Protein Quality – Why Some Proteins are Better than Others
When it comes to quality, not all proteins are created equally. The protein found in eggs for example is utilised twice as effectively within the body as the protein found in wheat. In this article we explain why some proteins are utilised within the body better than others and what protein foods offer the best quality.
What is protein quality?
To understand the quality of protein concept you must first learn some basic nutrition. Protein is made up of building blocks called amino acids. There are 20 different amino acids found within the body, 9 of which are consider essential and must be obtained through your diet. The other 11 amino acids are considered non-essential as these can be manufactured within the body as long as your diet is otherwise adequate.
The following table summarises the essential and non-essential amino acids. Don't worry, unless you're studying biochemistry you won't need to remember them, they are listed simply because they are an important part of understanding the concept of protein quality.
* Histidine is now listed as an essential amino acid as it is evidently required in childhood, even though its role in adulthood is less clear.
You can obtain all of the amino acids (both essential and non-essential) through a good diet, however it is the essential amino acids which determine the protein quality of a food.
Protein Quality of Food
There are 2 factors which determine the protein quality of a food:
In summary, the protein quality of a food is determined by how closely the ratio of essential amino acids in the food matches the human requirement and by how easily the protein is digested and utilised within the body.
There are a number of the methods of determining the protein quality of food. These methods work by either comparing the amino acid content of the food with the human requirement (through chemical estimation) or by measuring the amount of protein absorbed and retained by the body (Net Protein Utilisation).
The table below summarises the protein quality score of common foods using Net Protein Utilisation (NPU). The NPU is determined by measuring the amount of protein in a food and then accounting for any loses (in urine and faeces) after ingestion. The percentage of protein that is retained within the body is the NPU value. In summary, the higher the score the more effectively the protein will be absorbed and retained (and in theory utilised) within the body.
As can be seen in the table above, human breast milk has the highest protein quality (94% utilisation) which makes sense given the role of human milk in the growth and development of babies. This also helps to explain why eggs and cow's milk are high in protein quality as again these foods provide infant animals the nutrition required for growth and development in the early stages of life.
Supplementary protein powders often use milk proteins such as whey protein and casein, or egg protein as their main ingredient as these proteins are high quality and are easily utilised within the body to build muscle. To make it easy to compare the protein quality of food supplements to that of real foods a measure of biological value is used.
Biological Value of Protein
The Biological Value (BV) of protein is the measure of protein quality typically used within the body building industry. The biological value of protein can be measured using either of the methods (chemical estimate or Net Protein Utilisation) described above. The NPU method is considered the gold standard of measuring protein retention and in an ideal world all food and food products would use this method to determine their true protein Biological Value. Given however that this method is also both expensive and invasive it is rarely used in the food supplement industry.
A true Biological Value will be displayed as a percentage and uses the Net Protein Utilisation method to calculate how much protein is retained within the body. Theoretically you cannot have a true BV (or NPU) greater than 100%.
The other more common and cheaper method of determining BV is to use the chemical estimate technique. This technique allows you to easily compare the protein quality of food supplement items to that of whole foods. Given that eggs have the highest NPU value of any freely available food (seen in the table above), by comparing the amino acid profile of a specific food, to that of eggs, a chemical estimate of the relative Biological Value can be determined.
Using the relative BV technique, eggs are assigned a BV = 100. By comparing the amino acid profile of a test food to that of eggs a relative BV can be determined.
Whilst in theory you cannot have a true BV value great then 100%, it is possible to have a relative BV greater than 100. The best example is that of whey protein, which in its most natural state as whey protein concentrate has a relative BV of 104. In comparison some of the more desirable whey protein isolate products can have a relative BV of up to 159.
In summary it is not uncommon for whey protein supplements to have true biological value of almost 100%, making them the most complete and desirable sources of protein sold in the market place. This is achieved by isolating the essential amino acids that make up whey protein (found in cow's milk) and removing the unwanted properties to ensure a higher utilisation within the body. To learn more please refer to the article on the benefits of whey protein.
Protein Quality of General Food Items
As a general rule the protein found in meat and meat products are of higher quality than those found in vegetarian sources as there is a higher percentage of essential amino acids, and in ratios that are similar to the human requirement. This is not to say that plant based foods are bad sources of protein, they are just less complete meaning that you need to consume more of them and more variety in order to obtain all of the essential amino acids. Soy is widely accepted as the most complete plant source of protein and therefore vegetarians are encouraged to consume soy based foods on a regular basis.
The best way to obtain your daily protein requirement is through a balanced diet made up of a mixture of plant and animal foods. Whilst the protein in animal foods is of higher quality, a diet that consists of only of meat products is associated with a higher incidence of chronic disease, therefore ensuring that you consume adequate amounts of fruits and vegetables is important.
Whilst vegetarians typically have a lower incidence of chronic disease, some of most common nutrient deficiency pathologies reported among vegetarians are associated with either low dietary protein or low iron. To ensure vegetarians are obtaining their daily requirement of protein it is important that they consume a variety of high protein plant sources. Additionally because the quality of plant protein is not as high as that found in meat, vegetarians will typically have to consume a higher portion size to ensure they obtain all of their essential amino acids. Please refer to the article high protein foods for good examples of both meat and plant foods that are high in protein.
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These articles will provide you with the necessary tools to start a successful gym training or bodybuilding program. These principles are only the beginning. Remember that discipline, knowledge and consistency are the keys to achieving your goals. Good luck!
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