Understanding Dietary Protein Quality: Digestible Indispensable Amino Acid Scores and Beyond

Dietary protein quality refers to the capacity of a food to meet the human metabolic needs for essential amino acids (EAAs) and nitrogen. This is critical in low- and middle-income countries, where severe protein malnutrition occurs, and relevant in higher-income countries, where increasing dietary EAA intake may improve health and function. There are several methods to assess protein quality, each with different objectives. Chemical scoring metrics, like the digestible indispensable amino acid score, describe the EAA composition and digestibility of a protein source. However, these methods do not capture the metabolic activity of food-derived amino acids. Overreliance on a single metric leads to generic dietary recommendations lacking individual context. This review draws on chemical score and stable isotope methods to provide a comprehensive assessment of dietary protein quality. We translate these findings into practical recommendations for improving protein quality in the context of whole diets. High-quality protein sources are characterized by high EAA density (%EAAs/kcals), digestibility, bioavailability, and the capacity to stimulate protein synthesis. Practically, protein quality improves when using processing and cooking methods that reduce antinutrients, denature proteins, and reduce food particle size and structure. Conversely, protein quality decreases when exposing foods to prolonged storage, heat sterilization, and high surface temperatures. Diet modeling studies show that EAA density and protein quality are higher in omnivorous and lacto-ovo-vegetarian diets, and diets high in whole food plant-derived proteins may require greater total protein and energy intakes to compensate for lower protein quality. For incomplete plant-derived proteins, consuming complementary proteins may be beneficial. Considerations for dietary protein quality in older adults include chewing efficiency, food particle size, and higher EAA density and leucine intakes to maximize muscle protein synthesis. Recognizing dietary protein quality as a multifaceted, modifiable metric is essential to improving dietary recommendations and public health outcomes.