Understanding Protein Quality in Plant‑Based Foods Through DIAAS

What DIAAS Measures

The Digestible Indispensable Amino Acid Score (DIAAS) is the gold‑standard method recommended by the Food and Agriculture Organization (FAO) since 2013 for assessing the quality of dietary protein in foods intended for human consumption. It evaluates how effectively a protein source supplies indispensable (essential) amino acids after accounting for how well those amino acids are digested and absorbed in the small intestine. This makes DIAAS a more accurate reflection of true, bioavailable protein quality.

Why DIAAS Matters

• Provides a more precise measure of bioavailable amino acids, including the impact of plant anti‑nutritional factors such as phytates that reduce absorption.
• Distinguishes high‑quality proteins more clearly (e.g., whey ≈ 109 vs. soy isolate ≈ 90).
• Avoids score truncation, allowing superior proteins—often animal-based or purified plant isolates—to be fully recognized.
• Helps optimize mixed diets by identifying limiting amino acids (e.g., lysine in grains, sulfur amino acids in legumes).

DIAAS is especially relevant for groups with higher or more specific amino‑acid needs — including children (who require more indispensable amino acids for growth), older adults (who experience reduced digestive efficiency and anabolic resistance), and athletes (who need high‑quality protein to maximize muscle protein synthesis). For these populations, the quality of protein can be just as important as the total amount consumed.

Complete Proteins in Plant‑Based Diets

A complete protein contains all nine essential amino acids—histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine—in amounts sufficient to meet human nutritional needs. Most plant proteins fall short in one or more of these amino acids (for example, grains are typically low in lysine, while legumes are often low in sulfur‑containing amino acids, abbreviated Met + Cys or SAA), which is why complementary food pairing is so important. It’s also worth noting that complete does not necessarily mean high‑quality by DIAAS standards; for instance, quinoa is technically complete but still scores modestly because digestibility often matters more than amino‑acid completeness alone. Among common whole foods, potatoes stand out as a rare exception that are both complete and relatively well‑digested.

Within vegan and plant‑based nutrition, soy and potato (or potato protein) emerge as the most reliable standalone complete protein options from plants. Other foods such as quinoa, buckwheat, hemp, and chia come close, but they don’t consistently reach the highest DIAAS benchmarks without strategic pairing. When the focus is strictly on whole foods rather than isolates, potato becomes the unexpected—but scientifically supported— “complete” standout among everyday plant sources.

Key Findings from the Analysis

This analysis asked AI to rank leading plant‑based protein sources using the DIAAS framework. The findings are summarized in the chart above and the table below.

Plant‑based eaters can improve overall protein quality by combining complementary foods — for example, pairing grains with legumes to raise the effective DIAAS of a meal. The highest‑scoring plant proteins are typically processed isolates such as soy, pea, and potato, which achieve higher scores because processing removes fiber and anti‑nutrients that otherwise reduce amino‑acid digestibility. Whole grains and legumes score lower by comparison, yet they still offer valuable nutrition when eaten in variety.

DIAAS remains the most reliable routine method for assessing protein quality, though ongoing research — including more human ileal data and refined in‑vitro models — continues to strengthen the field. In everyday practice, aim for a diverse mix of protein‑rich foods; quality matters, but meeting total protein needs remains the foundation.

Plant Source	DIAAS Range (approx.)	Typical Form	Notes / Limiting AA
Potato	100–118	Concentrate or cooked whole	Excellent; balanced profile, often the top whole-plant source
Soy	90–100	Isolate/concentrate	Good/excellent; Met+Cys sometimes limiting in whole forms
Pea	82–91	Concentrate/isolate	Good; Met+Cys limiting
Quinoa	68–85	Cooked whole	Pseudocereal; good lysine, borderline good quality
Canola/Rapeseed	70–90+	Concentrate	Good in processed forms; balanced
Lentils	68–75	Cooked	Borderline good; often SAA limiting
Chickpeas	67–83	Cooked	Variable; Met+Cys often limiting
Buckwheat	47–68	Cooked groats	Moderate digestibility; complete-like profile
Oats	43–77	Dehulled/cooked	Variable, up to good; better lysine among cereals
Chia seeds	40–60	Whole/raw or soaked	Lower; digestibility limits, good lysine balance
Hemp	40–60	Seeds/protein	Lower; lysine often limiting
Rye	47–56	Grain/dehulled or flour	Lower; lysine-limited
Barley	47–59	Dehulled	Lower; similar to rye, lysine-limited
Rice	37–64	Cooked	Lower; low lysine
Corn/Maize	36–54	Various	Lower; low lysine/tryptophan
Wheat	40–48	Whole grain / flour	Lower; lysine severely limiting; pig-model consensus ~43–48
Sorghum	29–54	Various	Lower; poor digestibility, lysine-limited
Millets (general, e.g., proso/foxtail/finger/pearl)	7–55	Cooked/dehulled	Mostly lower; lysine severe in most varieties (e.g., proso/foxtail ~7–10)

Summary

Many people in developing regions appear to meet the 50‑gram protein RDA, but once digestibility and amino acid quality are taken into account, their true usable intake often falls below physiological needs. In wealthier countries, where overall protein consumption is typically higher, this gap is less problematic. For athletes or anyone aiming to build muscle on a plant‑based diet, however, protein quality becomes especially important for supporting optimal muscle protein synthesis. 

How to Use This Information

DIAAS is a helpful tool for comparing the quality of different protein sources, but it should be used as a guide rather than a strict rule. Because DIAAS values can vary depending on processing methods, cooking techniques, and the specific study used, the numbers you see in charts or tables represent general ranges—not fixed, universal scores. Use these patterns to make informed choices: include a few higher‑quality plant proteins (such as soy or potato protein), pair complementary foods like grains and legumes, and aim for overall dietary variety.

A Note of Caution

While DIAAS is currently the most accurate routine method for assessing protein quality, it is not the only factor that matters. Total protein intake, overall diet diversity, micronutrient density, and personal health goals all play important roles. Think of DIAAS as one useful lens—valuable, but not the whole picture.

References

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