Protein synthesis underpins adaptations of muscle mass, muscle strength, and all other adaptations to training. Protein quality can be an important factor to increase protein synthesis. The quality of a protein source is determined by 3 main factors:
Essential amino acid (EAA) content
Leucine content, and
Bioavailability (i.e., the availability of ingested amino acids for protein synthesis)
These factors all contribute to the degree in which a protein source can increase protein synthesis (see this previous blog). A higher quality protein will do this more effectively. But how exactly can we assess protein quality? And how do these assessments help determine a protein as low or high quality? How can we predict or estimate the effect of protein intake on protein synthesis? Below we will discuss the various ways we can assess protein quality. We will start with the simple and cheaper methods and move towards more complex, time consuming and expensive methods. The purpose is too give an idea of the methods available and a rough idea of the principles. It is beyond the scope of this blog to discuss the details, the assumptions and the limitations of the techniques mentioned in great depth.
Protein, EAA, and leucine content
The easiest and cheapest way to get an impression of protein quality is to study its amino acid composition. Although we call it "easy", in reality most food labels only report total protein, without any EAA values and finding good values for the amino acid composition of various foods is not as straightforward as it probably should be.
Protein sources with a more favourable EAA and leucine content are considered more effective for protein synthesis. Although the exact numbers may vary, there appears to be a threshold for EAAs (~10 g) and leucine (~3 g) required to optimally stimulate protein synthesis (1, 2). Below is a summary of the amount of food needed to obtain ~3 g protein of leucine.
Protein quality scoring systems
The next methods combines information about amino acid composition with digestibility of the protein. Protein quality can be ‘scored’ using techniques such as the protein digestibility-corrected amino acid score (PDCAAS) and the digestible indispensable amino acid score (DIAAS). Both techniques compare the digestibility of a dietary protein against a reference protein to determine a score of protein quality; a higher score = a higher quality protein.
PDCAAS
The PDCAAS is now considered an outdated technique. Although appropriate for lower quality proteins, PDCAAS values are capped at a score of 1.00 (or 100%). This capped value makes it impossible to categorise protein scores that exceed the reference protein (egg protein). In other words there is no differentiation between proteins that are high quality. PDCAAS estimates protein digestibility crudely from faeces, and this will not always accurately reflect amino acid absorption. In the infographic below you can find a selection of foods and their PDCAAS values.
DIAAS
The DIAAS is the newer, preferred technique that compares a dietary protein source to a theoretical protein based on current EAA requirements. This theoretical protein can detect higher quality proteins (i.e., those ≥1.00). In addition, the DIAAS assesses the digestion of individual EAAs in the ileum (i.e., at the end of the small intestine). Assessing individual EAA digestion in the ileum (rather than crude protein digestion in the faeces) is considered a better reflection of amino acid absorption. However, there are a limited number of proteins assessed for quality using the DIAAS.
In short, PDCAAS and DIAAS techniques provide a score of protein quality, as determined by the digestibility of a dietary protein source. A higher score indicates a higher quality protein, that may be more effective for increasing protein synthesis
Additional reading: Minimum requirements vs. optimal
Protein quality scoring systems are based on minimum requirements, defined as the lowest level of dietary protein that will balance nitrogen loss from the body. This begs the question: Are there benefits to consuming above the minimum requirements?
The PDCAAS and DIAAS calculate protein quality based on one limiting EAA. That is, the lowest EAA in a protein source relative to requirements. This calculation tells us whether an appropriate amount of EAAs is provided. What it doesn’t tell us is if the dietary protein was higher in certain EAAs, we would see a greater benefits. For example, leucine is recognised as a key regulator of protein synthesis. Although leucine may not strictly be limiting in a protein source, providing more leucine could increase the muscle’s response to a given protein.
Blood amino acids and muscle protein synthesis
Blood amino acid concentrations
The next level up in the assessment of protein quality is measuring it in blood after absorption. These so called ‘feed and bleed’ studies involve the consumption of a specific protein source (feed) followed by frequent blood sampling to determine blood amino acid concentrations (bleed).
Blood amino acid concentrations act as a surrogate measure of protein digestion. Protein sources with a rapid, and a greater elevation in blood amino acids are considered higher quality. These sources provide a greater availability of amino acids for the muscle to use for protein synthesis. In contrast, sources with a slower, and lower rise in blood amino acids are considered lower quality. Although insightful for what is happening in circulation, changes in blood amino acid concentrations leave us to speculate what this means for muscle.
Muscle protein synthesis
Therefore the most relevant measurement would be the measurement of protein synthesis. If we want to understand how protein-derived amino acids are used in protein synthesis, we can use labeled amino acids and study the incorporation of that amino acid into muscle proteins (or other tissues). Small muscle samples are collected usually 3-5 hours apart: one before ingestion and (at least) one after. It is possible to measure the label in muscle protein and more label means more of the amino acid is incorporated and thus protein synthesis was higher. There are many assumptions and considerations with this technique but for simplicity reasons we stick to the principle of the measurement here.
The most common used method form research purposes is a stable isotope tracer method that combines the infusion of a tracer (usually a “labelled” amino acid) with biopsies to detect amino acid incorporation into muscle proteins. High-quality protein sources are those that stimulate a greater incorporation of amino acids, and therefore have a higher rate of protein synthesis. In a previous blog we discussed a study that used this method. It was reported that whey protein was more effective than equal amounts of casein and casein more effective than soy protein.
Studies obtaining human tissue (blood and muscle) are complex, and invasive. Therefore, the quality of evidence is high. However, the nature of these studies limit the number of protein sources investigated.
Anabolic signalling
There is one other technique worth mentioning. Researchers can also use mechanistic techniques in combination with measures of protein synthesis. Briefly, protein synthesis is regulated by activation of mTORC1 signalling following protein consumption (see this blog). Techniques including ‘Western Blot’ are used to detect the expression of mTORC1 signalling proteins. The greater expression of signalling proteins, the greater the expected effect on protein synthesis. For example, leucine-rich protein sources activate mTORC1 signalling proteins, which may result in a greater stimulation of MPS. However, whether enhanced mTORC1 activation results in greater adaptations (muscle mass or strength) is unclear, as discussed in this previous blog.
Summary
All techniques discussed above estimate of the anabolic effect of a protein source on muscle protein synthesis. The accuracy of this prediction depends on the technique used. Although difficult to say for certain, the techniques/measures can be ranked as follows:
MPS > blood EAA concentration > DIAAS and PDCAAS > EAA and leucine content > Protein content
A combination of all assessments is optimal to determine protein quality. However, PDCAAS and DIAAS scores are probably less informative for those wanting to optimise the effects of protein on protein synthesis. Most of the other techniques, although useful in research, are not very practical. Good tools for athletes are unfortunately not available. It is therefore recommended, that athletes, at a minimum, should look for the EAA and leucine content of their food to inform their protein source choices to get an idea of protein quality.
Reference
Witard, O.C., Jackman, S.R., Breen, L., et al. (2014) Myofibrillar muscle protein synthesis rates subsequent to a meal in response to increasing doses of whey protein at rest and after resistance exercise. The American Journal of Clinical Nutrition [online], 99 (1): 86–95.
Churchward-Venne, T.A., Breen, L., Di Donato, D.M., et al. (2014) Leucine supplementation of a low-protein mixed macronutrient beverage enhances myofibrillar protein synthesis in young men: a double-blind, randomized trial. The American Journal of Clinical Nutrition [online], 99 (2): 276–286.
Phillips, S.M. (2016) The impact of protein quality on the promotion of resistance exercise-induced changes in muscle mass. Nutrition & Metabolism [online], 13 (1).
