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Proteins are an important component of diet and their everyday intake secures optimal functioning of many processes in our body. On the market, you may find a wide variety of sources of protein either in plant-based products or in animal products, but also in form of nutritional supplements. However, the quality of protein is not the same in all sources, therefore we should be able to evaluate it. When evaluating the quality of protein, two basic criteria are essential – biological availability and amino-acid profile. Based on these two criteria, the international scales DIAAS and PDCAAS have emerged and they can evaluate the sources of protein and their contribution for human body in a complex way.
Biological availability of protein
Protein molecules are made up of a chain of amino acids that the human body uses for various purposes. In general, the body works with approximately 20 amino acids, of which 9 are essential. This means that the body cannot produce them by itself and therefore they need to be taken from food. After consumption, proteins are digested in the stomach and small intestine, where they are broken down into separate amino acids. The organism adjusts them in a process called protein synthesis. It is a very complex process, but to put it simply, the body can adjust the amino acids in such a way to be able to compile from them some usable proteins. Biological availability, or Bioavailability therefore expresses how well the body can use the protein it consumes. 
Bioavailability of the protein is determined by three measurements based on nitrogen calculations in the body    :
- Biological value of protein (BV) – measures how much nitrogen from protein is theoretically used in muscle tissue.
- Net Protein Utilization (NPU) – determines the proportion of amino acids in a diet that the body converts into proteins.
- Nitrogen Balance (NB) – can be positive, negative or zero. At a positive value, dietary proteins are used to build muscle. A negative value means that the body uses muscle protein as an energy source.
You are not a fan of calculations and formulas? BV, NPU and NB measurements are based on the nitrogen that is excreted by a person and from this it is determined how much protein it represents. The final number can be compared to the consumed protein and the result is bioavailability. Overall bioavailability includes BV, NPU and NB results. The most interesting value is the biological value of proteins. 
Biological value of proteins
The human body receives proteins in the diet, and these are then processed in the digestive tract. Not every type of protein is used to build muscle to the same extent. Biological value (BV) is the percentage of nutrients that the body can use. It is derived from the calculation of protein intake, which indicates the difference between the increase and excretion of nitrogen. Simply put, BV indicates how quickly and well the body can use the consumed protein. 
A natural source of protein that are the easiest to digest are whole eggs and the organism can use up to 94 % of them. For this reason, eggs are assigned a value of 100, and all other protein sources are then compared to eggs. Nowadays, there are proteins in form of nutritional supplements that are easier and better to digest than eggs and therefore have a value of more than 100. Among the most popular ones are the whey concentrate and isolate. For a better idea, we present you a table with the protein evaluation :
Would you like to know the difference between the whey concentrate and whey isolate? Get to know all the necessary information about these sources of protein in our article – What protein to choose? Whey concentrate, isolate or hydrolysate?
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Amino acid profile
As mentioned above, protein molecules are composed of amino acid chains. Some of them are created by the body itself, while others are taken from the diet. The type of amino acids which the body cannot synthesize is called essential. These include 9 amino acids :
The most important amino acids for muscle mass growth and maintenance are the so-called BCAA, branched chain amino acids. These include leucine, isoleucine and valine, with leucine having the most significant anabolic effect. A very important criterion for a protein source selection is the amino acid profile, which represents the content of amino acids in the protein. For a better orientation, we present you a table with an overview of the amino acid content in various protein sources and their recommended daily dose for humans. 
* amino acid content per kilogram of body weight per day
Would you like to know more about essential amino acids and their importance for a human body? You can find all the necessary information in our article – Essential amino acids EAA and their effects, sources and dosing.
According to the amino acid content, protein sources can be divided into 2 groups :
- Complete proteins – contain all essential amino acids, among them are mainly animal products such as meat, dairy products and eggs.
- Incomplete proteins – contain at least one essential amino acid, among them are proteins of plant origin, such as legumes and cereals.
Bioavailability is still a popular figure, especially in the media and as a part of promotional materials. It relies on two assumptions which have been challenged :
- Proteins are the only source of nitrogen in the body.
- Non-excluded proteins are used to produce body proteins.
The reason for this disbelief is the fact that the digested protein can be converted into glucose in the body, and this happens in the case of a quickly digested protein. The more objective ways of evaluating proteins are PDCAAS and DIAAS scales .
PDCAAS and DIAAS
Experts prefer new scales to bio-availability – PDCAAS and DIAAS, because they both address not only digestibility but also the amino acid profile. The PDCAAS (Protein Digestibility Corrected Amino Acid Score) scale can be translated as protein digestibility with respect to amino acids. The scale is used by the World Health Organization (WHO) and the Food and Agriculture Organization. PDCAAS has become a substitute for bioavailability since it takes into account the amino acid profile and digestibility. The abbreviation DIAAS (Digestible Indispensable Amino Acid Score) denotes in free translation a value for digestible necessary amino acids.  
The difference between these protein indicators is in the sample from which they determine the protein value. PDCAAS of protein is evaluated from the faeces and DIAAS from the contents of the ileum, which is the last part of the small intestine. PDCAAS expresses the digested protein content after passing through the small and large intestine, DIAAS measures the protein content after passing through the small intestine only. 
The DIAAS scale result is a percentage of protein quality that can be divided into 3 basic groups :
- 100 and more % – high (excellent) quality protein
- 75 – 100 % – good quality protein
- less than 75 % – low quality protein
DIAAS is the most modern way of evaluating protein quality, for better orientation we present DIAAS protein sources, which are evaluated on the bases of protein digestibility and their amino acid content .
|Whole milk powder||143||High|
|Milk protein concentrate||118||High|
What makes protein to be the “high quality” source?
Some people’s idea of the whole process begins with the protein shake and ends with the muscle mass growth. But to maintain the muscle mass, protein needs to overcome its journey. Proteins from food are digested by gastric juices, enzymes and other components, and they are subsequently cleaved to amino acids. Only then do they enter the bloodstream and are transported to tissues, where they “attach” to larger protein molecules. High-quality protein is determined by three basic functions :
- protein digestibility (Can your body break them down?)
- amino acid content (What amino acids are contained in proteins)
- availability of amino acids for the support of metabolic functions (Can your organism use amino acids?)
Whether you prefer protein with low, good or high quality, you should definitely remember the following :
- The amount of proteins you need depends on the quality of proteins you consume.
- If you consume proteins mainly of plant origin, you should increase your daily protein intake, since these are of lower quality.
- If you’re a vegetarian or vegan, try to have a varied mix of diet. It is also no harm to know the amino acid profile of each protein source.
The purpose of protein differentiation is not to navigate people only to a high-quality protein source, or to exclude food sources with a low content of protein. It is more important to ensure a varied diet and complex protein intake on a daily basis. Build your diet so that you take enough amino acids daily to ensure the proper functioning of your body and to achieve your fitness goals.
 Jillian Kubala - Essential Amino Acids: Definition, Benefits and Food Sources – https://www.healthline.com/nutrition/essential-amino-acids
 Alex Leaf - How can you assess protein quality? – https://examine.com/nutrition/rating-proteins/
 Yvette Brazier - How much protein does a person need? – https://www.medicalnewstoday.com/articles/196279.php
 PROTEIN BIOLOGICAL VALUE SCALE
 Schaafsma G - The protein digestibility-corrected amino acid score – https://www.ncbi.nlm.nih.gov/pubmed/10867064
 About DIAAS – https://www.diaascalculator.com/about-us/
 100% Amino Acid Score – https://www.mondoscience.com/blog/2017/10/25/100-amino-acid-score
 Protein Bioavailability and Digestibility: What You Should Know – http://beelitenutrition.com/2016/04/protein-bioavailability-what-you-should-know/
 PROTEIN QUALITY–THE 4 MOST IMPORTANT METRICS – https://labdoor.com/article/protein-quality-the-4-most-important-metrics
 Nitrogen Balance Calculator – https://intensivecarenetwork.com/Calculators/Files/Azote.html
 Jay R. Hoffman, Michael J. Falvo - Protein – Which is Best? – https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3905294/
 Pamela Nisevich Bede - WHAT IS A “HIGH-QUALITY” PROTEIN? – https://www.bornfitness.com/high-quality-protein/