How Do the Indices based on the EAT-Lancet Recommendations Measure Adherence to Healthy and Sustainable Diets? A Comparison of Measurement Performance in Adults from a French National Survey.

Background: Measuring adherence to EAT-Lancet recommendations for healthy and sustainable diets is challenging, leading to diverse methods and a lack of consensus on standardized metrics. Available indices vary mainly in scoring systems, food components, units, energy adjustments, and cut-off points. Objectives: To evaluate and compare the measurement performance of 9 dietary indices for assessing adherence to EAT-Lancet reference diet. Methods: This cross-sectional study utilized repeated 24-h dietary recall data from 1723 adults in the French Third Individual and National Study on Food Consumption Survey (INCA3, 2014-2015). Sociodemographic, nutritional, and environmental variables were analyzed to assess the validity and reliability of dietary indices. Results: The 4 indices assessing their food components with proportional scoring captured dietary variability, were less dependent on energy intake and converged to a large extent with nutritional indicators. Although the 3 binary indices showed a stronger correlation with environmental indicators, 1 proportional index converged with both domains. Indices had valid unidimensional structures, meaning that the combination of food components within each index accurately reflected the same construct, supporting the use of total scores. Furthermore, the indices differed between sociodemographic groups, demonstrating concurrent-criterion validity. Higher scores were associated with higher nutritional quality and lower environmental impact, but with unfavorable results for zinc intake, vitamin B12, and water use. A low concordance rate (32%-43%) indicated that indices categorized individuals differently. Conclusions: Researchers must align study objectives with the applicability, assumptions, and significance of chosen indices. Indices using proportional scoring allow a global understanding of dietary health and sustainability, being advantageous in precision-focused research (for example, clinical trials or epidemiological research). Conversely, indices based on binary scoring offer a simplified perspective, serving as valuable tools for surveys, observational studies, and public health. Recognizing their strengths and limitations is crucial for a comprehensive assessment of diets and their implications.