Proteins belong to a family of organic compounds, which serve many functions within the body. All proteins are made from building blocks called amino acids, which number 20 in total. These amino acids can be thought of as forming the protein alphabet, since they build proteins in a similar way as the various combinations of the 26 letters of the alphabet can be used to create individual words. Thus one protein will differ from another according to the number and sequence of its constituent amino acids.
Animal and plant cells join amino acids together to form peptides. This process results in the formation of chains of amino acids of varying lengths, which eventually become proteins.
|Two amino acids||Dipeptide||Di meaning two|
|Three amino acids||Tripeptide||Tri meaning three|
|4-9 amino acids||Oligopeptide||Oligo meaning few|
|10 or more amino acids||Polypeptide||Poly meaning many|
Proteins themselves are formed when the chain of amino acids total 100 or more, or when two or more polypeptide chains combine and repeatedly fold together to form specific three-dimensional shapes. The shape or structure of a protein will dictate its function within the body.
Essential amino acids
Of the twenty amino acids, nine are considered to be essential to the daily diet because the body is unable to produce or synthesise them itself. Only when sufficient quantities have been ingested, are we able to synthesise the remaining non-essential amino acids.
|Essential amino acids|
Non-essential amino acids
These are also present in many foods, but are not essential to the daily diet. So long as we successfully absorb sufficient amounts of the nine essential amino acids, the liver is able to synthesise the remaining eleven non-essential amino acids listed below.
|Non-essential amino acids|
|glycinealaninetyrosineserinecysteinproline||glutamic acidglutamineaspartic acidasparaginearginine|
Note: some authors now consider that due to the close relationship that exists between some of the essential and non-essential amino acids, that the latter are more accurately described as conditionally essential amino acids(Laidlaw and Kopple, 1987).
These foods contain all nine essential amino acids in sufficient amounts necessary for the liver to synthesise the remaining non-essential amino acids.
|Animal sources||Non-animal sources|
Below is a graph that represents the complete amino acid content in chicken:
Plants contain many nutrients, including protein. However, with the exception of soya based foods, these proteins are of an inferior nature, since they are deficent or ‘incomplete’ in one or more of the essential amino acids.
|cereals and grains (wheat, rye, barley, oats, rice)|
cereal products (bread, pasta etc)
pulses (beans, lentils, peas)
In order to gain sufficient protein from plant sources, we need to combine a variety of incomplete proteins together, either in the same meal or during the course of the same day.
Whilst these carbohydrate based foods contain energy in the form of glucose, it is important to remember that they also contain smaller amounts of protein. Including a variety of unrefined carbohydrate foods is, therefore, particularly important for anyone on a no meat or low meat diet. Good combinations include:
- rice and pulses
- vegetables and seeds
- nuts and vegetables
- grains and pulses
Quality of Protein Foods
Before we even purchase them at the supermarket, farmers and manufacturers can have a huge influence on the quality of protein-based foods. The treatment of the animals and their level of health are paramount to producing good meat. If any level of the cycle of food quality is not maintained at a high standard then quality will diminish. Factory processing can further damage and destroy animal produce by adding water, bulking agents, additives and preservatives, therefore increasing the potential for profit.
The current legal guidelines state that ‘meat’ can be composed of up to 25% connective tissue. The following minimum requirement of meat in certain food products gives a further indication as to their quality.
- economy burgers 41 – 50%
- luncheon meat 55 – 67%
- meat pies 12.5%
- pasties/sausage rolls 6%
- sausages 26 – 32%
The total amount of meat used can be alarming enough, but often the meat used in these types of products are lower grade off cuts and machine reclaimed meat collected at the end of the day.
Functions of Protein
The types of protein within the body can be placed under three headings, along with their corresponding functions:
- structural: form the main framework of many components of the body; collagen present in bone and connective tissue, keratin in the skin, and muscle tissue all provide structure. Muscle tissue is also contractile for movement.
- homeostatic: hormones regulate various processes, eg insulin controls blood sugar, enzymes speed up reactions, and white blood cells fight infection.
- fuel: although not the primary source, protein is a useable source of energy, especially during endurance events or periods of fasting. They can be converted into glucose, fatty acids or ketones to help produce ATP.
Catabolism relates to the breaking down of larger structures into smaller ones. Protein catabolism occurs to some extent all of the time, as existing proteins from damaged cells are broken down into their amino acids and recycled to build new proteins elsewhere. Further muscle catabolism occurs during intensive exercise, as a result of both micro-tear damage and the partial utilisation of key amino acids as fuel.
Anabolism can be defined as a building up process within the body. The anabolic phase mostly occurs during rest. Since proteins form a major component of most cell structures, adequate dietary protein is required to maintain both health and performance.
The amount of protein needed for effective function will vary significantly from person to person. It is very difficult to get it exactly right with a simple calculation. It will take some trial and error and ‘fine tuning’ to find what works best for an individual. In the UK, it is very common to find the general public lacking in this vital nutritent. Commonly, the only decent amount of protein is eaten at an evening meal. It should be a major part of every meal consumed. A good starting point is to consider the amount of protein needed dependent on body weight and the intensity of physical activity.
Data taken from American Collage of Sports Medicine, American Dietetic Association, and Dieticians of Canada Joint Position Statement. Nutrition and Athletic Performance. Medicine and Science in Sports and Exercise 32 (12): 2130 – 2145, 2000.
** Athletes attempting to lose body fat via the use of an energy restricted diet, will convert much of their dietary protein into glucose which is then burned as energy. This leaves less protein available for muscle repair, requiring the athlete to increase their protein intake.
Protein is best utilised when ingested in smaller, more regular meals. If a protein surplus is ingested the amino acids are taken to the liver, where a process called ‘deamination’ occurs.
Key learning points
- Proteins are made from amino acids
- there are 20 protein amino acids
- there are nine essential amino acids because we can not synthesise them ourselves
- there are 11 non essential amino acids that the liver can synthesise so long as we have sufficient essential amino acids in our diet
- Proteins are made from chains of amino acids
- shorter chains are called peptides
- proteins are chains of 100 or more amino acids
- large proteins are two polypeptide chains joined together an folded into specific shapes
- Complete proteins are foods, which contain large amounts of all 9 essential amino acids. Examples are:
- soy products
- Incomplete proteins are foods, which are deficient in two or more essential amino acids. Examples:
- Complementary proteins occur when a variety of incomplete proteins are eaten which will provide all nine essential amino acids. They need not be eaten with the same meal, but need to be eaten on the same day.
- the breaking down of structures within the body
- exercise is catabolic on muscle tissue
- the building up of a structure
- rest and feeding after exercise is anabolic
- Excess dietary protein is disposed of via two pathways:
- the nitrogen is excreted in urine via the kidneys
- the protein remnants are burnt by the tissue as fuel
Excess protein can suppress the use of fat as fuel, potentially causing an increase in body fat.