Healthy intentions: A daily diary intervention study on physical activity and unhealthy snacking at work.

Employees are often not sufficiently physically active and engage in unhealthy snacking during the workday. We address these unhealthy behaviors by evaluating an individual-level intervention based on the principle of mental contrasting with implementation intentions targeted at increasing physical activity and decreasing unhealthy snacking during the workday. In addition to evaluating intervention effectiveness (compared to a passive control group), we also contrast two intervention groups: Participants were either randomly allocated to refresher interventions focusing on physical activity versus unhealthy snacking each day (daily-assignment intervention group) or could decide which behavior to focus on each day (daily-choice intervention group). We employed a randomized controlled design within a daily diary study (73 employees, 516 days). Between-person results showed that the intervention successfully improved employees' accelerometer-assessed physical activity during the 2-week study phase. Moreover, participants in the daily-choice intervention group were more physically active than participants in the daily-assignment intervention group. Within-person results did not provide evidence of an additional benefit from daily intervention refreshers. The intervention was ineffective at reducing unhealthy snacking at work. Exploratory analyses suggested that the intervention was more effective in work environments characterized by higher levels of job stressors (for both physical activity and unhealthy snacking) and among individuals with higher baseline levels of unhealthy snacking. These findings highlight the potential of mental contrasting with implementation intention interventions in promoting health behaviors during the workday while suggesting that behavioral choice and individual boundary conditions (i.e., baseline levels and work context) can further enhance intervention effectiveness. (PsycInfo Database Record (c) 2026 APA, all rights reserved).