Methods to Measure Protein Quality of Dairy

What methods do we use to measure protein quality?

Currently, there are 2 main methods for calculating protein quality:

1. The Protein Digestibility-Corrected Amino Acid Score (PDCAAS)
2. The more recently developed Digestible Indispensable Amino Acid Score (DIAAS)¹

The Food and Agricultural Organization of the United Nations (FAO) oversees protein quality evaluation methods and makes recommendations based on the latest research.

1. The PDCAAS Method

This PDCAAS method, developed in 1991, evaluates quality by comparing the amino acid composition of a particular protein to the body’s requirements. 

This method attempts to determine protein quality by also considering digestibility and amino acid absorption. The amount of amino acids in a particular food is evaluated using a scoring pattern based on the essential amino acid requirements of a 2 to 5-year old child.² 

The PDCAAS uses a scoring system that evaluates protein quality based on a range from 0 to 1.0, with 1.0 being the highest quality and providing 100% or more of the amino acids in the diet. 

Whey and casein are both high-quality proteins with a score of 1.0, while tree nuts are less than 0.5.
 

2. The DIAAS Method

In 2013, the FAO proposed the DIAAS method as an improved and more accurate way of determining protein quality.³

The DIAAS method measures the ratio of the digestible amino acid content compared to the same amino acid in a reference pattern, taken from age-specific amino acid requirements. The lowest value across amino acids is multiplied by 100 to give a final value. DIAAS samples are taken from the ileum of a pig.

The majority of protein absorption takes place in the ileum, and pigs are thought to be more genetically similar to humans compared to other animals. PDCAAS samples come from faeces, which may not be an accurate depiction of absorption.⁵

With the DIAAS scoring method, values are used to identify protein quality. These values include the following categories:

• No protein quality claim – Score of <75%
• Good protein quality – Score ranging from 75% to 99%
• Excellent or high protein quality – Score of 100% or more

Using this method, both whey and casein score over 1.0, meaning they are of the highest quality and provide all essential amino acids in the required amounts. Consumers and practitioners would be helped with better availability of clear and concise information about the protein quality of products, e.g. on pack.

Figure 1.

Which method is preferred? PDCAAS vs. DIAAS

The DIAAS method was proposed by the FAO and is thought to be the preferred method of determining protein quality.¹

One limitation of the PDCAAS method is that the maximum achievable score is 1.0, and with this, proteins of higher quality may not be identified or highlighted.

On the contrary, the DIAAS method does not truncate scores, allowing for a more accurate reflection of protein quality. Additionally, the DIAAS sampling method is more accurate. DIAAS samples come directly from the ileum, which is where the majority of protein absorption is occurring.

DIAAS is considered the most accurate method to determine protein quality. 

When evaluating dietary adequacy, having a general understanding of protein quality scores can help a practitioner determine if a patient’s diet is meeting their daily protein needs. 

It may also help the practitioner provide guidance for improving the overall protein quality of the diet by including high scoring choices like whey or casein.