What does protein quality mean and how is it measured?
Protein quality measures how much of a source your body can actually use, combining its amino acid profile with how completely it is digested. Two scoring systems are in use: PDCAAS, which is older and caps at 1.0, and DIAAS, which measures digestion further down the gut and separates sources the older score treated as equal.
Key takeaways
- Quality combines two things: which amino acids a source carries, and how much of it survives digestion.
- PDCAAS is the older system and it truncates at 1.0, which hides real differences between good sources.
- DIAAS measures digestibility at the end of the small intestine instead of in stool, and does not truncate.
- Dairy and egg score highest. Most plant sources score lower, and the gap is larger under DIAAS than under PDCAAS.
- For a mixed diet at a sensible total, none of this changes the shopping. It matters when protein is tight or entirely plant-based.
A label says 20 g of protein. Your body may get the use of noticeably less than 20 g, and how much less depends on the source. Protein quality scoring is the attempt to put a number on that gap.
The two things being measured
Every scoring system combines the same two ingredients.
- The amino acid profile. Whichever essential amino acid a source has least of, relative to human requirements, sets the ceiling for the rest.
- Digestibility. How much of the protein is actually broken down and absorbed rather than passing through.
Multiply the limiting amino acid's adequacy by the digestibility and you have a score. The systems differ in how they do each half.
PDCAAS, and its ceiling
The Protein Digestibility Corrected Amino Acid Score has been the standard since 1991. It uses faecal digestibility and it is truncated at 1.0.
The truncation is the problem. Once a source clears the bar, it scores 1.0 and stops being distinguished from anything else that cleared it. Whey, egg, casein and soy protein isolate all sit at 1.0 under PDCAAS, which reads as equivalence and is not what the underlying data says.
The digestibility half has its own issue: measuring what arrives in stool includes the effect of gut bacteria further down, which consume and produce amino acids of their own. That flatters some sources.
DIAAS, and what it changed
The Digestible Indispensable Amino Acid Score measures ileal digestibility, at the end of the small intestine, before the large intestine's bacteria get involved. That is where absorption actually happens, so it is the right place to measure. It is also not truncated, so a source can score above 100.
| Under PDCAAS | Under DIAAS | |
|---|---|---|
| Milk and whey protein | 1.00, at the ceiling | Above 100, the highest group |
| Egg | 1.00, at the ceiling | High |
| Soy protein isolate | 1.00, at the ceiling | Clearly below the dairy proteins |
| Pea protein | High | Moderate, limited by methionine |
| Wheat and rice protein | Low | Lower again, limited by lysine |
The 2017 comparison of four animal and four plant proteins is the readable source for this, and its conclusion is in its title: DIAAS describes quality better than PDCAAS does.1
The practical effect of the change is that the animal and plant gap is wider than a generation of labels implied, and that some plant isolates that scored a perfect 1.0 do not hold that position when the ceiling is removed.
When the score changes what you eat
Rarely, and it is worth being blunt about that before anyone starts optimising groceries.
It does not matter if you eat a mixed diet and hit a reasonable total. Quality differences are a few tens of percent; the difference between a 15 g breakfast and a 30 g breakfast is a hundred. Amount dominates.
It starts to matter in three situations:
- Eating entirely plant-based, where the gap accumulates across every source in the day. The usual response is a higher total, which is what hitting a target without animal foods works through.
- Protein is genuinely tight, because appetite is suppressed or the day has little room in it. Then the usable fraction of a small intake is worth caring about.
- Choosing between powders, which is the one shopping decision where the number is printed and comparable. Whey, casein or plant is the page for that.
What the scores do not tell you
A quality score says nothing about anything else in the food. Collagen scores badly and may still be worth taking. Lentils score moderately and bring fibre that whey does not. Beef scores well and is not therefore a better dinner than fish.
The recommendations that this whole section is built on are stated as total protein, not as quality-adjusted protein, precisely because for ordinary diets the adjustment is smaller than the noise.2 Get the total right first, in the number your own body calls for, and treat the score as the tie-breaker it is.
More questions about Protein basics
What is the difference between DIAAS and PDCAAS?
Two things. PDCAAS measures digestibility from what turns up in stool, which is affected by gut bacteria and overstates how much you absorbed; DIAAS measures it at the end of the small intestine, where absorption actually happens. And PDCAAS is capped at 1.0, so whey and soy can both score 1.0 despite differing. DIAAS is not capped, which is why some dairy proteins score above 100.
Does a lower score mean plant protein does not work?
No. It means you need somewhat more of it to deliver the same usable amino acids. That is a quantity adjustment, not a disqualification, and it is one reason plant-based recommendations for protein tend to sit higher than the general figure rather than at it.
Should I choose foods by their DIAAS score?
Only if protein is genuinely tight. For most people the amount eaten swamps the score: 150 g of lentils at a lower score beats 40 g of chicken at a higher one. Quality scores are a tie-breaker between sources, not a ranking of meals.
References
- Values for digestible indispensable amino acid scores (DIAAS) for some dairy and plant proteins may better describe protein quality than values calculated using the concept for protein digestibility-corrected amino acid scores (PDCAAS)
- Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids
