A scoping review of approaches used to develop plant-based diet quality indices

Plant-based dietary patterns are comprised of a range of foods, and increasingly, diet quality indices are used to assess them and their associations with health outcomes. As the design of these indices varies, a review of existing indices is necessary to identify common features, strengths, and considerations. This scoping review aimed to synthesize the literature on plant-based diet quality indices by examining their 1) basis for development, 2) scoring methodology, and 3) validation approaches. MEDLINE, CINAHL, and Global Health databases were systematically searched from 1980 to 2022. Observational studies were included if they assessed plant-based diets in adults, using an a priori methodology with food-based components. Studies conducted among pregnant/lactating people were excluded. Thirty-five unique plant-based diet quality indices were identified in 137 included articles published between 2007 and 2022. Indices were developed to reflect epidemiological evidence for associations between foods and health outcomes (n = 16 indices), previous diet quality indices (n = 16), country-specific dietary guidelines (n = 9), or foods from traditional dietary patterns (n = 6). Indices included 4 to 33 food groups, with fruits (n = 32), vegetables (n = 32), and grains (n = 30) the most common. Index scoring comprised of population-specific percentile cutoffs (n = 18) and normative cutoffs (n = 13). Twenty indices differentiated between healthy and less healthy plant-based foods when scoring intakes. Validation methods included construct validity (n = 26), reliability (n = 20), and criterion validity (n = 5). This review highlights that most plant-based diet quality indices were derived from epidemiological research, the majority of indices differentially scored healthy and unhealthy plant and animal foods, and indices were most often evaluated for construct validity and reliability. To ensure best practice use and reporting of plant-based dietary patterns, researchers should consider the basis for development, methodology, and validation when identifying appropriate plant-based diet quality indices for use in research contexts.