When it comes to animal or vegetable proteins, the term “protein quality” often comes into play. But what is it exactly? And how can protein quality in a portion of food be reliably assessed? In a general sense, protein quality refers to how well or badly the body uses a food protein or its ability to achieve defined metabolic effects. Technically speaking, the protein quality indicates how well the profile of essential amino acids of protein can adequately meet the individual requirements for essential amino acids. Digestibility of the protein and the bio availability of the amino acids also play a role. Essentially, the protein quality is about how well or how badly the body uses a protein that is ingested once it has been digested. No process in the body works 100% effectively, including digestion. Some of the nutrients ingested will escape the digestive process for various reasons and continue through the gut. In this article we are going to look at the different types of protein and their sources.

There are considerable differences in the digestibility of the nutrients. The digestibility of fats is said to be around 97 percent. Digestibility of carbohydrates is a maximum of 98 percent, but depending on the source of carbohydrates, there can be large differences. The presence of fiber with a carbohydrate character (e.g., digestive-resistant starch) can significantly reduce digestibility. And what about proteins? Digestibility of plant proteins are, on average, 85 percent, that of animal proteins 95 percent. Under our eating conditions, protein digestibility of 94 percent can be assumed. The higher fiber content and anti-nutrient factors can also reduce the digestibility of proteins or amino acids and fats by another 4-6 percent. Processing foods and ingredients can reduce (but not eliminate) the activity of anti-nutrient factors and affect protein quality on its own. Types of processing that can affect protein quality include germination (soaking seeds until germination), wet or damp-heat treatment (cooking, steaming), dry heat treatment (drying, drying), smoking, roasting and grilling, spray drying, Extrusion, radiation, and fermentation. How well a protein is ultimately digested depends on various factors, such as the source, the amino acid composition, and the preparation and other nutrition. Digestibility of amino acids compared to rough protein digestibility is similar, but not identical digestibility of the individual amino acids also varies. However, protein digestion is generally extremely efficient, with the result that only a few percent of the protein nitrogen in the food are found in the stool.

The requirements of amino acids can vary depending on the age group and physiological conditions. The consequences of insufficient protein consumption, which does not meet the need for essential amino acids, include growth disorders, increased susceptibility to infections, muscle weakness, and reduced mental performance (from retardation to apathy). An accurate assessment of a protein source’s ability to meet the organism’s need for certain amino acids enables better use of an increasingly scarce resource. For evaluating the protein quality, multiple measurement scales, and techniques are available. The best known and most recognized methods include biological value, the Protein Digestibility Corrected Amino Acid Score (PDCAAS), and the newer Digestible Indispensable Amino Acids Score (DIAAS). However, the Protein Efficiency Ratio (PER) and the Net Protein Utilization (NPU) are also used in some sources to assess the protein quality of the source.

Protein quality, protein value, E protein quality, assessment of the nutritional value of dietary proteins. In general, P. provides information on the extent to which absorbed protein can replace degraded body protein. Decisive criteria for nutritional P. are the amino acid pattern of the protein and the digestibility of the amino acids’ protein/availability. The values determined are also dependent on the methodology used (in vivo studies on the species and the organism’s physiological state). The P. is determined using biological or chemical methods (Tab.). If biological methods are used, the P. is often stated as biological value. Data are available both from standardized feeding experiments on animal models and from human studies. The P. is often given relatively by comparison with a reference (standard) protein (whole egg protein or cow’s milk casein). From this, values of over 100% can be derived (supplementary value of different proteins in combination, see biological value tab.). In the chemical evaluation, amino acid concentrations determined analytically in the acidic protein hydrolysate are related to various reference values (Tab.). In addition to the reference protein, the required values for individual essential amino acids can also be used to assess. The amino acids with the lowest amino acid score (AAS) are the limiting amino acids for P. (usually the sulfur-containing amino acids methionine and cysteine). The digestibility of proteins and the availability of the released amino acids depends on the primary and tertiary structure of the proteins from. Besides, there are possible reactions between other food components (e.g., Maillard reactions with carbohydrates) and hydrolysis or absorption-inhibiting substrates (e.g., protease inhibitors).


The formula for calculating the protein efficiency ratio is:

PER = weight gain (measured in g) / protein intake (in g)

The ration of protein Efficiency (PER) determines the effectiveness of a protein by measuring a growing animal (usually laboratory rats). In this method, rats are fed a test protein, and then the weight gain in grams per gram of protein consumed is determined. The calculated value is then compared to a standard value of 2.7, which corresponds to the standard value of casein protein. Any value exceeding 2.7 is considered an excellent source of protein. Foods such as eggs, whey protein, milk, and casein cut with the highest protein quality after the PER. However, PER is a measure of growth in rats and is not a strong correlation to human growth requirements. Although PER is a relatively simple and economical method of measuring protein quality, the data derived from animal experiments are not always directly related to humans transferable. Individual amino acid requirements are not the same as in rats. And, in rats, the requirements can vary between young and adult animals. From 1991 until recently, PER was a popular method used n the United States for assessing the quality of proteins in food. The food industry in Canada currently uses PER as the standard for evaluating the protein quality of food.


The Net Protein Utilization (NPU) or Net protein utilization is the ratio of amino acids that have been converted into proteins concerning the supplied amino acids. The formula is calculated as the quotient of the amount of nitrogen retained and the nitrogen uptake, multiplied by 100: The NPU can take values from 1 to 0, where a value of 1 indicates 100% utilization of dietary nitrogen as protein and a value of 0 means that no nitrogen contained therein has been converted into protein. According to the NPU, eggs, whey protein, and milk have the highest protein quality.


When comparing animal and vegetable protein directly in Germany, the so-called “biological value” is often mentioned. The biological value measures the protein quality by calculating the nitrogen used for tissue formation (retained or retained nitrogen), divided by the nitrogen absorbed from the diet, that is, absorbed. This product is multiplied by 100 and expressed as a percentage of the nitrogen used.

Biological value = (retained nitrogen) / (absorbed nitrogen) *100

If you compare the formula for biological value with the formula for Net Protein Utilization (NPU), you can see that the formulas are similar. Both methods measure the same parameter of nitrogen retention. However, the difference is that the biological value is calculated from the nitrogen absorbed, while the NPU refers to the nitrogen absorbed. The biological value provides a measure of how efficiently the body can convert food proteins into the body’s proteins. A food with a high value correlates with a high supply of essential amino acids. The whole egg was arbitrarily set as the reference value at that time (value of 100 or 1) because, at the time, it was the protein source with the highest known biological value. Food proteins that can be used better in the body than egg protein have a biological value of over 100, food proteins with a poorer protein use than egg have minimal protein sources typically have a higher biological value because they are more like body proteins in their amino acid composition. Vegetable protein sources often lack one or more essential amino acids, which is why they rank worse. For example, whey protein has a biological value of 104, soybean only 74. However, several key factors that affect the digestion of proteins and the interaction with other foods before absorption are not considered in the biological value. Besides, the biological value measures the maximum potential quality of a protein and not its assessment at the requirement level.


Due to these deficiencies in the significance of biological value, the so-called PDCAAS (Protein Digestibility Corrected Amino Acid Score) was published in a joint position paper by the Food & Agriculture Organization (FAO) and WHO in 1989 preferred method for measuring protein quality in human nutrition adopted. In addition to the amino acid content, this “amino acid index corrected for digestibility” also considers the digestibility. The contribution of food to essential amino acids according to human needs. To calculate the PDCAAS, each essential amino acid of a food protein is first compared with the content of the corresponding amino acid in an ideal reference protein (for human use). The result is the amino acid index or Amino Acid Score (AAS) or Chemical Score (CS). The Amino Acid Score or Amino Acid Index is a way to predict how efficiently the amino acid content in a food protein will meet human amino acid needs. This concept assumes that tissue protein synthesis is restricted if not all of the required amino acids are available at the same time and in suitable amounts. The method is a comparison of the concentration of the first limiting essential amino acid in the test protein with the concentration of this amino acid in a reference protein:

AAS or CS = [mg of the limiting amino acid in 1 g test protein) / mg of the same amino acid in 1 g reference protein]

This is the amino acid in a protein that deviates the most from the amino acid requirement. It is “limiting” because the body can only synthesize as much protein as the amino acid with the lowest concentration allows.

Its biological value expresses the extent to which a portion of food contributes to meeting individual protein requirements. The biological value of the whole egg (egg yolk + egg white) is set to 100 because the egg protein is very similar to the human amino acid pattern. All other foods are compared. The biological value of cereals ranges from 60-70, for legumes 60-80, nuts and seeds come to 66-76 and buckwheat even to 80-93.

Protein Takeway:
Protein Quality

Since we almost always eat different foods together in a meal or the course of a day, the proteins of the individual foods complement each other. If the limiting amino acid of one food is found in excess in another, the biological value is upgraded. Grains with legumes or buckwheat are a good combination of plants, for example. In cereals, lysine is the limiting amino acid, and methionine is present in excess. The reverse is true in legumes. For example, a meal made from corn with beans or from buckwheat with wheat can have a biological value of 100. A varied vegetable diet with whole grains, legumes, vegetables, nuts, and seeds is, therefore, the best guarantee. Different protein source gives the body different protein quality. Depending on who is taking the protein, e.g., old people or sportsmen and women building muscles, it is good to check with your nutritionist the best source of your protein to maximize its protein intake. Young people and children also require different amounts of proteins in their body, which is important to consider when preparing their meals.