189-OR: Prepregnancy Plant-Based Diet and the Risk of Gestational Diabetes Mellitus: A Prospective Cohort Study of 15,999 Women

Objective: To identify novel modifiable factors that may help prevent gestational diabetes mellitus (GDM) by examining associations of plant-based diet indices with GDM risk.

Methods: We included 15,999 women (22,524 singleton pregnancies) in the Nurses’ Health Study II (1991-2001), who reported at least one singleton pregnancy between 1991 and 2001. Pre-pregnancy plant-based diets (i.e., overall plant-based diet index (PDI), healthful plant-based diet index (hPDI), and unhealthful plant-based diet index (uPDI)) were assessed every 4 years using a food frequency questionnaire. Incident first time GDM was ascertained from a self-reported physician diagnosis of GDM, which was previously validated to have high concordance with medical records. We used log-binomial models with generalized estimating equations to estimate relative risks (RR) and 95% confidence intervals (CI) for quartiles of PDI, hPDI, and uPDI and GDM with adjustment for demographic, lifestyle, and other factors.

Results: We documented 891 GDM cases over the 10 years of follow-up. Both PDI and hPDI were significantly and inversely associated with GDM (PDI: adjusted RR for extreme quartiles (highest vs. lowest) 0.75, (95% CI: 0.62, 0.92), p trend = 0.01; hPDI: RR for extreme quartiles 0.76, (0.61, 0.94), p trend = 0.01). Further adjustment for pre-pregnancy BMI attenuated the associations (RR for extreme quartiles for PDI: 0.87 (0.71, 1.06); for hPDI: 0.81 (0.65, 1.00)). We estimated that pre-pregnancy BMI mediated 61.3% (95% CI: 21.7%, 90.1%) of the association between PDI and GDM, and 44.4% (18.1%, 74.3%) of the association between hPDI and GDM. uPDI was not significantly associated with GDM.

Conclusions: Our study suggests that pre-pregnancy adherence to overall and healthful plant-based diets may be associated with a lower risk of GDM. These associations were substantially mediated through pre-pregnancy BMI.