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What is protein quality?

Dietary protein is essential to support muscle maintenance or growth. In previous blogs (1 and 2) we addressed the significance of the amount of protein in each meal, and the pattern of protein consumption throughout the day. We also discussed the importance of protein quality. But what exactly does protein quality mean? And what makes a protein source high quality compared to low?


Factors determining protein quality infographic

In a series of blogs, we will delve into the topic of protein quality. Starting with an overview of what protein quality is, followed by a blog on how protein quality can be assessed, and finally an outline of how athletes or everyday exercisers can use this information.


What is protein quality?

Protein quality considers factors that influence the magnitude and speed at which we can make new proteins (protein synthesis rate) following protein consumption (see a previous blog for a detailed insight into muscle protein synthesis). These factors include the essential amino acid (EAA) content, leucine content, and digestion/absorption kinetics of a given protein source. As the occurrence of training adaptations, such as muscle hypertrophy, are ultimately underpinned by a net positive protein balance, protein quality is considered an important aspect of protein nutrition to enhance such adaptations.


What distinguishes high- from low-quality proteins?

There are 9 EAAs that must be obtained from the diet in sufficient amounts to stimulate protein synthesis. They are essential because the body cannot produce them. A noteworthy EAA is leucine, as this acts as both a building block for the formation of new skeletal muscle proteins, but also as a signal for protein synthesis to be ‘switched on’. Following ingestion, proteins must be digested into constituent amino acids, which are then absorbed and subsequently delivered to peripheral tissues (e.g. skeletal muscle) for protein synthesis.


In this way, high quality proteins are those that:

1). Provide all 9 EAAs in sufficient amounts

2). Are abundant in leucine, and

3). Are easily digested and absorbed into circulation.


As mentioned in a previous blog, it is generally thought that animal-based proteins are superior to plant-based sources in terms of protein quality. However, there are exceptions and this should be considered as a general principle rather than a rule.


Is high quality good and low quality bad?


Gram for gram? Yes... Well, kind of.

Protein quality is considered most important when the total dose of protein consumed is low to moderate. For example, say we consume 10 g of low-quality protein providing a low EAA content and leucine content. The amount of AA provided here is unlikely to maximally stimulate the protein synthetic response, even if the digestion and absorption was great. So, combined with poor digestion and absorption kinetics, we are unlikely to see a pronounced increase in protein synthesis with this lower quality source. In contrast, consuming 10 g of high quality protein, with a more favourable EAA profile and leucine content, as well as better digestion/absorption kinetics, we would likely induce a more robust increase in protein synthesis. However, simply consuming more of a low quality protein source may compensate for the aforementioned inadequacies.


Fast or slow? It depends.

The differential digestion speed of protein sources has promoted the labelling of ‘fast’ and ‘slow’ proteins. Exactly how protein digestion can be assessed will be discussed in another blog. However, fast proteins (such as whey protein) are thought to be most effective in promoting a rapid increase in protein synthesis. This may be an ideal strategy after a training session. On the other hand, slow proteins (such as casein protein) may be beneficial in scenarios where frequent protein feeding is not possible, such as overnight, with a busy travelling schedule or intentional fasting (e.g. during Ramadan).


In the context of other aspects of protein nutrition? It also depends.

Another consideration here is the role of total protein intake and/or the pattern of protein consumption. For example, in the presence of a moderate-high daily total protein intake (1.6-2.2g protein/kg/day), or frequent protein feedings (≥20 g every ~3 hours) the importance of protein quality for protein synthesis is likely diminished. On the other hand, a low total protein intake and skewed pattern of consumption could emphasise the role of protein quality. Therefore, protein quality should be considered in the context of all aspects of protein nutrition.


In whole-food sources? The jury is still out.

There are also other non-protein related aspects of a food or drink that may influence the effect of a given protein source on the acute protein synthetic response. For example, the food matrix (i.e. constituents that make up a food, including water, fibre, macronutrients, macronutrients etc. and how these interact) may either enhance or impair the muscle’s response to protein consumption. Although much of the research into protein quality uses isolated protein sources (usually powdered supplements), most of us obtain a large proportion of protein from whole-food sources. We are also unlikely to always consume protein-rich foods alone, as they usually fit within a meal. Therefore, the protein quality of whole-foods is an emerging area of research, and we will no doubt have more answers regarding the importance in the coming years.


Summary

In summary, protein quality considers the EAA content, leucine content, and digestion/absorption of a protein source. These factors influence the muscle’s immediate response to protein consumption. High quality proteins appear important to enhance the protein synthetic response to a given protein source. However, whether this translates to measurable differences in muscle-related adaptations (e.g. hypertrophy) is unclear, and is discussed in a separate blog.



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