The purpose of this study is to help Internet users to adopt and maintain a healthy lifestyle, it is imperative to increase self-management competencies to improve healthy eating and regular physical activity. Aim of this research project is to evaluate an evidence- and theory-based computerized expert system in comparison to a standard program. In addition, two different tailoring criteria are compared to each other with regard to the misclassification of study participants. Internet users will be treated psychologically and followed up over 12 weeks. The computerized expert system is expected to help users better than the standard program. Both interventions are hypothesized to improve self-management competencies over time.

Two experimental studies are planned over a time period of 12 weeks. In the nutrition study, participants will randomly be allocated to either a waiting control group (WCG) receiving a tailored intervention after the last follow-up questionnaire, an active control group (ACG), receiving an interactive quiz on all areas of healthy nutrition or to one of two intervention groups (IGs). Intervention group 1 receives one of four computerized tailored interventions based on a standard recommendation on vegetable and fruit intake, intervention group 2 receives one of the same four tailored interventions based on another criterion.

In the physical activity study, study participants in the intervention group receive an interactive, computerized expert system (Intervention Group, IG). Individuals in the Active Control Group (ACG) get a standard program, which does not tailor treatment components to the individual needs of the patients. Internet users (N = 1000) will be recruited in in the internet via press releases, personal invitations and advertised likes at well known web-sites. Study participants will be followed up over three measurement points: One to eight weeks after T1, T2 will take place. Four to 12 weeks after T1 the T3 measurement will be conducted.

The hypotheses are: Both the IG and the ACG are expected to have a higher motivation, to adopt a healthy lifestyle, to perform more health behavior and to be less likely to relapse into previous unhealthy routines at T2 and T3 than at T1. Also, IG and ACG will be healthier as well as they will report more quality of life and rehabilitation satisfaction at T2 and T3 than at T1. In comparison to ACG, the IG is hypothesized to be more effective than the ACG regarding motivation, behavior and social-cognitive predictors of behavior. Moreover, the interventions (ACG and IG) are supposed to be equally effective for different age and sex groups, obese and non-obese individuals. In the nutrition study, the IG2 will report higher on all outcome measures than IG1.

After successful evaluation and some adoptions the intervention will be implemented as a self-help program in all eligible Internet users in the Internet.

Inclusion Criteria:

• to be capable of exercising on their own at a minimum level and/or to consume fruits and vegetables
• able to fill out a questionnaire (no illiteracy)
• adequate German language ability

Exclusion Criteria:

• no internet access
• no computer with keyboard