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

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. ### Competing Interest Statement The authors have declared no competing interest. ### Funding Statement This study is part of the FEAST (Food systems that support transitions to healthy and sustainable diets) project funded by the European Union's Horizon Europe research and innovation program under grant agreement number 101060536 and by Innovate UK under grant number 10041509. Swiss participant in FEAST is supported by the Swiss State Secretariat for Education, Research and Innovation (SERI) under contract number 22.00156. More details in https://www.feast2030.eu/. ### Author Declarations I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained. Yes The details of the IRB/oversight body that provided approval or exemption for the research described are given below: The INCA3 study was carried out in accordance with the Declaration of Helsinki and received approval from the French Data Protection Authority (Decision DR 2013-228) on May 2, 2013, following a favorable opinion from the Advisory Committee on Information Processing in Health Research on January 30, 2013 (Opinion 13.055). Verbal informed consent was obtained from all participants before their voluntary inclusion in the study. Verbal consent was witnessed and formally recorded. I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals. Yes I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance). Yes I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable. Yes Data from the Third French Individual and National Food Consumption Survey (INCA3) in available on the [data.gouv.fr][1] platform. Data on the environmental impacts of foods consumed in France is available on the [agribalyse.ademe.fr][2] platform. [1]: http://data.gouv.fr [2]: http://agribalyse.ademe.fr