Mediterranean diet reduces gestational diabetes risk by 40% in Hispanic women

Methodology

This prospective randomized controlled trial was conducted at Hospital Clínico San Carlos in Madrid, Spain from 2015-2017. Normoglycemic pregnant Hispanic women attending their first gestational visit at 8-12 weeks were screened for eligibility. Inclusion criteria required fasting blood glucose <92 mg/dL, age ≥18 years, and singleton pregnancy. Women were randomized by age, ethnicity, body mass index, and parity before 14 weeks gestation to either intervention or control groups. Following the RCT, a real-world implementation study enrolled 1000 consecutive pregnant women who received the intervention as standard care.

The control group received standard nutritional advice recommending fat restriction with extra virgin olive oil limited to <40 mL/day and nuts consumed <3 days per week. The intervention group received Mediterranean diet recommendations emphasizing ≥40 mL/day of extra virgin olive oil and a handful of pistachios (25-30g) at least 3 days per week, with free supplies provided at visits 1 and 2. The real-world group received identical Mediterranean diet recommendations but without free food provision.

Participants attended four visits during pregnancy: Visit 1 (12-14 weeks) for baseline assessment and nutritional intervention initiation, Visit 2 (24-28 weeks) for dietary reinforcement and 75g oral glucose tolerance test for GDM screening using IADPSG criteria, Visit 3 (36-38 weeks), and Visit 4 (at delivery). Dietary adherence was assessed at each visit using two semi-quantitative food frequency questionnaires: a 14-point Mediterranean Diet Adherence Screener (MEDAS, adapted for pregnancy by excluding alcohol and juice) and a validated 12-item Nutrition Score from the Diabetes Nutrition and Complications Trial. Physical activity was evaluated using a separate scoring system assessing daily walking and stair climbing.

The primary outcome was GDM diagnosis at 24-28 weeks. Secondary maternal outcomes included gestational weight gain, pregnancy-induced hypertension, preeclampsia, urinary tract infections, mode of delivery, and perineal trauma. Secondary neonatal outcomes included birthweight, gestational age at delivery, large-for-gestational-age and small-for-gestational-age classification, Apgar scores, and neonatal complications. Statistical analysis used chi-square tests for categorical variables and t-tests or Mann-Whitney U tests for continuous variables. Logistic regression adjusted for age, parity, and BMI.

Interventions

The intervention consisted of early implementation of a Mediterranean diet-based nutritional therapy beginning at 12-14 weeks gestation. Women randomized to the intervention group received specific dietary recommendations emphasizing increased consumption of extra virgin olive oil (EVOO) to at least 40 mL per day and regular pistachio intake of 25-30 grams (approximately one handful) at least 3 days per week. To ensure adequate consumption and adherence, the RCT intervention group received free supplies of 10 liters of EVOO (approximately 1 liter every 10 days) and 2 kilograms of roasted pistachios (approximately 160 grams per week) at visits 1 and 2.

The intervention was reinforced at visit 2 (24-28 weeks gestation) through an approximately 2-hour meeting with a registered dietician who provided detailed Mediterranean diet education and encouraged continued adherence to EVOO and pistachio consumption. The real-world group received identical Mediterranean diet recommendations and dietician counseling but without free provision of EVOO or pistachios, reflecting real-world clinical implementation.

The control group received standard nutritional care consisting of advice to restrict dietary fat intake, with EVOO consumption limited to a maximum of 40 mL per day and nuts consumed less than 3 days per week, as is typically recommended in routine prenatal care. The pistachio was specifically chosen because it is produced in Spain, and the requirement to shell each nut was thought to stimulate the cerebral cephalic phase, potentially enhancing satiety compared to pre-shelled nuts.

Key Findings

The Mediterranean diet intervention significantly reduced gestational diabetes mellitus incidence compared to standard care. In the RCT, GDM occurred in 14.8% of the intervention group versus 25.8% of controls (p=0.021), representing a 28% adjusted relative risk reduction (RR 0.72, 95% CI 0.50-0.97, p=0.037). The real-world implementation group showed similar benefit with 13.4% GDM incidence (p=0.011 vs. control) and 23% relative risk reduction (RR 0.77, 95% CI 0.61-0.97, p=0.008). Both intervention groups demonstrated significantly lower HbA1c levels at 24-28 weeks and 36-38 weeks gestation compared to controls. Among women who developed GDM, those in intervention groups required insulin therapy less frequently: 21.1% in the RCT intervention group and 23.7% in the real-world group versus 35.3% in controls.

Multiple secondary maternal and neonatal benefits were observed. Urinary tract infection rates were significantly lower in both intervention groups (7.0% RCT intervention, 6.3% real-world) compared to control (18.9%, p<0.003 for both). Emergency cesarean section rates were reduced from 7.6% in controls to 1.6% in the intervention group (p=0.020) and 1.8% in the real-world group (p=0.004). Perineal trauma rates decreased from 11.4% in controls to 3.1% in intervention (p=0.009) and 1.6% in real-world groups (p=0.001). In the RCT intervention group specifically, rates of large-for-gestational-age infants (0.8% vs. 6.1%, p=0.020) and small-for-gestational-age infants (0.8% vs. 5.3%, p=0.036) were both significantly reduced compared to controls, though these benefits were not replicated in the real-world group likely due to lower dietary adherence without free food provision.

Comparison with other Studies

These results align with and extend previous research on Mediterranean diet interventions during pregnancy. The St. Carlos group's earlier RCT in a predominantly Caucasian population with lower baseline GDM risk demonstrated similar efficacy for GDM prevention. The current study confirms these benefits extend to Hispanic women, a known high-risk ethnic population with GDM rates approaching 26% in controls compared to typical rates of 10-15% in general populations. This ethnic specificity is important as Bardenheier and colleagues documented that Hispanic women in the United States have the highest age-standardized increase in GDM diagnosis among all ethnic groups.

The findings are consistent with recent meta-analyses showing that lifestyle interventions, particularly those emphasizing low glycemic index foods and healthy fats characteristic of Mediterranean dietary patterns, can reduce GDM incidence. However, this study contrasts with some large trials like LIMIT and UPBEAT, which showed no benefit of dietary interventions in obese pregnant women, possibly because the Mediterranean diet's emphasis on healthy fats and overall dietary quality differs from simple calorie restriction approaches. The real-world validation component of this study addresses a critical gap identified by recent systematic reviews, which noted that while efficacy trials show promise, evidence of effectiveness in routine clinical practice has been limited. The successful translation to real-world practice, even with reduced adherence, suggests Mediterranean diet recommendations are feasible and effective beyond controlled research settings.

The 40% relative risk reduction observed in this Hispanic population is notable given that previous intervention studies in Hispanic women with or at risk for GDM, reviewed by Carolan-Olah and colleagues, showed mixed results. The provision of culturally appropriate, accessible intervention components (EVOO and pistachios rather than foods unfamiliar to the population) may have contributed to better adherence and outcomes. The reduced rates of urinary tract infections, emergency cesarean sections, and perineal trauma align with proposed mechanisms whereby Mediterranean diet components, particularly phenolic compounds in olive oil, may modulate immune function and optimize metabolic health during pregnancy.

Journal Reference

Melero V, García de la Torre N, Assaf-Balut C, et al. Effect of a Mediterranean Diet-Based Nutritional Intervention on the Risk of Developing Gestational Diabetes Mellitus and Other Maternal-Fetal Adverse Events in Hispanic Women Residents in Spain. Nutrients. 2020;12(11):3505. doi:10.3390/nu12113505