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).

INTRODUCTION

To interrogate whether reported associations between body mass index (BMI) and workplace bullying (WPB) reflect an impact of weight on bullying, an impact of bullying on weight or confounding.

METHODS

We use data from working-age participants in two large, general population Norwegian surveys, the Trøndelag Health Study (HUNT, N=32 826) and the Norwegian Mother, Father and Child Cohort Study (MoBa mothers, N=14 076). To interrogate causal mechanisms, we apply genetic instrumental variable analysis (Mendelian randomisation) and, in HUNT only, bidirectional longitudinal analysis.

RESULTS

In cross-sectional analysis in both surveys, participants with a higher BMI were slightly more likely to report concurrent WPB, adjusted for confounders including socioeconomic position and psychological distress (meta-analysed absolute risk difference, ARD: 0.22% per kg/m2, 95% CI 0.15 to 0.29, meta-analysed OR: 1.02 per kg/m2, 95% CI 1.01 to 1.03). Genetic analysis found strong evidence for an impact of BMI on WPB only in MoBa, but not overall (meta-analysed ARD: 0.32% per kg/m2, 95% CI -0.29 to 0.93). In longitudinal analyses using HUNT data, there was less evidence that earlier BMI affected WPB onset (ARD: 0.08% per kg/m2, 95% CI -0.10 to 0.25, with raised risk for participants with class I but not class II obesity at baseline). However, WPB in 2006-2008 was associated with higher BMI in 2017-19 (beta: 0.16 kg/m2 (95% CI 0.02 to 0.29)). Stratified analyses found inconsistent evidence of age, sex and socioeconomic modification, but suggested stronger associations pre-2020.

CONCLUSIONS

In two large Norwegian surveys, we found inconclusive evidence from genetic models that higher BMI increased the risk of WPB. Longitudinal analysis (possible only in HUNT) found mixed evidence that higher BMI increased risk of WPB, but clearly suggested that WPB affected BMI over time, consistent with an impact of psychosocial stress on BMI. Results underscore the negative consequences of WPB for physical as well as mental health.