Effect of food swap recommendations on the healthfulness and carbon footprint of grocery purchases in a naturalistic online store: A randomized parallel trial

Background: Modeling studies suggest that simple dietary “swaps” in which just one food is replaced with another could yield both health and environmental benefits, but it remains unclear whether consumers make such changes when prompted. We tested whether recommending swaps improved the healthfulness and reduced the carbon footprint of simulated online grocery selections. Methods and findings: In May–June 2025, we recruited 1,201 US adults ages 18+ for a randomized parallel trial. Participants completed 3 weekly study visits. In the first visit, participants shopped in a naturalistic online grocery store with no intervention. They were then randomized in parallel to 1 of 4 trial arms: (1) climate swaps (n = 301), (2) health swaps (n = 300), (3) climate + health swaps (n = 300), or (4) control (n = 300). In the second and third visits, participants completed the shopping task in their randomly assigned trial arm. Participants were not masked to their trial arm. Using intent-to-treat analyses, we assessed 2 primary endpoints: the healthfulness (operationalized as the average Ofcom Nutrient Profiling Model score across the selected items [range: 1–100]) and carbon footprint (operationalized as the average kg of carbon dioxide equivalents [CO2-eq] per kg associated with producing the selected items) of participants’ grocery selections. We examined whether outcomes changed from the first (no-intervention) visit to the second and third (intervention) visits in the swaps arms compared to the control arm. Conclusions: Encouraging swaps to more healthful and lower-carbon foods improved the healthfulness and reduced the carbon footprint of online grocery selections. Clinical trials registration: ClinicalTrials.gov (NCT#06648226), first submitted October 16, 2024. Why was this study done?: What did the researchers do and find?: What do these findings mean?: In this randomized parallel trial, Anna Grummon and colleagues examine whether simple food-swap suggestions can influence online grocery choices. They show that recommendations focused on health, climate impact, or both were associated with healthier selections, while climate-focused and combined recommendations were also linked to lower-carbon grocery baskets in a simulated online shopping environment.