A Protein Quality-Adjusted Density Index for Ranking Protein-Containing Foods Within Exchange List: A Proof-of-Concept Methodological Study

Exchange lists group foods for structured practical meal planning but may obscure differences in protein density and protein quality. This proof-of-concept methodological study developed a protein-focused extension of the conventional exchange-list framework by ranking protein-containing foods within five exchange categories using the Protein Quality-Adjusted Density Index (PQDI), while retaining the provenance and evidence status of the applied protein-quality factors (PQFs). The final analytical dataset comprised 59 foods: Lean Meat and Substitutes (n = 16), Milk and Dairy (n = 13), Medium-Fat Meat and Substitutes (n = 11), Legumes/Starch–Protein (n = 14), and High-Fat Meat and Substitutes (n = 5). Food-specific protein and energy values were derived primarily from USDA FoodData Central. Protein density was calculated as grams of protein per 100 kcal, and PQDI as protein density multiplied by the applied PQF. PQFs were classified as Measured, Estimated, or Assigned according to the food specificity and strength of the supporting evidence. Observed PQDI values ranged from 5.47 to 24.19 in Lean Meat and Substitutes, 5.46 to 18.58 in Milk and Dairy, 8.30 to 15.31 in Medium-Fat Meat and Substitutes, 1.93 to 9.52 in Legumes/Starch–Protein, and 3.77 to 8.00 in High-Fat Meat and Substitutes. Sensitivity analysis showed category-dependent concordance between protein-density and PQDI rankings, with the lowest concordance in Medium-Fat Meat and Substitutes. PQDI therefore provides a transparent proof-of-concept comparative descriptor of protein quality-adjusted density within the familiar exchange-list structure; its incremental practical and clinical value beyond protein density alone requires further validation.