM are Marasmus (infants under 2 years) and Kwashiorkor (infants 1-3 years) which are most prevalently found in children living in poverty where there is insufficient energy and protein available in their diet. 

Marasmus symptoms – slow development, severe weight loss, severe muscle waste, small for age, thin and dry hair, thin and dry skin.

Kwashiorkor symptoms – muscle wasting, small for age, Oedema, enlarged fatty liver, loss of appetite, straight brittle dry hair, skin lesions.

Deficiencies can also be seen in other vulnerable groups such as 

  • People suffering from eating disorders
  • Patients with HIV/AIDS or cancer
  • Addiction to drugs or alcohol
  • Gastrointestinal malabsorption disease
  • Elderly – poor diet, poverty, living alone
  • (Whitney et al., 2014, p.199-201)

An excess intake of protein, particularly animal-sourced protein, can also increase fat intake and therefore lead to chronic disease.

  • Osteoporosis, Calcium Homeostasis and other disorders of bone
  • Impaired renal function, kidney stones, gout and kidney disease
  • Increased risk of Colon cancer
  • Cardiovascular disease 
  • (Delimaris, 2013, Abstract)


To guard against chronic disease and ensure the body repairs and functions at its optimal level 15 – 25% of energy intake should be made up of first class proteins. If following a vegan plant based diet, protein complementing is advised.


Australian Government Department of National Health and Medical Research Council. (2014). Nutrient reference values for Australia and New Zealand 2006. Retrieved from

Delimaris, I. (2013). Adverse effects associated with protein intake above the recommended dietary allowance for adults Abstract. Hindawi international scholarly research notice, 126929.

Timberlake, K. (2013). General, organic, and biological chemistry structures of life. Glenview, IL: Pearson.

Whitney, E., Rolfes, S., Crowe, T., Cameron-Smith, D., & Walsh, A. (2014) Understanding Nutrition: Australian & New Zealand edition 3.  South Melbourne, Australia: Cengage.

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Tracey Talbot
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