Determining Protein Quality: the Current State of Play

Over the years, there has been considerable debate around how the relative quality of different proteins can be accurately measured. With a number of different methods having been used over the years, the debate has been reignited in recently as to what the best standard method should be. So what’s the current state of play in measuring dietary protein quality?

Lindsey Ormond
Lindsey Ormond Owner, LO Health Solutions

By Lindsey Ormond, Owner, LO Health Solutions

Why is protein quality important?
Let’s take a step back for a moment, to look at what exactly is meant by protein quality and why it is important. Dietary proteins are the building blocks for the human body. So ‘protein quality’ describes how well a particular protein provides the right amount and types of essential amino acids to meet the needs of a human or animal – and how well these amino acids are then absorbed.

Determining the quality of a protein is important for many reasons, including knowing how much of a certain protein may be needed, which source of proteins may be better in certain circumstances (such as recovering from illness or heavy, prolonged exercise sessions) and which proteins can be matched up to compensate for lower quality. Also, if protein intake is limited for some reason, then higher-quality protein will be optimal.

Yesterday’s best practice
In the early nineties, the US Food and Drug Administration (FDA) and the Food and Agricultural Organization of the United Nations/World Health Organization (FAO/WHO) adopted something called the PDCAAS as a method of evaluating protein quality - Protein Digestibility-Corrected Amino Acid Score [pronounced Pee-Dee-Kass].

PDCAAS is still the current best practice in protein quality measurement. At its core, as with other methodologies, is the idea that protein quality is best judged by examining the amino acid needs of humans and their ability to digest it. However, one of its shortcomings is that it is based on a rat model. This is just one reason why food scientists and regulatory authorities alike are seeking to put a new method in place.

What’s wrong with PDCAAS?
The PDCAAS of a dietary protein is calculated by comparing its amino acid composition against a reference pattern that roughly represents human nutritional needs. Each amino acid contained in the protein is scored in relation to this pattern, then this preliminary score is corrected for digestion availability. The most limiting amino acid in the protein, reflected by the lowest PDCAAS, then gives us the protein’s final score. Animal proteins, particularly dairy proteins, score highest under this system, while other proteins, such as those contained in rice or cereal, score far lower.

Since protein contains nitrogen, PDCAAS looks at the amount of nitrogen ingested in the diet against the amount excreted. But one of the problems with this is that it doesn’t take into account the microbial influence on nitrogen excreted, and can therefore lead to inaccurate estimation of amino acid availability.

As a reference point, PCDAAS uses the amino acid requirement pattern of preschool-age children, determined in the early eighties in a small group of subjects recovering from malnutrition, rather than that of more typical or representative populations. Plus, it doesn’t take into account anti-nutritional factors that could adversely influence amino acid absorption.

One of the most obvious shortcomings, however, is the issue of truncation. PDCAAS suggests that any value greater than 100% is irrelevant and will not be taken up by the body. Therefore, all values are truncated at 1. But this doesn’t allow us to really look at the difference between proteins. For example, although milk protein scores up to 1.3 under PDCAAS it must be truncated to 1, while soy protein scores around 0.97 and is rounded to 1, so they appear to be the same quality. Truncation means, therefore, that we simply can’t pull individual proteins apart for closer examination.

Tomorrow’s best practice
In 2011, the FAO met to review protein quality assessment, with three main goals in mind: 

  • To decipher if PDCAAS was still the best available protein quality methodology
  • To examine recommendations for potential alternatives
  • To highlight necessary research avenues

In 2013, a new methodology was recommended by the body as the new ‘preferred best method’ for measuring protein quality: DIAAS (Digestible Indispensable Amino Acid Score).

The FAO Expert Consultation’s chair, Paul Moughan, captured the spirit of this change and its key benefit when he stated: “The recommendation of the DIAAS method is a dramatic change that will finally provide an accurate measure of the amounts of amino acids absorbed by the body and an individual protein source’s contribution to a human’s amino acid and nitrogen requirements. This will be an important piece of information for decision makers assessing which foods should be part of a sustainable diet for our growing global population.”

What makes DIAAS better than PDCAAS? For one thing, it’s based on a pig model, which is closer to humans than the rat model underlying PDCAAS. For another, rather than estimating digestibility across the entire digestive tract, it focuses on ileall absorption of essential amino acids rather than the total protein. And it uses an older child as the reference point rather than the previous two year-old reference. Moreover, it doesn’t truncate values – so you can really see the differences in protein values for different protein types. For example, whey protein is 1.25, soy protein 0.98 and pea at 0.93, allowing us to really see the difference in protein quality.  

Nothing’s perfect - yet
DIAAS is certainly progress. But it’s not perfect. While DIAAS uses the older child as a reference point, it would be highly beneficial to get to a place where we can use values defined for specific populations, including teenagers, men versus women, pregnant women and older age groups.  And there are added complexities with using an animal model, including both ethical and cost challenges. Furthermore, with only a limited number of proteins having been tested via DIAAS, more work is needed before this is fully utilised as the best preferred method.

DIAAS does, however, enable us to analyze the digestibility of individual amino acids, treating each of them as an individual nutrient. While a mixed diet is always the best option, DIAAS can help us determine which sources to use in certain circumstances, for example,  assisting aging populations with reduced appetites or athletes looking to recover from heavy training.

With the world’s population increasing and growing older, protein quality is going to become even more important as we look to feed more people and keep them healthy for longer. 


(1) The Protein Digestibility–Corrected Amino Acid Score, Gertjan Schaafsma, 2000, The American Society for Nutritional Sciences.

This blog contains material and information intended for B2B customers, suppliers and distributors, and is not intended as information to the final consumers. 

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