What makes a good protein?

Protein is arguably one of the most discussed macronutrients in health, fitness and nutrition, and increasingly among everyday consumers.

Its rising prominence has been driven, not only by the long standing fitness trends and industry growth, but also through the increasing use of GLP-1 receptor agonists for weight management (e.g. Wegovy and Ozempic).

Whilst effective at promoting fat loss, research has shown that these medications can also contribute to a reduction in muscle mass, promoting recommendations for higher protein intake to help preserve muscle. Similarly, ageing populations are becoming more aware of the increased risk of sarcopenia – the age-related loss of muscle mass which affects an estimated 10-16% of older adults worldwide.

Growing interest in longevity has placed further focus on the role of protein in long term health. Those adopting plant-based diets are also paying closer attention to the quantity and quality of protein in their diets.

At the same time, social media and the wider wellness culture have amplified proteins’ perceived role in satiety, body composition and metabolic health.

Good vs bad protein

While the headlines often allocate protein into either ‘good’ or ‘bad’, the reality of this macronutrient is more nuanced. Understanding protein quality, how it’s scored and what to look out for can help the general consumer make more educated choices.

Protein, in its simplest form, is broken down into amino acids, building blocks our body needs for essential everyday function. Of the 20 standard amino acids, 11 are non-essential, whilst nine are essential, because the body is unable to produce them on its own.

It is these nine essential amino acids that we often refer to when discussing protein quality or whether it is considered a ‘good’ protein.

Several factors determine protein quality; these are as follows:

Animal vs plant protein: Animal based protein, such as eggs, meat, and dairy, often contains all nine essential amino acids. Most plant proteins, however, are naturally low or missing one or more of these. By combining complementary sources, for example, pea protein (low in methionine and cysteine but high in lysine) with faba bean protein, (high in methionine and cysteine but low in lysine) it is possible to create a protein profile comparable to that of an animal based source.

Digestibility: Even if a protein source contains all nine essential amino acids, the body must be able to absorb them efficiently. Proteins vary in this respect, which can affect their nutritional value to the body.

Functionality: Beyond the basic nutrition, some proteins can offer further functional benefits, for example, whey protein is highly digestible and higher in Branch Chain Amino Acids (BCAAs), helping to support muscle growth, recovery and immune health, making it popular with athletes and sports individuals.

How protein quality is scored

Protein quality is determined by two main factors: amino acid composition and digestibility. A protein that is rich in essential amino acids but poorly digested may be less beneficial and score less than a slightly lower quality but higher absorbed one.

Other factors that can affect quality include:

Processing and preparation: Cooking, fermentation, and other forms of processing can alter both digestibility and protein availability. For example, legumes naturally contain compounds known as anti nutrient which can reduce nutrient availability (e.g. phytates). The soaking and cooking process can help break down these compounds ensuring more protein is available to the body.

Protein interaction in food: The food matrix, which is the complex physical and chemical structure of a food, including how nutrients and other components are organised and interact, can significantly impact protein absorption. When consumed alongside other macronutrients such as fats or fibre, protein digestibility and utilisation can be affected.

How useful a protein is for the human body can be quantified using various scoring systems, of which there are two commonly used: Protein Digestibility Corrected Amino Acid Score (PDCAAS) and Digestibility Indispensable Amino Acid Score (DIAAS).

What is PDCAAS?

Introduced in the early 1990s and adopted by the Food and Agricultural Organization (FAO) and World Health Organization (WHO) as the standard measure for protein quality, PDCAAS combines amino acid composition with digestibility to give a score between zero and one.

The score of 1.0 represents the highest protein quality and score, indicating the protein provides all essential amino acids in sufficient amounts and is well digested in the body.

Most animal-based proteins, such as whey, milk, and eggs, will score 1.0. Many plant-based proteins will score lower, but combining complementary sources they can achieve a score close to, or even equivalent to, 1.0.

PDCAAS is a useful determinant of protein quality, however, it does have its limitations. Scores are truncated at 1.0 and, therefore, it cannot differentiate between very high quality proteins. Additionally, it measures digestibility over the whole digestive tract, rather than specifically where amino acids are absorbed, which can overestimate the availability of amino acids that are actually absorbed.

What is DIAAS?

DIAAS, introduced by the FAO in 2013, addresses some of the limitations associated with PDCAAS. Instead of truncating at 1.0, it allows for scores above this, helping to differentiate higher quality proteins. It also focuses on amino acid digestion, specifically in the small intestine, where the absorption actually occurs, providing a more accurate measure of protein utilisation.

Overall, DIAAS is considered more accurate, particularly when it comes to evaluating plant and novel protein sources. PDCAAS, however, remains more widely used for food labelling and nutrition guidelines.

Key factors to watch out for

When evaluating protein sources, there are some key practical considerations:

Complete vs incomplete proteins

Animal proteins are generally considered complete, containing all essential amino acids, whilst plant sources are not; however, combining complementary plant sources, such as grains and legumes, can ensure all essential amino acids are obtained.

Digestibility

Select proteins that are easily digested and absorbed. Plant proteins, especially when raw or minimally processed, can contain anti nutrients such as phytates that interfere with protein digestion and reduce amino acid availability.

Amino acid goal

Certain proteins may be more beneficial depending on your goals. For example, leucine, a branched-chain amino acid (BCAA), is a primary trigger for muscle synthesis.

Processing and additives

Some protein powders may be fortified with stabilisers or other compounds that could negatively affect overall nutrient absorption.

Protein scoring labels

Understanding PDCAAS and DIAAS can help identify higher quality protein. This is particularly important when comparing plant-based proteins, sources that can vary widely in amino acid composition.

The bottom line

Overall, a ‘good’ protein is not just about the quantity you consume; it also must take into consideration what ratio of amino acids it contains and how well your body can absorb it. When evaluating protein quality, it is important to look at the amino acid profile, digestibility and factors such as the preparation and source.

About the author

Matthew Balkin is the founder of Balkin Nutrition, a consultancy supporting food, beverage, and supplement brands with product formulation, regulatory compliance, and evidence-based nutrition strategy.

He works with companies across the FMCG sector to develop functional products from concept through to launch, helping bridge the gap between scientific research and commercially successful innovation.

His work focuses particularly on functional nutrition, emerging ingredients, and creating products that deliver both efficacy and consumer appeal.