Protein plays a central role in exercise recovery, muscle repair, immune function, and countless metabolic processes. For individuals who train regularly, increasing total daily protein intake is often one of the first evidence-based steps to support adaptation and performance.

Beyond quantity, however, many discussions focus on “protein quality.” Certain protein sources are frequently described as superior based on established scoring systems. But how meaningful are these scores in real-world settings – particularly in countries where protein intake is already adequate?

To answer that, it’s important to understand what “protein quality” actually measures – and what it doesn’t.

What do protein quality scores actually measure?

Proteins are made up of amino acids. Of the 20 amino acids found in the diet, nine are considered essential because the body cannot synthesise them in sufficient amounts and must obtain them from food.

A man puts a scoop of protein powder into a shaker
Protein is a macronutrient that is important for muscle growth. Photo - Canva

In the United States, protein quality has traditionally been assessed using the Protein Digestibility-Corrected Amino Acid Score (PDCAAS). This method:

  1. Compares the essential amino acid profile of a food to a reference pattern (based on requirements of young children, considered the most nutritionally demanding group (source)).
  2. Adjusts that score based on digestibility, typically determined using animal models (source).

Newer systems such as DIAAS attempt refinement, but similar limitations remain.

Importantly, these scoring systems were designed to prevent protein deficiency in vulnerable populations – not to predict muscle growth or long-term health outcomes (source, source).

How well do protein rankings predict athletic performance?

Protein scoring systems are often used as shorthand for “muscle-building potential.” However, current research suggests that this interpretation is overly simplistic.

A person does exercise in a gym
Equating protein scores to muscle growth is overly simplistic. Photo - Canva

Muscle adaptation appears to be driven primarily by:

  • Proper training stimulus
  • Total daily protein intake
  • Adequate energy intake

While proteins with higher quality scores can sometimes stimulate slightly greater short-term increases in muscle protein synthesis, these acute responses do not reliably translate into greater long-term gains in lean mass (source, source).

Several controlled trials show that plant-based proteins support increases in muscle size and strength comparable to animal-based proteins – provided total protein intake is sufficient (source, source, source, source). Meta-analyses and longer-term training studies reinforce this conclusion (source, source, source).

In practical terms, hitting overall protein targets matters much more than maximising a quality score.

Why fibre, phytonutrients, and carbs don't show up in protein rankings

A key limitation of current scoring systems is their narrow focus on essential amino acid content and digestibility. They do not account for the broader nutritional context of the food delivering that protein.

Various plant-based proteins
Plant-based proteins provide a range of nutritious benefits. Photo - Canva

For example, many plant protein sources provide:

  • Dietary fibre
  • Phytochemicals and antioxidants
  • Carbohydrates that support glycogen replenishment
  • Micronutrients such as potassium and magnesium

These components can meaningfully influence strength performance and recovery, as well as cardiovascular health and other long-term metabolic outcomes (source, source, source).

Conversely, some protein sources that score highly for amino acid composition may also be higher in saturated fat or lower in beneficial phytonutrients.

When evaluating foods in the context of overall health and performance, the total nutritional package matters.

A bowl of cooked tofu
Tofu is a popular form of plant-based protein. Photo - Canva

A high protein score doesn't mean a healthier food

Protein quality scoring may be highly useful in contexts of food insecurity or malnutrition, where essential amino acid sufficiency is a primary concern.

However, in economically developed countries – where average protein intake exceeds recommended requirements and most have access to a wide range of foods – the relevance of these scoring systems is more debatable (source, source).

A person chops meat in a meat market
Many people in economically-developed countries over-consume protein. Photo - Canva

Some experts have described current definitions of protein quality as misleading and outdated, arguing that they disproportionately elevate animal-based proteins while ignoring broader health outcomes (source).

Large prospective cohort studies suggest that higher intake of certain animal proteins – particularly red and processed meat – is associated with increased risk of chronic disease (source). In contrast, replacing animal protein with plant protein is consistently associated with improved cardiometabolic outcomes and, in some analyses, reduced all-cause mortality (source, source, source, source).

This divergence between amino acid scoring and long-term health data highlights the limitations of defining “quality” solely through essential amino acid ratios.

A more useful way to think about protein quality

The idea that protein quality determines muscle-building potential has become widespread, but leading researchers increasingly emphasise that:

  • Adequate total protein intake is the primary driver of adaptation
  • Both plant and animal proteins can support strength and hypertrophy when properly structured
  • Health outcomes depend on overall dietary patterns, not isolated amino acid scores
Choose your protein sources based on a range of factors, not just protein scores. Photo - Canva

If the goal is to define a high-quality protein source, a more useful question may be:

How does this food influence real-world outcomes – muscle mass, metabolic health, and disease risk – over time?

A narrow focus on amino acid ratios may obscure the bigger picture.

What actually makes a protein source worth eating?

Protein quality scoring systems were developed to address protein deficiency, not to rank foods for athletic performance or long-term disease prevention in well-nourished populations.

While amino acid composition and digestibility are relevant, they represent only a small part of the story.

In practice, a high-quality protein source is one that:

  • Contributes meaningfully toward daily protein needs
  • Supports muscle adaptation when paired with training
  • Enhances – rather than undermines – long-term health
  • Fits within a balanced, sustainable dietary pattern

When viewed through that broader lens, total diet quality matters far more than a single protein score.