Plain language summary
Food prices are a key driver of food purchasing behaviours, and The Supermarket Healthy Eating for Life (SHELf) study has shown that price discounts on healthier foods can increase their purchase. The aim of this study was to quantify whether participants in the SHELf study switched to study supermarkets for purchasing their FV during the intervention because of the discounted price. 642 participants completed self-reported surveys on the proportion of FV purchased at the study supermarket at baseline, 3 months and 6 months post intervention. This study found that it is likely that participants switched supermarket choice during the intervention to purchase their FV from the study supermarkets. The authors conclude that unless food-purchasing data are available from all sources, differential changes in purchasing patterns can make it difficult to discern the effectiveness of nutrition interventions.
Abstract
BACKGROUND The impacts of supermarket-based nutrition promotion interventions might be overestimated if participants shift their proportionate food purchasing away from their usual stores. This study quantified whether participants who received price discounts on fruits and vegetables (FV) in the Supermarket Healthy Eating for Life (SHELf) randomized controlled trial (RCT) shifted their FV purchasing into study supermarkets during the intervention period. METHODS Participants were 642 females randomly assigned to a 1) skill-building (n = 160), 2) price reduction (n = 161), 3) combined skill-building and price reduction (n = 160), or 4) control (n = 161) group. Participants self-reported the proportion of FV purchased in study supermarkets at baseline, 3- and 6-months post-intervention. Fisher's exact and χ2 tests assessed differences among groups in the proportion of FV purchased in study supermarkets at each time point. Multinomial logistic regression assessed differences among groups in the change in proportionate FV purchasing over time. RESULTS Post-intervention, 49% of participants purchased ≥50% of their FV in study supermarkets. Compared to all other groups, the price reduction group was approximately twice as likely (RRR: 1.8-2.2) to have increased proportionate purchasing of FV in study supermarkets from baseline to post-intervention (p< 0.05). CONCLUSIONS Participants who received price reductions on FV were approximately twice as likely to shift their FV purchasing from other stores into study supermarkets during the intervention period. Unless food purchasing data are available for all sources, differential changes in purchasing patterns can make it difficult to discern the true impacts of nutrition interventions. TRIAL REGISTRATION The SHELf trial is registered with Current Controlled Trials Registration ISRCTN39432901, Registered 30 June 2010, Retrospectively registered ( http://www.isrctn.com/ISRCTN39432901 ).
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