Key Takeaways
- Protein quality depends on amino acid completeness and digestibility, not just gram count.
- Nine essential amino acids must come from food because the body cannot synthesize them.
- Animal-based proteins are generally complete; most plant proteins are not, but can be combined.
- DIAAS is the current gold-standard scoring method for evaluating dietary protein quality.
- Varied diets — including well-planned plant-based ones — can meet all essential amino acid needs.
- Consult a registered dietitian for personalized protein guidance, especially with health conditions.
Protein Quality
Protein quality refers to how well a food's protein can be used by the human body. It depends on two factors: which amino acids the protein contains, and how efficiently the body can digest and absorb them. A food can be high in protein grams yet still be considered lower quality if it lacks certain essential amino acids or is poorly digested.
Protein quality is formally measured using the Digestible Indispensable Amino Acid Score (DIAAS), which accounts for both amino acid composition and true ileal digestibility — currently the standard recommended by the Food and Agriculture Organization of the United Nations.
Why Grams on a Label Don't Tell the Whole Story
Most people track protein intake by counting grams — a reasonable starting point, but an incomplete one. Two foods can both show 20 grams of protein per serving and deliver meaningfully different nutritional value depending on which amino acids they contain and how well the body absorbs them. Understanding protein quality bridges the gap between what a label reports and what your body actually receives.
Proteins are made of amino acids, 20 of which are relevant to human nutrition. Of these, nine are classified as essential — meaning the body cannot synthesize them and must obtain them through food. These are histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. A protein source that provides all nine in adequate proportions is called a complete protein; one that lacks or provides insufficient amounts of one or more is considered incomplete.
For a broader grounding in how protein fits alongside other macronutrients and food groups, see our nutrition fundamentals primer.
Complete vs. Incomplete Proteins: A Practical Breakdown
Animal-based proteins — including eggs, dairy, meat, poultry, and fish — are generally complete. They supply all nine essential amino acids in ratios that align well with human physiological needs. Plant-based proteins, with a few notable exceptions, tend to be limited in one or more essential amino acids:
- Legumes (lentils, chickpeas, black beans) are typically low in methionine but rich in lysine.
- Grains and seeds (rice, wheat, oats) tend to be low in lysine but provide adequate methionine.
- Soy and quinoa are exceptions — both are considered complete plant proteins with DIAAS scores comparable to many animal sources.
This complementary amino acid relationship between legumes and grains is the nutritional rationale behind traditional food pairings — rice and beans, lentil soup with bread — found across many food cultures. The body maintains a short-term pool of amino acids, so these complementary foods do not need to be eaten at the same meal; meeting needs across a full day is what matters.
How Protein Quality Is Measured: PDCAAS and DIAAS
Two scoring systems are widely referenced in nutrition science. The older Protein Digestibility-Corrected Amino Acid Score (PDCAAS) compares a food's amino acid profile to a reference pattern and adjusts for overall digestibility. It was the standard for decades, but it caps scores at 1.0 — meaning a food cannot score above its reference even if it exceeds requirements — and uses fecal digestibility measurements, which can overestimate absorption.
The Digestible Indispensable Amino Acid Score (DIAAS), introduced by the FAO in 2013, addresses these limitations. It uses ileal digestibility (measured at the end of the small intestine, before bacterial fermentation occurs) and does not cap scores. A DIAAS above 100 means the food provides more than sufficient amounts of all essential amino acids; below 100 indicates a limiting amino acid.
9
Essential amino acids humans must get from food
Defined by the Food and Agriculture Organization; the body cannot synthesize these in sufficient quantities regardless of overall protein intake.
~1.0
DIAAS score for whole eggs
Whole eggs consistently score at or near 1.0 on DIAAS across multiple studies, making them a reference-point protein source in nutrition research.
0.8 g/kg
Recommended Dietary Allowance for protein in healthy adults
Established by the Institute of Medicine; individual needs may be higher for athletes, older adults, or those with certain health conditions.
These scores help researchers and clinicians compare protein sources with more precision than gram counts alone allow.
Digestibility: The Absorption Factor
Even a protein with an ideal amino acid profile must be physically broken down and absorbed in the small intestine to be useful. Several factors influence digestibility:
- Food matrix: Plant cell walls and fiber can limit enzyme access to protein, reducing digestibility compared to animal sources.
- Anti-nutritional factors: Compounds like phytates and tannins in legumes and grains can bind to amino acids and reduce their absorption. Cooking, soaking, and fermentation substantially reduce these compounds.
- Processing: Moderate heat generally improves plant protein digestibility; highly processed proteins can behave differently depending on the method used.
Simple Ways to Improve Plant Protein Digestibility
Soaking dried legumes for several hours before cooking, then discarding the soaking water, reduces phytates and oligosaccharides that inhibit protein absorption. Fermentation — as seen in tempeh and miso — further breaks down anti-nutritional compounds, which is why fermented soy products tend to have better digestibility profiles than unfermented whole soybeans.
Digestibility differences explain why whole food plant protein sources often score lower on DIAAS than their isolated counterparts — isolated soy protein, for instance, scores higher than whole cooked soybeans because the fiber and matrix are removed.
Putting Protein Quality Into Everyday Practice
Understanding protein quality informs practical eating decisions without requiring calorie counting or advanced nutrition knowledge. A few general principles from the research literature:
- Diversify protein sources. Rotating between animal and plant sources, or varying plant sources daily, naturally balances amino acid intake across the week.
- Don't dismiss plant proteins. Lentils, tofu, edamame, and quinoa are nutrient-dense and, when eaten in variety, can fully satisfy essential amino acid needs for most healthy adults.
- Consider total dietary pattern. Individual foods are rarely eaten in isolation. What matters most is the cumulative amino acid and digestibility profile of what you eat across a day.
People with higher protein needs — including those engaged in resistance training, older adults experiencing age-related muscle loss (sarcopenia), or those recovering from illness — may benefit from prioritizing higher-quality protein sources. See our fitness and movement hub for context on how exercise interacts with protein needs.
Nutrition labels can be a useful starting point. Our guide on reading a nutrition label without getting lost in the fine print explains how to interpret what you see on packaging.
This article is for general informational and educational purposes only and does not constitute medical or dietary advice. Individual protein needs vary based on health status, age, and other factors. Consult a registered dietitian or qualified healthcare professional before making significant changes to your diet.