There were no differences at baseline between experimental (n = 42) and control (n = 38) conditions on any of the included outcome variables or by age or gender, all ps > .05. Further, there were no differences between those who remained in the study and those who dropped out (n = 17) on any outcome variable, age or gender, all ps > .05. However, participants were more likely to drop-out of the control condition (n = 12) than the experimental condition, n = 5; χ2 = 4.615, p = .032.
Wisconsin card sort task
There were main effects of time and condition which were qualified by a significant time by condition interaction, F (2,122) = 26.736, p < .001, Cohen’s d = 1.325. Contrasts examining these effects separately for each time comparison revealed that WCST perseverance errors reduced in the CRT-O condition, MD = − 5.854, SD = 4.216, but not the control group, MD = − 0.226, SD = 4.455, from baseline to post-intervention, F (1,61) = 22.928, p < .001, Cohen’s d = 1.086. Similarly, perseverance errors reduced in the CRT-O condition, MD = − 6.108, SD = 4.532, but not the control group, MD = 1.038, SD = 5.087, from baseline to follow-up, F (1,61) = 34.311, p < .001, Cohen’s d = 1.500, see Fig. 1.
Trail making task
There was a main effect of time which was qualified by a significant time by condition interaction, F(2,116) = 4.788, p = .010, Cohen’s d = 0.574. Contrasts examining these effects separately for each time comparison revealed that TMT performance improved in the CRT-O condition, MD = − 19.673, SD = 35.442, but not in the control group, MD = 0.870, SD = 24.858, from baseline to post-intervention, F (1,58) = 5.842, p = .019, Cohen’s d = 0.637. Similarly, TMT performance improved in the CRT-O condition, MD = − 19.791, SD = 37.945, but not the control group, MD = 0.467, SD = 32.538, from baseline to follow-up, F (1,58) = 4.869, p = .031, Cohen’s d = 0.578, see Fig. 2.
Unhealthy eating habit
There were no significant main effects of time or condition, nor was the interaction between time and condition significant. However, planned contrasts- which analysed these effects separately for baseline to post-intervention and for baseline to follow-up- revealed that habit strength significantly reduced in the CRT-O group, MD = − 1.588, SD = 0.986, compared to the control group, MD = 0.032, SD = 1.021, from baseline to follow-up, F (1,60) = 39.101, p < .001, Cohen’s d = 1.616, but not from baseline to post-intervention, F (1,60) = .083, p = .778, Cohen’s d = 0.063, see Fig. 3.
Sedentary behavior habit
There were main effects of time and condition which were qualified by a significant time by condition interaction, F (2,120) = 30.770, p < .001, Cohen’s d = 1.432. Contrasts examining these effects separately for each time comparison revealed that sedentary habit strength reduced in the CRT-O condition, MD = − 1.412, SD = 0.946, but not in the control group, MD = 0.199, SD = 0.825, from baseline to post-intervention, F (1,60) = 65.326, p < .001, Cohen’s d = 2.086. Similarly, sedentary habit strength reduced in the CRT-O condition, MD = − 1.660, SD = 1.306, but not in the control group, MD = 0.170, SD = 0.591, from baseline to follow-up, F (1,60) = 42.949, p < .001, Cohen’s d = 1.692, see Fig. 4.
Mediation analyses
The indirect effect of condition on change in unhealthy eating habit, through change in WCST perseverance errorsFootnote 1 was significant, β = 0.396, 95% [CI: 0.119, 0.790]. This indirect effect accounted for 31.56% of variance in the total effect and an additional 8.58% of the variance in unhealthy eating habits. The indirect effect of condition on change in unhealthy eating habit, through change in TMT performance was not significant, β = 0.026, 95% [CI: -0.108, 0.199], and only accounted for 1.62% of the variance in the total effect. The effect of intervention condition on change in habit strength was partially mediated by change in WCST performance, as the direct effect of condition on habit change remained significant once change in WCST was added to the model, β = 0.838, 95% [CI: 451, 1.687]. Overall, the direct and indirect effects accounted for 46.97% of the variance in change in unhealthy eating habits. See Fig. 5 for standardised coefficients between all variables in the model.
The indirect effect of condition on change in sedentary behavior habit, through change in WCST perseverance errors was significant, β = 0.277, 95% [CI: 0.033, 0.626]. This indirect effect accounted for 21.21% of variance in the total effect and an additional 3.91% of the variance in sedentary behavior habits. The indirect effect of condition on change in sedentary behavior habit, through change in TMT performance was not significant, β = − 0.010, 95% [CI: -0.140, 0.193], an only accounted for 0.8% of the variance in the total effect. The effect of intervention condition on change in habit strength was partially mediated by change in TMT performance, as the direct effect of condition on change in habit remained significant once change in TMT was added to the model, β = 1.037, 95% [CI: 0.758, 2.148]. Overall, the direct and indirect effects accounted for 44.95% of the variance in change in sedentary behavior habits. See Fig. 6 for standardised coefficients between all variables in the model.