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March 2017
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PROTEIN

PROTEIN

What are proteins?

Fundamentally, proteins are made up of 4 elements: carbon, hydrogen, oxygen and nitrogen, and protein molecules are composed of units of amino acids. There are many, known amino acids but only around 20 are necessary in the human diet (common amino acids). Proteins are essentially living organisms and they are the main ‘building blocks’ in the architecture of the human body (protein structures) and in the functioning of living beings (functional protein), for example, enzymes, hormones, growth factors, coagulation pathways, cellular respiration, carrier proteins, and so on. Proteins make up more than 50% of organic components in a human body and about 14-18% (according to age) of one’s total weight.

Requirements

Our protein requirements are determined by a series of factors, amongst which being how much nitrogen we need to expel, the quality of the protein, physical activity, our calorie intake and physiological state. Here is a table of the daily protein requirements according to the Italian Association of Human Nutrition (SINU – Società Italiana di Nutrizione Umana).

CATEGORY AGE PROTEIN REQUIREMENTS (*)
Males > 18 (**)

0.95g/kg/day

Females > 18 (**) 0.95g/kg/day
Pregnant  0.95g/kg/day+6g
Nursing 0.95g/kg/day+17g

(*) Values from L.A.R.N. were originally calculated for the population of Italy 

(**) During adolescence, it is recommended that you increase your intake by 30%

The chemistry of protein

The amino acid components of proteins are linked together with peptide bonds (-CO-HN-) resulting in long chains called peptides. The number of polypeptides that can form from the 20 common amino acids is enormous, hence the large amount of existing and edible proteins. From a functional point of view, the amino acids used by the human body are classified as either essential, non-essential or semi-essential.

ESSENTIAL NON-ESSENTIAL SEMI-ESSENTIAL
These are the amino acids that the body is not able to synthesise itself and, therefore, they must be introduced through food. These are the amino acids that the body can create adequate amounts of, under physiological conditions. Cysteine and tyrosine can be synthesised by the body by using phenylalanine and methionine, when there is enough of these two amino acids available.
Isoleucine (Ile) Alanine (Ala) Cysteine (Cys)
Leucine (Leu) Arginine (Arg) ** Tyrosine (Tyr)
Lysine (Lys) Asparagine (Asn)
Methionine (Met) Aspartic acid (Asp)
Phenylalanine (Phe) Histidine (His) **
Threonine (Thr) Glutamic acid (Glu)
Tryptophan (Trp) Glutamine (Gln)
Valine (Val) Glycine (Gly)
Proline (Pro)
Serine (Ser)
** Arginine and Histidine are very good for children

The quality of the protein

From a chemical point of view, proteins can be regrouped in to two categories:

  • Simple proteins, made up of just amino acids;
  • Conjugated proteins, made up of amino acids and other compounds (for example, cell membranes and nucleic acid (DNA)).

The quality of proteins is also important as it indicates the nutritional effectiveness of them, regarding the amount of amino acids they provide and their bioavailability. The quality of proteins can be measured according to the following:

  • chemical composition
  • digestibility
  • biological value
  • net protein utilisation.

AMINO ACID COMPOSITION  

This is calculated by taking the ratio of one given amino acid in 1g of the protein in question, and the ratio of the same amount of the amino acid in 1g of protein that comes from a biological source (for example, egg, milk). The result is used to estimate the ability of the given protein, or even a mix of proteins contained in the given food, in providing the essential amino acids. However, this does not k  eep in mind biological factors such as, digestibility and how the body will use the protein.

DIGESTIBILITY

This is worked out by taking the ratio of the amount of nitrogen absorbed and the amount of nitrogen expelled in the faeces. Normally animal proteins have a higher digestibility ‘score’ than vegetable proteins and wholegrain products can decrease of absorption because of the high amount of fibre in them.

BIOLOGICAL VALUE 

This indicates the quality of the absorbed nitrogen from a protein that will be used for bodily functions, for example growth. The biological value expresses the completeness of the protein, or more precisely, which essential amino acids are present and the optimum proportions for bodily protein synthesis. Animal proteins have a higher biological value in comparison to plant proteins and from a composition point of view, animal proteins are considered complete, whilst plant ones are incomplete. Complete and incomplete proteins can be eaten in the same meal however, and doing so will provide complete amino acid intake.

NET PROTEIN UTILISATION 

This is the ratio of the amount of nitrogen ingested and the amount of nitrogen used by the body and, when calculated, both the biological value and digestibility of the protein is kept in mind. It is used to work out daily protein requirements considering a population’s mixed diet that is made up of both animal and plant proteins.

PROTEIN

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