BACKGROUND: Web-based computer-tailored interventions often suffer from small effect sizes and high drop-out rates, particularly among people with a low level of education. Using videos as a delivery format can possibly improve the effects and attractiveness of these interventions. OBJECTIVE: The main aim of this study was to examine the effects of a video and text version of a web-based computer-tailored obesity prevention intervention on dietary intake, physical activity, and body mass index (BMI) among Dutch adults. A second study aim was to examine differences in appreciation between the video and text version. The final study aim was to examine possible differences in intervention effects and appreciation per educational level. METHODS: A three-armed randomized controlled trial was conducted with a baseline and 6 months follow-up measurement. The intervention consisted of six sessions, lasting about 15 minutes each. In the video version, the core tailored information was provided by means of videos. In the text version, the same tailored information was provided in text format. Outcome variables were self-reported and included BMI, physical activity, energy intake, and appreciation of the intervention. Multiple imputation was used to replace missing values. The effect analyses were carried out with multiple linear regression analyses and adjusted for confounders. The process evaluation data were analyzed with independent samples t tests. RESULTS: The baseline questionnaire was completed by 1,419 participants and the 6 months follow-up measurement by 1,015 participants (71.53%). No significant interaction effects of educational level were found on any of the outcome variables. Compared to the control condition, the video version resulted in lower BMI (B = -0.25, p = 0.049) and lower average daily energy intake from energy-dense food products (B = -175.58, p < 0.001), while the text version had an effect only on energy intake (B = -163.05, p = 0.001). No effects on physical activity were found. Moreover, the video version was rated significantly better than the text version on feelings of relatedness (p = 0.041), usefulness (p = 0.047), and grade given to the intervention (p = 0.018). CONCLUSIONS: The video version of the web-based computer-tailored obesity prevention intervention was the most effective intervention and most appreciated. Future research needs to examine if the effects are maintained in the long term and how the intervention can be optimized. CLINICAL TRIAL: Netherlands Trial Register NTR3501; http://www.trialregister.nl/trialreg/admin/rctview.asp?TC=3501.

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