Body mass index and workplace bullying: exploring causality in two general population Norwegian surveys, the HUNT study and the Norwegian Mother, Father and Child Cohort Study (MoBa)

Background: Previous studies suggest people with a higher BMI are at increased risk of bullying victimization in the workplace. It is unclear whether this reflects an impact of weight on bullying, an impact of bullying on weight, or confounding by other factors. Methods: We explore the impact of BMI on workplace bullying victimization (WPB) using data from working-age participants in two large, general population surveys from Norway, the Trøndelag Health Study (HUNT, N=32,826) and the Norwegian Mother, Father and Child Cohort Study (MoBa mothers, N=14,076). We use genetic instrumental variable analysis to interrogate causal directionality in associations between BMI and WPB. In HUNT, we also conduct longitudinal analysis, examining whether BMI in 2006-08 predicts onset of WPB in 2017-19. Results: Participants with a higher BMI were more likely to report WPB, adjusted for confounders including socioeconomic position and psychological distress (absolute risk difference, ARD: 0.22% per kg/m2, CI: 0.13,0.31, p<0.001 in HUNT, 0.22%, CI: 0.11, 0.33, p<0.001 in MoBa). Genetic instrumental variable analysis found strong evidence for an impact of BMI on WPB only in MoBa (ARD: 0.65%, CI:0.06, 1.25, p=0.03 in MoBa, 0.03%, CI:-0.45,0.51, p=0.90 in HUNT). In longitudinal analyses, there was little evidence that BMI in 2006-08 affected WPB in 2017-19 (ARD: 0.01%, CI:-0.16,0.18, p=0.91), but WPB in 2006-08 was associated with slightly higher BMI in 2017-19 (0.16kg/m2, CI:0.02,0.29, p=0.02). Stratified analyses found inconsistent evidence of age, sex, and socioeconomic modification, but suggested a greater impact of BMI on WPB before 2020.Conclusions: In two large surveys from Norway, we found inconclusive evidence from genetic models that higher BMI increased risk of WPB. However, longitudinal analysis indicated that WPB affected BMI over time, consistent with an impact on BMI of psychosocial stress. Results underscore the negative consequences of WPB for physical, as well as mental, health.