Student Number: 1446235

Class: B401 year 1

Course (module): 4MNT0102-Introduction to Nutrition

Tutor’s name: Kevin Whelan

Essay title: Outline the digestion, absorption and metabolism of protein in the human body and explain briefly why some dietary proteins are utilised

more efficiently than others.

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Proteins represent a large group of organic compounds characterized by a sequence of

amino acids bound together by peptide bonds. Each protein fulfils a specific function that

could involve defence, transport, support, motion, regulation and storage.

Proteins are large molecules that, in order to be absorbed and metabolised, need to be

broken down to smaller units (dipeptides or tripeptides) or to single amino acids, during the

process of digestion.

Digestion.

The digestion of proteins, known as proteolysis, takes place in the stomach and in the small

intestine, in which digestive enzymes called proteases (also known as peptidases or

proteinases) progressively break down the peptide bonds between the amino acids that

form each protein.

In the stomach pepsin, a protease, initiates proteolysis cleaving several peptide bonds, in

particular those between hydrophobic and aromatic amino acids.

Pepsin is secreted by chief cells into the stomach in the form of a zymogen (inactive

precursor), pepsinogen and the acidic environment of the stomach activates a rapid

autocatalytic process that turns pepsinogen into pepsin.

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The peptides and amino acids liberated in the stomach are then sensed in the duodenum by

enteroendocrine cells, leading to the release of CCK (Cholecystokinin) that causes pancreatic

protease secretion.

Two of the major pancreatic proteases are trypsin and chymotrypsin. Trypsin is released in

the duodenum in the form of its inactive precursor, trypsinogen, which is activated by

enteropeptidase, a proteolytic enzyme that splits off a peptide from trypsinogen. Trypsin is

also a proteolytic enzyme that, after being activated, activates other pancreatic zymogens,

such as chymotrypsinogen.

After being subjected to the effect of pancreatic proteases, the peptides can be either

absorbed or further digested by pancreatic carboxypeptidases or aminopeptidases (located

on the brush border cells of the small intestine).

There are many other proteases which are involved in proteolysis, and each one of them

effectively breaks a particular peptide bond to produce smaller peptides or free amino acids

(Fig. 1).

Absorption.

The end products of proteolysis are free amino acids, dipeptides and tripeptides, which are

differentially absorbed by the epithelial cells of the villi of the small intestine.

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Dipeptides and tripeptides are absorbed by H+ dependant secondary active transport, while

free amino acids can be absorbed by Na+ dependant secondary active transport or by

primary active transport.

Once inside the cytosol of the epithelial cells, the dipeptides and tripeptides are hydrolysed

to amino acids (cytosolic peptidase). Free amino acids, then, enter the interstitial fluid

through the basolateral membrane by facilitated diffusion via specific facilitated amino acids

transporters and diffuse into the venous capillaries through capillary pores.

Metabolism.

Amino acids are not stored in the human body and there is only a small pool of free amino

acids within the body, in equilibrium with proteins catabolised and synthesised (1); thanks

to this dynamic free amino acid pool, metabolic need for amino acids can be continuously

satisfied.

The amino acid pool can be defined as a continuous, flux of amino acids (Fig.2).

The proteins of the body are constantly being broken down, synthetized and replaced with a

constant daily rate (300 to 400 g per day in adults)(2) which is highly superior to the level of

protein intake (50 to 80g per day). This means that amino acids released by endogenous

protein breakdown can be reutilized and reconverted to protein synthesis (2). Furthermore,

both protein synthesis and degradation consume energy (4Kj/g of protein of average

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composition)(3). Protein turnover is extended to all proteins within the body and is

regulated by nutritional and hormonal factors and exogenous stimuli.

In addition, (dispensable) amino acid synthesis occurs on the ribosomes of almost every cell

in the body when there is a need due to a poor dietary intake or associated pathologies and

is directed by the RNA and DNA of the cell. This process is called transamination (catalysed

by aminotransferases) and is characterized by the transfer of an amino group from an amino

acid to pyruvic acid or to an acid in the Krebs cycle.

Amino acids can also be oxidised in the Krebs cycle in order to release energy as ATP,

indeed, in the liver and the small intestine amino acids are the major source of energy. Yet,

before this process is possible, it is essential that the amino group of each amino acid is

removed by a process known as deamination (catalysed by deaminases) which occurs in

hepatocytes and produces NH3 (ammonia). Nevertheless, human blood can only contain a

limited concentration of ammonia; consequently, the excess enters the urea cycle, is

converted to urea and is excreted through urine.

The carbon skeleton (amino acid deprived of the amino group) may also be utilized for the

synthesis of other metabolic intermediates and products such as glucose (gluconeogenesis),

ketone bodies (ketogenesis) and triglycerides (lipogenesis). Furthermore, amino acids are

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the precursors for many products such as nucleotides, neurotransmitters, porphyrin and

hormones.

Why some dietary proteins are utilised more efficiently than others.

As each protein is a sequence of different amino acids, their efficiency is determined by the

particular amino acids of which they are composed.

Protein quality can be defined by biological and chemical methods and represents the

efficiency with which a dietary protein is utilised in the body.

Biological methods (adopted by FAO and WHO in 1991) measure the biological response to

diets containing only a set amount of test proteins. Fig. 3.

Chemical methods (4), instead, enable to predict protein quality utilising the reference

protein pattern, which is the amount of essential amino acids needed by pre-school children

to reach optimal nitrogen retention. Comparing the reference protein with the amino acid

composition of a dietary protein the amino acid score is obtained (Fig. 4) and, applying it for

each essential amino acids 8 figures will be produced: the lowest represents the amino acids

score of the protein and the amino acid with the lowest score is the first limiting amino acid.

The acknowledgment of the first limiting amino acid has a crucial importance as it is the

amino acid that limits the value of the protein as once it has all been utilised within the

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body, the remaining amino acid assortment is incomplete and so no more tissue protein can

be synthetized. (5)

The aim of this essay was to highlight the fundamental pathways of protein digestion,

absorption and metabolism and to explain what influences protein efficiency. Further

research is still ongoing to attain more specific values and guidelines to determine the

requirements and efficiency of this macronutrient that has various roles and is essential to

maintain good functioning and structure of all living cells and to support growth and repair

within the human body.

(1100 words)

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Fig. 1 (6)

Name of protease Peptide bond on which it acts

Pepsin Whole protein

Endopeptidases Bonds with peptide chain

Trypsin Peptides

Exopeptidases C-terminal bonds

Carboxypeptidase Successive amino acids at C terminus

Aminopeptidase Successive amino acids at N terminus

Fig. 2 (7)

Fig. 3 (8)

Net Protein Utilization (NPU)= nitrogen retained/nitrogen intake

Digestibility= nitrogen absorbed/nitrogen intake

Fig. 4 (5)

Amino acid score =

mg amino acid per g test protein x 100mg amino acid per g reference protein

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Reference

1. Geissler C. and Powers H. 2011; Human Nutrition 12th edition; Elsevier Ltd.; Protein

metabolism and requirements 8: 157.

2. Schutz Y. 2011; Protein turnover, ureagenesis and gluconeogenesis; Department of

Physiology, University of Lausanne; Lausanne, Switzerland; PubMed.

3. Mann J. and Truswell A. S. 2012; Essentials of Human Nutrition 4th edition; Oxford

University Press; Protein 5: 77.

4. recommended in “Dietary protein quality evaluating in human nutrition”, FAO Food and

Nutrition Paper n.92 published on 15 February 2013

5. Sanders T. and Emery P. 2003; Molecular Basis of Human Nutrition; Taylor & Francis;

London; Protein 3: 41.

6. Mann J. and Truswell A. S. 2012; Essentials of Human Nutrition 4th edition; Oxford

University Press; Protein 5: 76.

7. Mann J. and Truswell A. S. Essentials of Human Nutrition 4th edition; Oxford University

Press 2012; Protein 5: 81.

8. Sanders T. and Emery P. Molecular Basis of Human Nutrition; Taylor & Francis; 2003

London; Protein 3: 39.

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Bibliography

Geissler C. and Powers H. Human Nutrition; 12th edition; Elsevier Ltd. 2011.

Mann J. and Truswell A. S. Essentials of Human Nutrition 4th edition; Oxford University Press

2012.

Sanders T. and Emery P. Molecular Basis of Human Nutrition; Taylor & Francis 2003; London.

Widmaier/Raff/Strang; Vander’s Human Physiology, The Mechanisms of Body Function 13th

edition; McGraw-Hill 2014.

Tortora G. J. and Derrickson B. H.; Principles of Anatomy and Physiology 12th edition vol.2;

Wiley 2008.

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