Protein Quality
What It Actually Means — and Why It Matters
Most people ask how much protein they eat. Very few ask how much of it their body can actually use. Those are two very different questions, and the answer to the second one changes considerably depending on where your protein comes from.
This page covers protein quality science for healthy, active adults. It is educational information, not personalised dietary advice. If you have a medical condition — including kidney disease, liver disease or diabetes — speak to your GP or a registered dietitian before making significant changes to your protein intake.
How Much Protein Are You Actually Using?
There is a significant difference between how much protein a food contains and how much of that protein your body can actually use to build and repair muscle. A food might list 42 grams of protein per 100 grams on the label. But if only 61 per cent of that protein is digestible and usable by the body, the effective yield is closer to 26 grams. That gap matters — and it varies dramatically between protein sources.
Protein quality is determined by two things working together: the amino acid profile of the protein (whether it contains all the essential amino acids in adequate proportions), and how digestible and bioavailable those amino acids are once the food is consumed. A protein source can be rich in nitrogen — which is what most protein measurements detect — but poor in usable amino acids. The number on the label tells you quantity. Protein quality tells you what your body can actually do with it.
The original archive — written in the mid-1990s from library research — included both a protein rating table and a net protein utilisation table covering common food sources. The data was accurate then and remains consistent with current research. What has changed is not the ranking of foods but the sophistication of the measurement methods used to assess them.
The Protein Rating Table
Archive data: The tables below were compiled from library research in the mid-1990s before the internet existed. The food rankings remain consistent with modern research — but the measurement methods have evolved considerably since then. See the section below the tables for how protein quality science has advanced.
Whole egg is set at 100 — the reference point against which all other protein sources are measured. Every other food is scored relative to egg in terms of the completeness and quality of its protein. The higher the score, the more your body can use per gram of protein consumed.
Source: original archive data, consistent with established biological value and protein rating literature. Animal-source proteins dominate the upper range due to their complete essential amino acid profiles and high digestibility. Plant-source proteins tend to be limited by one or more essential amino acids and lower digestibility — though combining sources addresses this.
Net Protein Utilisation
This table reveals something the protein rating alone does not: the relationship between the percentage of protein a food contains by weight and the percentage of that protein the body can actually retain and use. The gap between those two numbers is where the real story lies.
| Food | Protein by Weight | Net Protein Utilisation | NPU at a Glance |
|---|---|---|---|
| Eggs | 12% | 94% | |
| Milk | 4% | 82% | |
| Fish | 18–25% | 80% | |
| Cheese | 22–36% | 70% | |
| Brown rice | 8% | 70% | |
| Meat and poultry | 19–31% | 68% | |
| Soybean flour | 42% | 61% |
Egg has 12% protein by weight. Soybean flour has 42% protein by weight. Yet egg is consistently the superior protein source — because 94% of egg protein is retained by the body, versus 61% of soybean's.
The practical yield per 100g: egg provides approximately 11g of usable protein. Soybean flour provides approximately 26g — more in absolute terms, but per gram of protein consumed, egg is dramatically more efficient. There is a big difference between how much protein a food contains and how much of that protein you can actually use to build muscle. The archive stated this in the mid-1990s. The science confirms it.
How Protein Quality Is Assessed
The methods for measuring protein quality have evolved considerably over the past century — from crude nitrogen balance experiments to sophisticated amino acid digestibility scoring. Here is what each method measures and what its limitations are.
Biological Value (BV)
Plain English: of the protein your gut actually absorbs, how much does your body keep and use? Egg scores highest. The limitation is that it starts from what is absorbed, not what you ate — so a poorly digested protein can score deceptively well.
Net Protein Utilisation (NPU)
Plain English: of all the protein you eat, how much does your body actually keep? This is a more honest number than BV because it starts from what went in your mouth, not what made it through your gut wall. The tables in the archive used NPU — which was, at the time, the most rigorous commonly available measure.
Protein Digestibility-Corrected Amino Acid Score (PDCAAS)
Plain English: does this protein contain all the essential amino acids in the right proportions, adjusted for how well we digest it? The cap at 100% was a significant limitation — it treated all top-tier proteins as identical, which is not quite accurate. Replaced by DIAAS as the preferred regulatory standard.
Digestible Indispensable Amino Acid Score (DIAAS)
Plain English: the most accurate measure yet. Rather than treating all protein in a food equally, it measures how well your small intestine absorbs each individual essential amino acid separately. Egg and whey typically score above 100%. This means they don't just meet requirements — they exceed them, providing a surplus that helps cover shortfalls in amino acids from other foods in the same meal. The rankings have not fundamentally changed from earlier measures — eggs and animal proteins remain at the top — but the measurement is now considerably more precise.
The archive used NPU — the most rigorous measure available to a researcher working from library sources in the mid-1990s. The data in the tables is consistent with modern DIAAS rankings. The names and methods have evolved; the relative hierarchy of protein sources has not changed significantly. Animal proteins remain the most complete and most bioavailable. Plant proteins remain useful but require either higher quantities or complementary combinations to achieve equivalent amino acid delivery.
Complete and Incomplete Proteins
Protein quality is inseparable from amino acid completeness. A protein source that lacks one or more essential amino acids — or provides them in insufficient amounts — limits muscle protein synthesis regardless of how much total protein it contains.
Contain all nine essential amino acids in proportions adequate for human requirements. Generally animal-sourced.
- — Eggs
- — Chicken and poultry
- — Fish and seafood
- — Beef and red meat
- — Dairy — milk, yogurt, cheese
- — Whey and casein protein
Lack one or more essential amino acids in sufficient amounts. Generally plant-sourced. Combining two or more incomplete proteins can produce a complete amino acid profile.
- — Wheat and grains (low in lysine)
- — Legumes (low in methionine)
- — Rice (low in lysine)
- — Peanuts (low in methionine)
- — Most vegetables
Note: soy protein and quinoa are plant-source exceptions — both provide complete amino acid profiles, though digestibility is lower than most animal proteins.
Practical Implications
Prioritise high-quality sources
Eggs, chicken, fish, lean beef and dairy provide the most usable protein per gram consumed. When total daily protein targets are the primary concern, these sources get you there most efficiently — less food, more usable amino acids.
Adjust targets for lower-quality sources
If plant protein makes up a significant portion of your intake, your total protein target needs to be higher to account for lower utilisation rates. A 68% NPU source requires you to consume roughly 1.5 times more protein to achieve the same usable amino acid delivery as egg.
Combine plant proteins strategically
Rice and beans together provide a complete amino acid profile — rice is low in lysine but adequate in methionine; legumes are the reverse. Combining them across a meal or a day produces a nutritional result closer to a complete animal protein than either provides alone.
Dairy earns its place
Milk, yogurt and cheese score well on both quality and digestibility. Yogurt in particular — high in both fast-digesting whey and slow-digesting casein — is a practical, versatile protein source for athletes at any time of day, including post-training.
A note on protein supplements. Whey protein isolate scores among the highest of any measured source on both PDCAAS and DIAAS — consistently at or above 1.0. It is a complete protein with high digestibility and an excellent leucine content. Its use is justified for athletes who struggle to meet protein targets through whole food alone. It is not magic — it is simply a convenient, high-quality protein source. Whole food first, supplements where genuinely needed.
Related Topics
References
FAO/WHO/UNU. Protein Quality Evaluation. Report of a Joint Expert Consultation. FAO Food and Nutrition Paper 51, Rome, 1991.
The foundational reference for NPU and BV methodology as applied to human nutrition. The protein utilisation values in the archive tables are consistent with the data presented in this consultation.
Schaafsma G. The protein digestibility-corrected amino acid score. Journal of Nutrition, 2000; 130(7): 1865S–1867S.
Provides a clear account of PDCAAS methodology and its limitations — including the capping at 1.0 and the use of faecal rather than ileal digestibility measurements.
FAO. Dietary Protein Quality Evaluation in Human Nutrition. Report of an FAO Expert Consultation. FAO Food and Nutrition Paper 92, Rome, 2013.
The report that formally established DIAAS as the preferred method for assessing protein quality in human nutrition, replacing PDCAAS. Provides the amino acid reference patterns and the methodology for measuring true ileal digestibility of individual amino acids.
Wolfe RR, Church DD, Ferrando AA, Moughan PJ. Consideration of the role of protein quality in determining dietary protein recommendations. Frontiers in Nutrition, 2024.
A 2024 review confirming that protein quality — not simply quantity — must be considered when establishing dietary protein recommendations. Supports the use of high-quality, complete protein sources as the primary means of meeting protein targets in active individuals.
Van Vliet S, Burd NA, van Loon LJC. The skeletal muscle anabolic response to plant- versus animal-based protein consumption. Journal of Nutrition, 2015; 145(9): 1981–1991.
Demonstrates that plant-based proteins elicit a lower muscle protein synthetic response compared to animal-based proteins when consumed in equal amounts, due to differences in amino acid profile and digestibility. Higher intakes of plant protein are required to achieve equivalent anabolic effects.
