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

Abstract

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. We aimed to evaluate and compare the measurement performance of six dietary indices for assessing adherence to EAT-Lancet reference diet. Food consumption data of 1,723 adults were obtained from the French Third Individual and National Study on Food Consumption Survey (INCA3, 2014-2015). Sociodemographic, nutritional, and environmental data were used to assess the validity and reliability of dietary indices. Results showed that the four indices assessing their food components with quantitative scoring captured dietary variability, were less dependent on energy intake and converged to a large extent with nutritional indicators. While the two binary indices showed a stronger correlation with environmental indicators, one quantitative index converged with both domains. Indices had valid unidimensional structures, meaning that the combination of food components within each index accurately reflects the same construct and 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 unfavourable results for zinc intake, vitamin B12 and water use. A low concordance rate (from 32% to 43%) indicated that indices categorized individuals differently. Researchers must align study objectives with the applicability, assumptions, and functional significance of chosen indices. Indices using quantitative scoring allow a global understanding of dietary health and sustainability, being advantageous in precision-focused research, such as clinical trials or epidemiological research. Conversely, indices based on binary scoring offer a simplified perspective, being valuable tools for surveys, observational studies, and public health. Recognizing their strengths and limitations is crucial for a comprehensive assessment of diets and understanding their implications.