Can nudge interventions targeting healthy food purchases in real-world grocery stores reduce diet-related health disparities? A pooled analysis of four controlled trials
Authors: Stuber Josine M. , Beulens Joline WJ. , Ayala Guadalupe X. , Crozier Sarah R. , Dijkstra S. Coosje , Lin Shih-Fan , Vogel Christina , Mackenbach Joreintje D. 
Published: December 2024

Abstract
Background
Healthy food nudges may be more, or especially, effective among groups experiencing socioeconomic disadvantage. We investigated the modifying role of socioeconomic and demographic characteristics in the effectiveness of nudge interventions targeting healthy foods in real-world grocery store settings on food purchasing patterns.

Methods
We pooled individual participant data from multiple trials. Eligible trials were identified via a PubMed search and selected based on having a controlled real-world design, testing a nudging intervention promoting healthy purchases, while collecting participants’ sociodemographic and purchasing data. Out of four eligible trials, three had longitudinal measurements, one consisted of a single time point, two were randomised and two were not. Applied nudges consisted of a combination of placement nudges (focussing on availability or positioning) and property nudges (presentation and/or information). Harmonised data included dichotomised socioeconomic and demographic variables and the percentage of purchased fruits and vegetables of total purchases. Multilevel meta-regression based on linear mixed-effects models were used to explore modifying effects using two approaches: longitudinal and cross-sectional analyses.

Results
The analytical sample in the longitudinal analysis comprised of 638 participants, who were predominantly female (76.3%), had a lower education attainment (67.7%), and a mean age of 46.6 years (SD 13.5). These characteristics were similar in the cross-sectional analysis (n = 855). Compared to control group participants, there was no main effect of healthy food nudges on the percentage of fruit and vegetable purchases by intervention group participants in the longitudinal analysis (β = 0.00; 95%CI -0.03, 0.09). This main effect was not modified by educational attainment (βgroup*higher education = -0.06; -0.40, 0.02), sex (βgroup*females = 0.13; -0.00, 0.61) nor age (βgroup*older adults = -0.05; -0.39, 0.02). Results from the cross-sectional analysis were comparable.

Conclusions
This pooled analyses of four controlled trials did not find evidence supporting the hypothesis that grocery store nudge interventions of healthy foods work more effectively among groups experiencing socioeconomic disadvantage. Future studies are needed to address the identified limitations through rigorous trial design using comprehensive interventional strategies, standardised outcome measures, while also evaluating context-specific approaches. Such insights will help to better understand the equity of nudging interventions in grocery store settings and the potential for reducing diet-related health disparities.

Trial registrations
The trial of Ayala et al. (2022) was retrospectively registered at ClinicalTrials.gov (NCT01475526; at 14 November 2011, https://clinicaltrials.gov/study/NCT01475526), the of Huitink et al. (2020) was retrospectively registered in the ISRCTN registry (ISRCTN39440735; at 5 September 2018, https://doi.org/10.1186/ISRCTN39440735), the of Vogel et al. (2024) was retrospectively registered at ClinicalTrials.gov (NCT03518151; at 24 April 2018, https://clinicaltrials.gov/study/NCT03518151), and finally of Stuber et al. (2024) was registered in the Dutch Trial Register (ID NL7064, at 30 May 2018, https://www.onderzoekmetmensen.nl/en/trial/20990).