Nudging towards sustainable and healthy diets: A randomized trial of young adults

Abstract Background Young adulthood is an optimal time for intervening in dietary behaviors. Young adults in the United States strongly value environmental sustainability, so appealing to these values could be a promising strategy to improve dietary quality. Objective We examined whether a "stealth" intervention that explicitly appealed to sustainability (climate-impact labels, climate-focused social comparison messages) while also promoting healthier options (through healthy and sustainable swap recommendations) improved the healthfulness of food selections. Methods Participants (US young adults ages 18-25) were randomized in Qualtrics to the experimental (n=1,074) or control (n=1,075) arm and selected one protein, frozen meal, and snack they would most like to purchase. All foods varied by healthfulness (Nutri-Score algorithm, range A-E). Participants in the experimental arm received nudges appealing to sustainability, including viewing front-of-package labels showing whether the food had low-, medium-, or high-climate-impact and, if they selected a high-climate-impact food, viewing a climate-focused social comparison message and swap recommendation to low- or medium-climate-impact foods with a healthy Nutri-Score (A or B). Control arm participants did not receive any nudges. Results Experimental arm participants selected foods that were 3.9 points healthier than participants in the control arm (p<0.001, d=0.67). Effects of the nudges on healthfulness did not differ by gender, education, financial situation, red meat consumption, political beliefs, or interest in sustainability (ps for interaction≥0.13). Experimental arm participants selected foods with a carbon footprint 0.76 kg CO₂-eq/100g lower than control arm participants (p<0.001, d=0.93). Compared to the control, experimental arm participants thought more about sustainability (p<0.001), less about taste (p<0.001) and cost (p=0.02), and similar amounts about healthfulness (p=0.32) when selecting foods. Conclusions "Stealth" nudges appealing to sustainability while also incorporating health considerations could encourage a wide range of young adults to select more healthful and sustainable foods. Trial registration ClinicalTrials.gov (NCT06119165) and AsPredicted.org (https://aspredicted.org/QXH_MR3).