Protein Quality and Its Importance

Adequate and quality protein intake is indispensable for the continuation of a healthy life. Do you take into consideration the quality of protein while striving for sufficient protein intake?
Protein is an essential macronutrient for our bodies. The amount of protein we consume is as crucial as the quality of the protein for maintaining a healthy body. Let's explore what protein quality is, why it matters, how it is measured, and many other related topics together.
What is Protein Quality?
Foods differ in protein quality. All proteins contain essential amino acids and serve as a nitrogen source. However, the amounts of essential amino acids in proteins can vary. The quality of a food's protein depends on its amino acid composition, the quantity of amino acids, and the digestibility of these amino acids in the upper gastrointestinal system.
There are various methods for calculating the protein quality of foods. Some methods focus on supporting optimal growth, while others concentrate on amino acid balance, protein digestion and absorption, or indispensable (essential) amino acids based on amino acid requirements.
How is Protein Quality Determined?
The Food and Agriculture Organization (FAO), the World Health Organization (WHO), and the U.S. Food and Drug Administration (FDA) proposed a protein quality assessment method called Protein Digestibility-Corrected Amino Acid Score (PDCAAS) in 1991, which became widely used for determining protein quality. However, in 2013, the FAO recommended a new method called Digestible Indispensable Amino Acid Score (DIAAS) for evaluating protein quality. These two methods are frequently used comparatively to better understand protein quality.
Apart from the PDCAAS and DIAAS methods, Protein Efficiency Ratio (PER) and Net Protein Ratio (NPR) methods can also be used in protein quality assessment.
PDCAAS Method
Protein Digestibility-Corrected Amino Acid Score (PDCAAS) is one of the methods used to evaluate the protein quality of a food.
PDCAAS is a method recommended by FAO and WHO in 1990 for assessing the protein quality of a food. In the PDCAAS method, the evaluation of protein quality is based on the ratio of essential amino acids to total protein, and it is determined through feces. In studies conducted using rat feces, the ratio of digested crude protein to the limiting amino acid score gives us the PDCAAS value. A PDCAAS score of 1 for a food indicates that it has the potential to meet 100% of an individual's essential amino acid needs.
For example, when comparing the PDCAAS scores of Food X, if the lysine amino acid has the lowest percentage (e.g., 40%), unless another protein with high lysine content is consumed, only 40% of this protein can be used for protein synthesis.
The highest PDCAAS score any protein can achieve is 1. This means that 100% of the protein can be used for protein synthesis; that is, it contains all the necessary amino acids in appropriate proportions per gram of protein.
The digestibility percentages of proteins in foods directly affect the PDCAAS value. Sulfur-containing amino acids, lysine, threonine, tryptophan, valine, isoleucine, and leucine have been identified as limiting amino acids in some studies.
DIAAS Method
Digestible Indispensable Amino Acid Score (DIAAS) is the current protein quality ranking method recommended by the Food and Agriculture Organization (FAO). The calculation logic of this method is based on predicting the digestibility (quality) of proteins relying on real ileal digestibility (i.e., determined at the end of the small intestine where amino acids are absorbed) and ideally conducted on humans.
DIAAS is calculated by comparing the ratio of the digestible amino acid content in food (mg/g protein) to the same ratios of amino acids obtained from a reference model based on age-specific amino acid requirements. To convert the ratio to a percentage, it is multiplied by 100, representing the DIAAS value of the food. Comparison samples are taken from the ileum of humans and also collected from a pig. Pigs have more biological similarity to humans than mice.
DIAAS scores obtained with the IAAS method are classified as follows:

• Low or medium protein quality – Score <%75
• Good protein quality – Score ranging from 75% to 99%
• Very good protein quality – Score of 100% or higher

Health organizations, particularly FAO, have been advocating for the widespread use of the DIAAS method since 2013 to better analyze protein quality. Therefore, when you want to learn about the protein quality of a food, looking at the DIAAS score may provide more accurate insights.
Differences Between PDCAAS and DIAAS Methods
There are significant differences between Protein Digestibility-Corrected Amino Acid Score (PDCAAS) and Digestible Indispensable Amino Acid Score (DIAAS) methods. Especially since 2013, considering these differences, the more widespread use of the DIAAS method is recommended.
The PDCAAS method falls short, especially in cases related to excessive prediction in the elderly (likely associated with reference values based on young individuals), the impact of ileal (small intestine) digestibility, and factors related to anti-nutrition (conditions that inhibit absorption).
Understanding the differences between these two methods and their applications can assist you in making more accurate plans when choosing protein sources in your dietary regimen. For example:
In a study on protein quality, milk protein concentrate, corn-based breakfast cereal, and a mixture of these two sources were compared. While the DIAAS value of milk protein concentrate was 1.18, the DIAAS value of corn-based breakfast cereal was 0.012. The DIAAS value of the meal content obtained by mixing these two foods in a 60% milk protein concentrate and 40% breakfast cereal ratio was determined as 1.07. This situation demonstrates the ability of milk to complement corn/wheat, resulting in a balanced meal that meets all essential amino acid requirements.