2.1 i) Protein


All proteins are made up of amino acids, these are simple organic compounds that are vital for normal health and are the building blocks of proteins.

Amino acids contain the carbon (C), hydrogen (H), oxygen (O) and nitrogen (N); many also contain sulphur (S), such as methionine, giving them further forms and functions. They are characterized by the presence of a carboxyl group (COOH) and an amino group (NH 2) attached to the same carbon at the end of the compound.

Proteins are a source of energy (4 kcal/g)



Proteins have many essential functions in the body, which include:

  • structure, maintenance, repair and growth of tissue (such as skin, hair, nails) and organs(such as the kidneys, liver and heart)
  • pH balance of the body
  • formation of transport proteins, such as haemoglobin in the blood
  • synthesis of enzymes that are required throughout the body
  • immune cell components such as antibodies
  • production of muscle fibres
  • formation of hormones such as insulin

Sources include: animal tissue such as meat, poultry, fish; nuts, seeds, beans and legumes

Essential amino acids cannot be made by the body and must be obtained via the diet.
There are 9 essential amino acids:

    • lysine, leucine, isoleucine, valine, methionine, phenylalanine, threonine, histidine and tryptophan
  • Non-essential amino acids can be synthesised in the liver from essential a.a.
  • Conditionally Essential amino acids are considered essential in certain situations such as increased growth in children and demand state such as pregnancy. There are 8 conditionally essential amino acids:
    • arginine, cysteine, histadine, glutamine, glycine, proline, taurine (pre-term infants only) and tyrosine




  • Essential amino acids cannot be made by the body and must be obtained via the diet.
    There are 9 essential amino acids:
    • lysine, leucine, isoleucine, valine, methionine, phenylalanine, threonine, histidine and tryptophan

  • Non-essential amino acids can be synthesised in the liver from essential a.a.
  • Conditionally Essential amino acids are considered essential in certain situations such as increased growth in children and demand state such as pregnancy. There are 8 conditionally essential amino acids:
  • arginine, cysteine, histidine, glutamine, glycine, proline, taurine (pre-term infants only), tyrosine

Biological Value (BV) of proteins

  • Proteins are classified as having either low or high BV: in essence if a protein has a few or no essential amino acids, then it is of low BV, if a protein has several or more essential amino acids then it is of high BV.
  • Higher BV proteins have all nine essential amino acids in adequate amounts which are of great value in that they able are to promote growth in the body and for repair of tissues.
  • animal protein has all essential amino acids in differing quantities and are therefore of high BV
  • most plants proteins contain only some of the nine essential amino acids and are therefore of low BV
  • When eating only a vegetarian or vegan diet it is very important to combine foods so that all the essential amino acids are acquired. Consequently strict vegetarians or vegans should eat a wide range of vegetable foodstuffs to make sure that they are getting all nine EAAs in their diet.In order make plant proteins complete (i.e. comprise of all the essential amino acids), plant food can be combined to make complete proteins, which have high BV:
  • legumes and grains
  • legumes and nuts
  • vegetables and lentils/ beans

A well-balanced vegan diet supplies all the protein & essential amino acids required for good health.

  • Plant sources of complete proteins (contain all essential amino acids) and have high BV include:
    • soya beans
    • quinoa
    • millet
    • avocado
    • spirulina
    • chlorella


Insufficient protein

Protein deficiency usually occurs if there is:

  • undernourishment, by not meeting energy requirements and consequently dietary protein or protein stored in the body is as an energy source.
  • a state of negative nitrogen balance, this is where intake of protein is less than the amount of protein that excreted, such as:
    • trauma such as injury (e.g. burns), surgery , when the breakdown of protein has exceeded the body's ability to replace it
    • excessive protein loss resulting from, e.g. kidney disease
  • an inability to absorb or use protein because of a particular disorder, e.g. gastrointestinal or liver disease, where which results in impaired uptake of protein.

In the long term deficiency of protein can result in:

  • muscle wasting, both skeletal muscle and cardiac muscle loss
  • failure to thrive in children/ stunted growth
  • poor wound healing
  • susceptibility to infection
  • anaemia
  • oedema


Excessive protein

  • can exacerbate the symptoms of liver and kidney disease
  • can promote metabolic acidosis - an acid bias in the body
  • In order to balance the metabolic acidosis, the body leaches calcium from the blood and
  • bones to bring pH back toward neutrality


Protein requirements

  • It is estimated that in order to meet the needs for protein turnover in the body (the demand for protein for the array of bodily processes it is involved in) 15% of all food energy should be derived from protein. For a healthy adult this equates to 0.75 g of protein per kg of body weight per day.
  • Thus an adult man that weighs 60 kg requires 45g protein per day (60 x 0.75g).
  • This is adjusted to times of extra requirements, e.g. recover from a trauma, a period of growth and pregnancy and lactation.
  • However in affluent societies protein intakes are usually in excess of daily requirements leading to problems such as those described above.



VIDEO: Why Do We Need Protein?

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