Introduction: Higher adherence to a Mediterranean diet before pregnancy has been associated with lower risk of developing gestational diabetes mellitus (GDM). The importance of pre-pregnancy body mass index (BMI) as a pathway of this relationship remains unclear, but this may help inform targeted prevention strategies.
Objective: To quantify the extent to which pre-pregnancy BMI explains the relationship between a Mediterranean diet and development of GDM.
Methods: This study includes 3,378 women aged 25-30 years participating in the Australian Longitudinal Study on Women’s Health. Women were not pregnant at baseline in 2003 and reported at least one pregnancy during follow-up until 2012. GDM diagnosis was self-reported for each pregnancy and validated in a subsample. A Mediterranean diet score was created using a baseline validated food frequency questionnaire, and categorised as low (<25th percentile) and higher adherence (≥25th percentile). Mediation analysis was performed to estimate total, natural direct and indirect effects of the pre-pregnancy Mediterranean diet on risk of GDM and proportions mediated through pre-pregnancy BMI.
Results: A total of 240 women (7.1%) developed GDM. Women with low adherence to a Mediterranean diet were at higher risk of developing GDM (OR 1.35, 95% CI 1.02-1.60) after adjustment for education, parity, polycystic ovary syndrome, energy intake and physical activity. The higher GDM risk due to low adherence to the Mediterranean diet was partly explained by pre-pregnancy overweight (21%) and obesity (38%) compared with normal weight.
Conclusions: Overweight and obesity before pregnancy contribute substantially to the total effect of the Mediterranean diet on risk of GDM. These findings suggest that interventions successful in implementing a Mediterranean diet before pregnancy may reduce the risk of GDM substantially through optimising pre-pregnancy BMI. The most effective ways to optimise BMI prior to pregnancy should be explored and addressed in future intervention studies.