Objective: This study examined cross-sectional and longitudinal associations of intermittent fasting (IF) engagement with body mass index (BMI), both thinness-oriented and muscularity-oriented eating disorder (ED) psychopathology, eating-related psychosocial impairment, and intuitive eating.

Method: Using a longitudinal design, 491 Chinese adults (M_age = 30.33years, SD = 7.89) provided data regarding BMI, IF status, and eating variables at baseline (T1) and 8-month (T2) follow-up. One-way ANOVA was used to explore cross-sectional associations between IF engagement at T1 and study variables (i.e., BMI and eating variables) at T1. Cross-lagged regression analyses were used to examine the longitudinal associations between IF engagement at T1 and study variables at T2, after adjusting for covariates and study variables at T1.

Results: Cross-sectionally, compared with participants who never engaged in IF at T1, those with current or past engagement in IF at T1 exhibited significantly higher BMI, ED psychopathology, eating-related psychosocial impairment, and lower intuitive eating at T1. Longitudinally, relative to never engagement in IF at T1, both current and past engagement in IF at T1 were associated with higher ED psychopathology and greater eating-related psychosocial impairment at T2. Also, relative to never engagement in IF at T1, past engagement in IF at T1 was associated with higher BMI at T2 and lower unconditional permission to eat as a facet of intuitive eating at T2.

Discussion: The findings challenge the perception of IF as an effective approach for weight loss and also suggest IF might have enduring adverse impacts on eating behaviors, similar to traditional forms of dieting.