Prepregnancy plant-based diets and the risk of gestational diabetes mellitus: a prospective cohort study of 14,926 women.

Background: Emerging evidence suggests beneficial impacts of plant-based diets on glucose metabolism among generally healthy individuals. Whether adherence to these diets is related to risk of gestational diabetes mellitus (GDM) is unknown.

Objectives: We aimed to examine associations between plant-based diets and GDM in a large prospective study.

Methods: We included 14,926 women from the Nurses' Health Study II (1991-2001), who reported ≥1 singleton pregnancy and without previous GDM before the index pregnancy. Prepregnancy adherence to plant-based diets was measured by an overall plant-based diet index (PDI), healthful plant-based diet index (hPDI), and unhealthful plant-based diet index (uPDI) as assessed by FFQs every 4 y. Incident first-time GDM was ascertained from a self-reported physician diagnosis, which was previously validated by review of medical records. We used log-binomial models with generalized estimating equations to calculate RRs and 95% CIs for associations of PDIs with GDM.

Results: We documented 846 incident GDM cases over the 10-y follow-up among 20,707 pregnancies. Greater adherence to the PDI and hPDI was associated with lower GDM risk. For the PDI, the multivariable-adjusted RR (95% CI) comparing the highest and lowest quintiles (Q5 compared with Q1) was 0.70 (0.56, 0.87) (Ptrend = 0.0004), and for each 10-point increment was 0.80 (0.71, 0.90). For the hPDI, the RR (95% CI) of Q5 compared with Q1 was 0.75 (0.59, 0.94) (Ptrend = 0.009) and for each 10-point increment was 0.86 (0.77, 0.95). After further adjustment for prepregnancy BMI, the associations were attenuated but remained significant: for the PDI, the RR (95% CI) for each 10-point increment was 0.89 (0.79, 1.00) and the corresponding RR (95% CI) was 0.89 (0.80, 0.99) for the hPDI. The uPDI was not associated with GDM.

Conclusions: Our study suggests that greater prepregnancy adherence to a healthful plant-based diet was associated with lower risk of GDM, whereas an unhealthful plant-based diet was not related to GDM risk.