The use of food swaps to encourage healthier online food choices: a randomized controlled trial.BACKGROUND
 Online grocery stores offer opportunities to encourage healthier food choices at the moment that consumers place a product of their choice in their basket.  This study assessed the effect of a swap offer, Nutri-Score labeling, and a descriptive norm message on the nutrient profiling (NP) score of food choices in an online food basket.  Additionally explored was whether these interventions made it more motivating and easier for consumers to select healthier foods and whether potential effects were moderated by consumer health interest.


 Hypotheses were tested with a randomized controlled trial (RCT) in a simulated online supermarket.  Dutch participants (n = 550) chose their preferred product out of six product options for four different categories (breakfast cereals, crackers, pizza, and muesli bars).  Participants were randomly allocated to one of eight groups based on the interventions in a 2 (Nutri-Score: present, not present) X 2 (swap offer: present, not present) X 2 (norm message: present, not present) between subject design.  The primary outcome was the difference in combined NP score of product choices, for which a lower score represented a healthier product.


 RESULTS
 Swap offer (B = - 9.58, 95% CI: [- 12.026; - 7.132], Ƞ2 = 0.098) and Nutri-Score labeling (B = - 3.28, 95% CI: [- 5.724; -.829], Ƞ2 = 0.013) significantly improved the combined NP score compared to the control condition (NP score M = 18.03, SD = 14.02), whereas a norm message did not have a significant effect (B = - 1.378, 95% CI [- 3.825; 1.070], Ƞ2 = 0.002).  No evidence was found that interventions made it more motivating or easier for consumers to select healthier food, but situational motivation significantly influenced the healthiness score of food choices for both swap offer (b = - 3.40, p < .001) and Nutri-Score (b = - 3.25, p < .001).  Consumer health interest only significantly moderated the influence of Nutri-Score on ease of identifying the healthy food option (b = .23, p = .04).


 Swap offer and Nutri-Score labeling were effective in enhancing healthy purchase behavior in the online store environment.


 TRIAL REGISTRATION
 This study was retrospectively registered in the ISRCTN database on 02-09-2021 ( ISRCTN80519674 ).
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