Polypeptides and Protein

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Proteins are comprised of subunits called amino acids joined together by chemical bonds. There are typically up to a few hundred amino acids in a protein and collectively these long chains or groups of amino acids are known as polypeptides. Whilst carbohydrate and fat are composed of carbon (C), hydrogen (H) and oxygen (O) atoms, proteins are chemically distinct due to the additional presence of one or more nitrogen atoms (N) in each amino acid (see nitrogen balance below).

20 amino acids are needed by the human body to create a diverse range of components, for example muscle, skin, hair, blood, enzymes, hormones. Of these 20 amino acids, some can be produced by the body itself (non-essential), some can be produced by the body under certain conditions (conditionally essential) and the remainder cannot be produced by the body so must be included in the diet (essential) – see Table 1 below..

Chemical bonds which join the amino acids in dietary protein are broken down in the stomach and small intestine by enzymes. The resulting free amino acids are then absorbed into the blood stream through the wall of the small intestine.

A “pool’ of amino acids is available to create new proteins for the body. As well as amino acids from dietary protein and de novo synthesis, this pool is supplemented with amino acids arising from cellular degradation within the body as a result of regular tissue maintenance. If supply exceeds demand the amino acids can be used as a source of energy. Alternatively, if energy supplies are sufficient excess amino acids are converted to fatty acids and stored in fat cells. Unwanted nitrogen is converted to urea and excreted by the kidneys.

Table 1: Essential, conditionally essential and non-essential amino acids

Essential Conditionally essential Non-essential
Histidine Arginine Alanine
Isoleucine Cysteine Asparagine
Leucine Glutamine Aspartic acid (Aspartate)
Lysine Glycine Glutamic acid (Glutamate)
Methionine Proline Serine
Phenylalanine Tyrosine


To meet bodily requirements, dietary protein should be of ‘good’ quality. Foods which contain all the essential amino acids that are readily available after digestion are recognised as sources of complete protein, for example meat, poultry, dairy products and eggs.

Soybeans as one exception, plants are generally regarded as incomplete protein sources since one or more of the essential amino acids are either present in a low amounts or missing altogether. For example, lysine is only present in small amounts in wheat, whilst there are low levels of both lysine and tryptophan in corn. Plants are also less readily digested compared to animal products due to cell walls which are resistant to human digestive enzymes. It follows therefore that vegetarians must combine a range of different plant foods in their diet to meet their needs for all the essential amino acids.

Dietary protein requirements are calculated by determining the nitrogen balance in the body, i.e. where the dietary intake of nitrogen in amino acids is equivalent to the amount of nitrogen excreted. To achieve this…

0.83 g of good quality protein per kg of body weight per day, should be consumed2

…so, for a 70 kg adult this would be 0.83 x 70 = 58 g of protein per day.

It is generally accepted that athletes who wish to increase muscle mass and strength will need to increase their dietary intake of protein to 1.3 g – 1.8 g per kg of body weight per day3

  1.  Institute of Medicine, (2005). Dietary Reference Intakes for Energy, Carbohydrate, Fibre, Fat, Fatty Acids, Cholesterol, Protein and Amino Acids. Washington, D.C.: The National Academies Press, Page 593
  2. Rand, W. M., Pellett, P. L., Young, V. R. (2003). Meta-analysis of nitrogen balance studies for estimating protein requirements in healthy adults. The American Journal of Clinical Nutrition, 77 (1), pp 109-127
  3. Phillips, S. M. and Van Loon, L. J. C. Dietary protein for athletes: From requirements to optimum adaptation. Journal of Sports Sciences [Online],  Taylor Francis. Dec 2011, vol. 29 (sup1),  pp S29-S38, [viewed 10 March 2021]. Available from DOI: 10.1080/02640414.2011.619204

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