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Protein quality across food sources: understanding DIAAS and PDCAAS

This figure compares the quality of dietary proteins based on their essential amino acid (EAA) density and true ileal digestibility—core concepts underlying two major protein quality scoring systems: PDCAAS and DIAAS. These methods quantify how effectively a protein provides absorbable essential amino acids for human needs.

1️⃣ PDCAAS — older method based on f***l digestibility
The Protein Digestibility-Corrected Amino Acid Score (PDCAAS) estimates protein quality using total nitrogen digestibility measured at the end of the digestive tract. It compares the amino acid profile of a food to human amino acid requirements, correcting for digestibility.
🟢 Example: PDCAAS assigns milk, eggs, and soy a perfect score of 1.0, reflecting adequate amino acid content and high overall absorption.
🔹 Limitation: Because PDCAAS measures digestibility after colonic fermentation, it can overestimate true absorption—particularly for plant proteins that lose amino acids in the large intestine. It also “truncates” scores at 1.0, masking differences between excellent and exceptional proteins.

2️⃣ DIAAS — newer, more precise method using ileal digestibility
The Digestible Indispensable Amino Acid Score (DIAAS) improves upon PDCAAS by measuring amino acid absorption at the end of the small intestine (the ileum), where nutrients are actually taken up. It accounts for the digestibility of individual essential amino acids rather than total nitrogen.
🟢 Example: Using DIAAS, milk proteins (~1.25), eggs (~1.18), and pork (~1.10) score higher than soy (~0.90) or legumes (~0.70), revealing meaningful differences in bioavailability.
🔹 Advantage: DIAAS better reflects physiological absorption and distinguishes proteins based on limiting amino acids, giving a more accurate ranking for both animal and plant foods.

3️⃣ How these scores align with the figure
The x-axis represents true ileal digestibility, corresponding to DIAAS methodology, while the y-axis reflects EAA density, a measure of amino acid content per calorie. Foods in the upper right—fish, eggs, yogurt, and lean meat—excel in both quality and digestibility.
🟢 Example: White fish and eggs achieve near-complete amino acid absorption and high EAA density, giving them top DIAAS values.
🟢 Example: Legumes and grains, while rich in fiber and micronutrients, show lower digestibility and lysine content, producing lower DIAAS despite moderate PDCAAS.

4️⃣ Plant proteins and complementary strategies
Plant-based proteins are limited by one or more essential amino acids (often lysine or methionine) and lower digestibility due to fiber and antinutrients.
🟢 Example: Combining legumes (high lysine) with grains (high methionine) can achieve a balanced amino acid pattern that approximates animal protein quality on a DIAAS basis.
🟢 Example: Processing methods such as soaking, sprouting, or fermentation improve plant protein digestibility and raise DIAAS scores.

Summary:

PDCAAS provides a general measure of protein adequacy but can overestimate absorption.

DIAAS offers a more physiologically accurate index by using ileal amino acid digestibility and identifying limiting amino acids.

Animal proteins typically score higher on both metrics, while plant proteins benefit from combination strategies and processing improvements to close the gap in true bioavailability

Matthews JJ, Arentson-Lantz EJ, Moughan PJ, Wolfe RR, Ferrando AA, Church DD. Understanding Dietary Protein Quality: Digestible Indispensable Amino Acid Scores and Beyond.