Article Estimating Dietary Intake from Grocery Purchase Data - A Comparative Validation of Relevant Indicators. Jing Wu 1 , Klaus Fuchs 2, * , Jie Lian 1 , Mirella Haldimann 3 , Tanja Schneider 1 * , Simon Mayer 1 , Jaewook Byun 4 , Roland Gassmann 5, , Christine Brombach 5, , Elgar Fleisch 2,   Citation: Wu, Jing; Fuchs, Klaus; Lian, Jie; Haldimann, Mirella; Schneider, Tanja; Mayer, Simon; Byun, Jaewook; Gassmann, Roland; Brombach, Christine; Fleisch, Elgar Title. Preprints 2021, 1, 0. https://doi.org/ Received: Accepted: Published: Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. 1 University of St. Gallen; jing.wu@unisg.ch, jie.lian@unisg.ch, tanja.schneider@unisg.ch, simon.mayer@unisg.ch 2 ETH Zurich; fuchsk@ethz.ch, efleisch@ethz.ch 3 D-One Solutions AG; mirella.haldimann@d-one.ai 4 Sejong University; Jaewook Byun jwbyun@sejong.ac.kr 5 Zurich University of Applied Science; garo@zhaw.ch, broc@zhaw.ch * Correspondence: fuchsk@ethz.ch; Tel.: +41-78-858-7037 † These authors contributed equally to this work. Abstract: In light of the globally increasing prevalence of diet-related non-communicable diseases (NCDs), new scalable and non-invasive dietary monitoring techniques are urgently needed. Auto- matically collected digital receipts from loyalty cards have the potential to serve as an objective and automatically traceable digital biomarker for individual food choice behavior and do not require patients to manually log each individual meal item. Until recently, such electronic purchase records were hard to collect for researchers and were only validated in national empirical studies. Multiple quantitative indicators for purchase quality have been suggested, but so far no comparison has validated the potential of these alternative indicators to discriminate between health-beneficial and -detrimental food choices. With the introduction of the General Data Privacy Regulation in the European Union, millions of consumers gained the right to access their purchase data in a machine- readable form, representing a historic chance to leverage purchase data for scalable monitoring of food choices. This study hence is the first study comparing the calibration capacity and validating the discrimination potential of previously suggested purchase indicators for the nutritional quality of purchased groceries, incl. HEI-2015, HETI, GPQI, and FSA-NPS DI. To assess the indicators’ potential, 464 study participants were asked to complete a validated food frequency questionnaire (FFQ) and to donate their digital receipts from the loyalty card programs of the two leading Swiss grocery retailers, representing 69% of the national grocery market. 89 participants fulfilled the eligibility criteria, i.e. completed the FFQ and were frequent users of the loyalty card systems. Compared to absolute food and nutrient intake, correlations between density-based relative food and nutrient intake and food purchase data are stronger. Counterintuitively, although the frameworks of the HETI and the GPQI are centered around food groups, both indicators do not capture food group intake such as vegetables or sweets very well. The FSA-NPS DI has the best calibration and discrimination performance in classifying participants’ consumption of nutrients and food groups, and seems to be a superior indicator to estimate nutritional quality of a user’s diet based on digital receipts from grocery purchases in Switzerland. Keywords: dietary intake; diet monitoring; digital receipts; purchase quality indicators; FSA-NPS DI 1. Introduction The globally increasing prevalence of diet-related non-communicable diseases (NCDs) including obesity, diabetes, certain types of cancer, and cardiovascular diseases represents a continuously growing burden for affected patients and health-care systems alike [1–4]. Due to societal trends such as urbanization and the transformation of food systems towards more processed, convenience, and fast food, dietary patterns around the world show an increasing consumption of (added) sugar, sodium, saturated fats, and calorific energy [5], thereby elevating the risks of diet-related NCDs [6–8]. Besides the uptake of food items of low nutritional quality, the global demand for meat, fish, exotic fruits throughout the Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 2 July 2021 doi:10.20944/preprints202107.0055.v1 © 2021 by the author(s). Distributed under a Creative Commons CC BY license.