Testing an integrated model of the theory of planned behaviour and self-determination theory for different energy balance-related behaviours and intervention intensities

Objectives The aim of the study was to test the relations between constructs from the self-determination theory (autonomous and controlled motivation), the theory of planned behaviour (attitudes, self-efficacy, and intentions), and behaviour change within a theoretically integrated model. Additionally, the aim was to test if these relations vary by behaviour (physical activity or dietary behaviour) or intervention intensity (frequency). Design. It was a randomized controlled trial with a 'usual care' condition (medical screening only) and an intervention condition (medical screening + access to a website and coaching). Participants in the latter condition could freely determine their own intervention intensity. Methods. Participants (N = 287) completed measures of the theoretical constructs and behaviour at baseline and after the first intervention year (N = 236). Partial least squares path modelling was used. Results. Changes in autonomous motivation positively predicted changes in self-efficacy and intentions towards a healthy diet. Changes in controlled motivation positively predicted changes in attitudes towards physical activity, changes in self-efficacy, and changes in behavioural intentions. The intervention intensity moderated the effect of self-efficacy on intentions towards physical activity and the relationship between attitude and physical activity. Changes in physical activity were positively predicted by changes in intentions whereas desired changes in fat intake were negatively predicted by the intervention intensity. Conclusions Important relations within the theoretically integrated model were confirmed but others were not. Moderation effects were found for behaviour and intervention intensity.