What does protein quality mean?

Not all protein sources are equal. In fact, where you get your protein from can significantly impact your results in training. Let us explain…

protein sources

Roles of protein

Think of dietary protein as having 2 main roles when it comes to performance:

1. Triggering an adaptation (i.e. the leucine trigger)

2. Providing the building blocks to repair damaged tissue and build new tissue

We’ve preached it many times – 20-30g of protein every 3-4 hours. But why? Well, this dose of protein generally provides ~10g of essential amino acids and 3g of leucine. However, these quantities will vary depending on the source of protein you’re consuming.

Difference sources of protein vary in:

  • Their amino acid profile (essential amino acid content and leucine content),

  • Their digestion and absorption rate (i.e. "fast vs slow proteins")

Protein quality

Protein quality defines the usefulness of protein sources based on their amino acid profile and digestibility. There are many methods used to define protein quality.

A couple of these include:

  1. Biological value – Reflects the availability and digestibility of the protein source

  2. Amino acid score – Measures the essential amino acids present in a protein and compares to a reference protein. The protein is rated based on the most limiting essential amino acid

  3. Protein Digestibility Corrected Amino Acid Score (PDCAAS) – Amino Acid Score with an added digestibility component

  4. Digestible Indispensable Amino Acid Score (DIAAS) – Ratio of the digestible amino acids to the same amino acid in an age-specific reference pattern.

So where do your favourite foods sit?

Source: Burd N.A. et al. Food-First Approach to Enhance the Regulation of Post-exercise Skeletal Muscle Protein Synthesis and Remodelling. Sports Medicine. 2019; 49(Suppl 1):59-6

Protein quality

Plant vs animal protein

As you may have gathered from the table above, there is a stark difference between the scores of plant sources and animal sources.

I recognise that food choices are (and should be) based on more factors than just nutrition and performance goals alone. However, this article is solely focussed on the research around the quality and amino acid profiles of protein sources.


There are likely a number or reasons including the three listed below:

1. The lower digestibility of plant proteins.

2. Plant proteins are made up of less essential amino acids.

3. Plant proteins have a lower leucine content, and we know that leucine acts as a trigger for muscle protein synthesis.


Digestibility is a measurement that helps us understand how much of the protein from the protein source is actually accessible to the body. Plant proteins have a lower digestibility score, meaning that we actually don’t have access to as much of the protein in the food and will need to consume these foods in larger amounts.

Less essential amino acids

Amino acids are the building blocks that your body uses to repair and synthesis new tissue. Your body is able to synthesise some amino acids but not all of them. Those that the body can’t create itself, we call essential amino acids as it is essential we hit our requirements through food.

The figure below compares the essential amino acid content of different foods.

Source: van Vilet S. et al. The Skeletal Muscle Anabolic Response to Plant-versus Animal-Based protein Consumption. J Nutr. 2015; 145(9):1981-1991.

Essential amino acids in food

Lower Leucine content:

As we know, leucine triggers muscle protein synthesis and peak results tend to be seen at ~3g every 4 hours.

The figure below compares the leucine content of various food sources.

Source: van Vilet S. et al. The Skeletal Muscle Anabolic Response to Plant-versus Animal-Based protein Consumption. J Nutr. 2015; 145(9):1981-1991.

Leucine in food

Here’s a handy table to help plan your meals and snacks. This table shows the cost of consuming 3g of leucine from various foods. Note that animal products are some of the easiest and cheapest ways to meet your requirements.

In summary:

Protein source does matter and careful planning is needed in order to get optimal results. If you are following a plant-based or vegetarian diet, aim for a variety of protein sources at each meal and ensure that you are consuming ~10% more protein than