Quantifying the mediating effect of body mass index on the relation between a Mediterranean diet and development of maternal pregnancy complications: the Australian Longitudinal Study on Women’s Health.

BACKGROUND
The contribution of body mass index (BMI) to the observed associations between dietary patterns and risk of gestational diabetes mellitus (GDM) and hypertensive disorders of pregnancy (HDP) remains unclear.

OBJECTIVE
The objective of this study was to formally quantify the mediating effect of prepregnancy BMI in these associations.

DESIGN
Women (aged 25-30 y) participating in the Australian Longitudinal Study on Women's Health were not pregnant at baseline in 2003 and reported ≥1 pregnancy up to 2012. GDM and HDP diagnoses were self-reported for each pregnancy and validated in a subset. A Mediterranean diet score was created by use of a baseline-validated food-frequency questionnaire and dichotomized to reflect low adherence (<25th percentile) and higher adherence (≥25th percentile). A causal inference framework for mediation analysis was used to estimate total, natural direct, and natural indirect effects of the prepregnancy Mediterranean diet on incident GDM and HDP and proportions mediated through prepregnancy BMI.

RESULTS
In 3378 women without a history of diabetes, 240 (7.1%) developed GDM. HDP was reported in 273 (8.6%) of 3167 women with no history of hypertension. Low adherence to the Mediterranean diet was associated with higher risk of GDM (OR: 1.35; 95% CI: 1.02, 1.60) and HDP (OR: 1.41; 95% CI: 1.18, 1.56), after adjustment for education, parity, polycystic ovary syndrome, energy intake, and physical activity. Proportions mediated through prepregnancy BMI (per 1-kg/m(2) increase) were 32% and 22% for GDM and HDP, respectively.

CONCLUSION
These findings suggest that prepregnancy BMI as a single mediator contributes substantially to the total effects of the prepregnancy Mediterranean diet on GDM and HDP risk.