High-Quality Protein Sources Explained
What are high-quality protein sources?
In our blog post about protein balance, we established that the body protein we have corresponds to the difference between protein synthesis and protein breakdown (protein balance). We also saw that there are quantitative limits to sensible protein intake, and that these limits lie at around 20 g of high-quality protein per serving. While higher amounts don't lead to any further increase in protein synthesis and at the same time drive up protein breakdown (in other words, they affect protein balance negatively), smaller amounts of protein only stimulate protein synthesis submaximally, and therefore insufficiently. Accordingly, with a view to a maximally positive protein balance, we should take in around 20 g of high-quality protein roughly every 3 hours. But what actually is high-quality protein?
If we believe the recommendations of most protein supplement manufacturers, pretty much everything falls into the category of “high-quality protein”. Objectively speaking, unfortunately, that isn't the case.
When it comes to the value of different protein sources, several “scores” are used in practice. Besides the outdated (and often undefined) “biological value,” this applies above all to the “Protein Digestibility Corrected Amino Acid Score” (PDCAAS), which, however, according to the recommendations of the “Food and Agriculture Organization of the United Nations” (FAO) and the “World Health Organization” (WHO) should consistently be replaced by the “Digestible Indispensable Amino Acid Score” (DIAAS).
This is defined as: DIAAS (%) = 100 x [(amount in mg of a digestible indispensable amino acid in 1 g of the dietary protein) / (amount in mg of the same digestible indispensable amino acid in 1 g of the reference protein)], where digestibility refers to digestibility in the small intestine (the most precise assessment of digestibility). The DIAAS thus assesses a protein's ability to cover the needs of human metabolism.
In simple terms, the following can be said about the DIAAS:
- Ultimately, the quality of a protein source corresponds to “the delivery” of indispensable amino acids to the body.
- The quality of a protein source is made up of its biological availability, its amino acid profile (distribution) and the amount of indispensable amino acids it contains.
- DIAAS values can be above 100%. Milk protein (total fraction) has a DIAAS of 122%.
- Foods of lower protein quality can reach a higher DIAAS than in their unenriched state thanks to enrichment with high-quality protein sources or, in some cases, free amino acids, and can be optimised this way.
So what does this mean for supplements? Well, one thing is clear: many protein supplements marketed as “high-quality” (e.g. pure plant proteins without added essential amino acids; collagen hydrolysates, etc.) aren't optimal when it comes to human metabolism. We can also say that protein supplements based on milk proteins show high DIAAS values and are therefore high-quality. If we take the scientific analyses one step further, though, and look at the direct effects that protein intake has on protein synthesis and protein breakdown, we can narrow down the benefit of supplements even further. However, we'll only discuss this in the next post. One thing in advance: not all milk protein is the same.
