Protein Quality Index: Not All Proteins Are Equal

When it comes to building muscle, recovering from workouts, or simply meeting daily nutrition goals, the protein quality index (PQI) tells you far more than total grams on a label. A 2024 meta‑analysis published in Nutrition Reviews (Smith et al., 2024) found that athletes who chose proteins with a PQI ≥ 0.9 gained 23 % more lean mass over 12 weeks than those consuming equivalent amounts of lower‑quality sources, even when total protein intake was matched. This article breaks down what the protein quality index really means, how it’s calculated, and which foods and supplements deliver the highest returns on your investment.

What Is the Protein Quality Index?

The protein quality index combines two scientifically validated metrics: the Digestible Indispensable Amino Acid Score (DIAAS) and the Protein Digestibility‑Corrected Amino Acid Score (PDCAAS). Both evaluate how well a protein supplies the nine essential amino acids (EAAs) relative to human requirements and how digestible those amino acids are. A score of 1.0 (or 100 %) indicates a protein that meets or exceeds the EAA pattern for humans after accounting for digestibility; anything below reflects limiting amino acids or poor absorption.

Key points:

• DIAAS is the newer FAO‑recommended method (adopted 2013) and uses ileal digestibility data.
• PDCAAS, while older, is still used on many product labels and caps scores at 1.0.
• Both scores range from 0 to 1.0; values ≥ 0.9 are considered “high quality” for muscle‑building purposes.

How the Index Is Calculated: A Quick Look at the Math

To compute DIAAS for a given food:

Protein Quality Index Scores of Common Foods and Supplements