Spillover effects of a nudge‐based dietary intervention on physical activity and sleep among adults with type 2 diabetes: Evidence from a randomised controlled trial

AIMS: Behavioural changes in one lifestyle domain may lead to improvements in others, known as spillover effects. This study examined whether a nudge-based dietary intervention could generate short-term (3 and 6 months) and long-term (24 months) spillover effects on physical activity and sleep among adults with type 2 diabetes. MATERIALS AND METHODS: A 2 × 2 factorial cluster-randomised controlled trial was conducted in 12 primary healthcare centres in Beijing, China. Adults aged 18-59 years with type 2 diabetes were randomised to a 1-month nudge-based dietary intervention (n = 204) or a control group (n = 73). Physical activity was assessed using the International Physical Activity Questionnaire-Short Form and step-tracking mobile applications. Sleep quality was evaluated using the Pittsburgh Sleep Quality Index. Generalised linear mixed-effects models tested group, time, and interaction effects with covariate adjustment. The mediating effect of diabetes management self-efficacy was assessed for behaviour change. RESULTS: A significant short-term spillover effect was observed for daily step counts at 3 months (+1105 steps; p = 0.02), which was not sustained at 6 or 24 months. Exploratory mediation analyses indicated that diabetes management self-efficacy at 3 months did not statistically mediate the association between the dietary intervention and daily step counts at the same time point. Total physical activity and sleep quality showed modest but nonsignificant short-term improvements. CONCLUSIONS: The nudge-based dietary intervention produced observable but short-lived spillover effects on daily physical activity among adults with type 2 diabetes. These findings contribute empirical evidence on cross-behavioural spillover in diabetes management and underscore the need for future research to better elucidate the mechanisms and sustainability of such spillover effects.