Estimating Dietary Intake from Grocery Purchase Data - A Comparative Validation of Relevant Indicators

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. Automatically 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.