BACKGROUND: Web-based computer-tailored interventions for multiple health behaviors can improve the strength of behavior habits in people who want to reduce their cardiovascular risk. Nonetheless, few randomized controlled trials have tested this assumption to date. OBJECTIVE: The study aim was to test an 8-week web-based computer-tailored intervention designed to improve habit strength for physical activity and fruit and vegetable consumption among people who want to reduce their cardiovascular risk. In a randomized controlled design, self-reported changes in perceived habit strength, self-efficacy, and planning across different domains of physical activity as well as fruit and vegetable consumption were evaluated. METHODS: This study was a randomized controlled trial involving an intervention group (n = 403) and a waiting control group (n = 387). Web-based data collection was performed in Germany and the Netherlands during 2013 to 2015. The intervention content was based on the Health Action Process Approach and involved personalized feedback on lifestyle behaviors, which indicated whether participants complied with behavioral guidelines for physical activity and fruit and vegetable consumption. There were three web-based assessments: baseline (T0, n = 790), a posttest 8 weeks after the baseline (T1, n = 206), and a follow-up 3 months after the baseline (T2, n = 121). Data analysis was conducted by analyzing variances and structural equation analysis. RESULTS: Significant group by time interactions revealed superior treatment effects for the intervention group, with substantially higher increases in self-reported habit strength for physical activity (F[1,199] = 7.71, p = 0.006, Cohen's d = 0.37) and fruit and vegetable consumption (F[1,199] = 7.71, p = 0.006, Cohen's d = 0.30) at posttest T1 for the intervention group. Mediation analyses yielded behavior-specific sequential mediator effects for T1 planning and T1 self-efficacy between the intervention and habit strength at follow-up T2 (fruit and vegetable consumption beta = 0.12, 95% CI 0.09 to 0.16, p < 0.001; physical activity beta = 0.04, 95% CI 0.02 to 0.06, p < 0.001). CONCLUSIONS: Our findings indicate the general effectiveness and practicality of web-based computer-tailored interventions in terms of increasing self-reported habit strength for physical activity and fruit and vegetable consumption. Self-efficacy and planning may play major roles in the mechanisms that facilitate the habit strength of these behaviors; therefore, they should be actively promoted in web-based interventions. Although the results need to take into account the high dropout rates and medium effect sizes, a large number of people were reached and changes in habit strength were achieved after 3 months. TRIAL REGISTRATION: ClinicalTrials.gov NCT01909349; https://ClinicalTrials.gov/ct2/show/NCT01909349 and Nederlands Trial Register NTR3706 http://www.trialregister.nl/trialreg/admin/rctview.asp?TC=3706.

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