Modelling the compensatory and carry-over effects between physical activity and fruit-vegetable consumption in young adults.

Physical activity (PA) and fruit and vegetable consumption (FVC) are crucial factors jointly affecting young adults' physical and mental health. However, the psychosocial interactive mechanisms of these behaviors remain understudied. Using a two-wave prospective design over two months, this study investigated the psychological mechanisms underlying multiple health behavior change (MHBC) through a novel two-layer social-cognitive framework, the Compensatory Carry-Over Action Model (CCAM), among 322 Chinese college students (19.47 ± 0.99 years; 55.6% female). Structural equation modeling with path analysis was conducted using Mplus 8.8 for data analysis. Results indicated an acceptable model fit for the hypothesized model, with χ = 120.845, df = 41, χ/df = 2.947, CFI = 0.940, TLI = 0.901, RMSEA = 0.078 [90% CI = 0.062 to 0.094], and SRMR = 0.079. The model explained 54.9% and 50.0% of the variance in PA and FVC, respectively. In the first layer, focusing on the individual behavior change process, both volitional self-efficacy and planning were identified as significant mediators in the intention-behavior relationship for PA and FVC (indirect effects = 0.03 to 0.14, all p < .01). In the second layer, targeting the interaction between the two behaviors, the mediating role of compensatory cognition (e.g., PA/FVC can compensate for FVC/PA) in the relationship between intention and behavior was not supported (all p > .05). However, volitional self-efficacy and planning played a significant mediating role in the interaction between PA/FVC intention and FVC/PA behavior (indirect effects = 0.01 to 0.04, all p < .05). Future interventions aiming at promoting MHBC should address compensatory cognition and the carry-over effects between diverse behaviors.