Let’s talk protein digestibility! What even is that? I’m so glad you asked.
To truly understand what makes a quality protein, we need to have a strong handle on how well various proteins are digested in the body. After all, a protein is only as good as the amount it is used.
But first, let me tell you how protein quality is calculated.
For a long time, protein digestibility has been established using the Protein Digestibility Corrected Amino Acid Score (PDCAAS). In this model, researchers fed mice various protein sources and measured how much was absorbed through stool testing. The results were ranked on a 0-1 scale, theoretically indicating the digestibility of a given protein.
The PDCAAS has truly laid the groundwork for our current understanding of protein intake, but serious criticism exists:
Mice are dissimilar enough to humans in how they digest proteins that it isn’t truly reliable
The 0-1 ranking system scores based on the presence of all essential amino acids and does not assess assimilation
Enter the Digestible Indispensable Amino Acid Score (DIAAS). Unlike the PDCAAS, the DIAAS evaluates the presence of amino acids but ultimately - and this is key - establishes protein digestibility. For this model, researchers are testing the protein absorption from the small intestines of pigs, which is a much better comparison for humans than mice. Results are then mapped on a scale that goes above one to indicate how well the amino acids in a given protein were absorbed. This approach works to establish the usefulness of a given protein, not simply show that amino acids are present.
How do protein scores differ? Let’s use Whole Milk as an example:
PDCAAS = 1
DIAAS = 1.14
Even as I’m typing this out, it seems like a no-brainer that we’d want to base protein quality on how well it is absorbed. So, why isn’t it the norm yet?
Given the newness of this framework, there is more research is needed to measure a broad spectrum of foods. Then, nutrition guidelines will have to be adjusted to better reflect the latest findings. Given it takes 17 years for a study to go from the research phase to being applied in a clinical setting, I won’t be holding my breath for when (and if) it actually comes to fruition.
In the meantime, I’ll be working on Part 2, where we’ll evaluate whether or not all proteins are equal. It’s going to be a good one. Stay tuned!