November/December 2019 Issue

Prenatal Nutrition: Mediterranean Diet for GDM
By Jamie Santa Cruz
Today’s Dietitian
Vol. 21, No. 11, P. 18

While it’s unclear why, research shows this healthful eating pattern can help prevent gestational diabetes mellitus.

One of the most common complications associated with pregnancy is gestational diabetes mellitus (GDM), defined as diabetes or glucose intolerance that occurs for the first time during pregnancy.1 The condition occurs in 15% to 20% of pregnancies,2 but prevalence is on the rise, due in part to increasing rates of obesity and women waiting longer to become pregnant.3,4

GDM is a significant concern because it’s associated with several negative outcomes, including preeclampsia, large infant birth weight, and stillbirth.5 In addition, mothers who develop GDM have a substantially greater risk of progressing to type 2 diabetes after the birth of their infants.6

The good news? Recent research suggests that consumption of a Mediterranean-style diet before and during pregnancy has significant potential to reduce the likelihood of developing GDM.

Who’s at Risk?
Several factors increase the risk of developing GDM, including advanced maternal age, obesity, and excessive gestational weight gain.7 Ethnicity is another risk factor: Women of South or Southeast Asian ethnicity appear to be at the highest risk, but Hispanics, African Americans, Native Americans, Pacific Islanders, and Indigenous Australians are also at greater risk compared with whites. Importantly, it’s not just ethnicity but also country of birth that matters; women of high-risk ethnicities who have immigrated to Western countries are at higher risk than women of these same ethnicities who were born in Western countries.8

The Research
Repeated epidemiologic studies in recent years have examined the association between a Mediterranean-style diet and GDM, and these studies—which have been conducted in the United States, Australia, and various countries around the Mediterranean Sea—consistently have shown that adherence to a Mediterranean diet before or during pregnancy is correlated with a 15% to 38% lower likelihood of developing GDM.9-13 The weakness of these epidemiologic studies, as with all epidemiologic research, is that they demonstrate only correlation, not causation. However, the following two new randomized controlled trials have been published since 2017 and do demonstrate a causal relationship.

The St. Carlos GDM Prevention Study
The first of the two trials was the St. Carlos GDM Prevention Study in Spain, which involved 874 participants, all of whom were healthy and had normal glycemic levels at the start of the study. Between eight and 12 weeks of pregnancy, all participants were randomized into either a Mediterranean diet intervention group or a control group. Both groups were given the same basic dietary recommendations consistent with a Mediterranean dietary pattern: two or more servings of vegetables, three or more servings of fruit, and three servings of skimmed dairy products per day, and two to three servings of legumes per week. Both groups also were instructed to eat whole grain cereals, consume a moderate to high quantity of fish, consume minimal red and processed meat, and avoid refined/processed foods as well as sugary soft drinks and fruit juices.14

Women in the control group were advised to restrict dietary fat, including olive oil and nuts. By contrast, those in the intervention group were advised to consume at least 40 mL extra virgin olive oil and a handful (25 g to 35 g) of pistachios each day, and the researchers provided participants with these two foods as part of the study. Nutrition questionnaires and biomarker analyses were used to determine adherence to the diet.14

Those in the intervention group proved to have a 25% lower risk of developing GDM than those in the control group (23.4% of the women in the control group developed GDM vs 17.1% of those in the intervention group). The intervention group also had lower rates of premature birth, lower risk of emergency cesarean section, and lower rates of perineal trauma. The women also gained significantly less weight during pregnancy, and their newborns were more likely to be a normal size for their gestational age.14

Building on the findings from this trial, the same Spanish research group published a follow-up intervention study earlier this year, involving 932 participants. Once again, all of the participants enrolled were healthy and had normal glycemic levels, but this study didn’t include a control group. Instead, all of the participants received the intervention, which consisted of instructions to follow the same Mediterranean dietary pattern as those in the intervention group from the earlier trial. Again, the women were advised to consume plentiful amounts of extra virgin olive oil and consume nuts every day, but this time participants had to supply their own olive oil and nuts.15

In this follow-up study, 13.9% of the participants developed GDM—a rate comparable to the rate of GDM in the intervention group from the earlier trial. The obvious limitation of this follow-up study is the lack of a control group. Participants were compared with the two cohorts from the previous study—a comparison of consecutive rather than parallel groups. Still, the findings suggest that merely counseling pregnant women to follow a Mediterranean diet—without actually supplying any of the ingredients—is effective for reducing GDM risk.15

In July, researchers in the United Kingdom published findings from the ESTEEM trial, a second randomized controlled trial conducted in five UK maternity centers. This study enrolled 1,252 women who—unlike in the Spanish trial—had metabolic risk factors that put them at higher risk of GDM. All participants were randomly assigned to follow either a Mediterranean or a control diet; the instructions for those on the Mediterranean diet were similar to those for the intervention in the Spanish trial.16

As in the Spanish trial, the researchers provided both nuts and olive oil to the intervention group. In addition, since participants in the ESTEEM trial came from varied cultural backgrounds, the researchers provided a recipe book featuring recipes that were consistent with the Mediterranean diet but that reflected the traditional cuisines of the study participants.16

Once again, the intervention provided significant benefit: Those in the intervention group were 35% less likely than controls to develop GDM.16

A key strength of the ESTEEM study is that two-thirds of the participants were ethnic minorities—who are at greater risk of GDM than white individuals but also are often harder to engage in lifestyle interventions. The fact that the intervention was effective in this population is encouraging, according to Shakila Thangaratinam, PhD, MRCOG, a professor of maternal and perinatal health at Barts and The London School of Medicine and Dentistry at the Queen Mary University of London, who’s also the senior author of the ESTEEM study. “These women are not used to the Mediterranean-style diet. But they were able to follow it, and we saw a big benefit in reducing gestational diabetes.”

Not only did the Mediterranean diet intervention benefit women of various ethnicities but the two trials also showed that a Mediterranean diet benefits women in various risk categories, says Carla Assaf-Balut, PhD, MSc, a researcher in diabetes and nutrition at the Hospital Clínico San Carlos in Madrid, Spain, and the lead researcher for the St. Carlos GDM Prevention Study. “The results between studies are similar despite the fact that they included samples with different characteristics—women considered high risk (with metabolic risk factors) as well as those considered low risk (with no known disease).”

Why Does It Work?
It’s unclear exactly why the Mediterranean diet is preventive against GDM, since none of the research so far has examined that specific question, but there are several aspects of the diet that could account for the observed benefit. “The prevention against GDM could be thanks to the high-antioxidant, anti-inflammatory capacity of the Mediterranean diet as well as its effect in improving insulin sensitivity and managing weight gain,” Assaf-Balut says. “The Mediterranean diet could prevent GDM through the down-regulation of circulating inflammatory biomarkers and favoring glucose homeostasis. In addition, it could also play an important role in modifying gene expression and regulation of different signaling pathways associated with GDM onset.”

Counseling Strategies for Dietitians
According to Federica Amati, MPH, a PhD candidate and research postgraduate in the department of primary care and public health at Imperial College London, all of the available evidence supports encouraging pregnant women to follow a Mediterranean diet. “More research is needed to understand the mechanisms responsible for the Mediterranean diet’s potential to prevent GDM, but the current evidence is robust enough to recommend the Mediterranean diet to women trying to conceive or already pregnant,” says Amati, who wasn’t involved in either of the recent trials.

The following are among the key pieces of advice dietitians can confidently offer pregnant women:

• Use extra virgin olive oil as the main fat source. According to Assaf-Balut, the ideal is to consume at least four tablespoons per day.
• Eat a handful of nuts (25 g to 30 g) per day. According to Thangaratinam, walnuts, almonds, and hazelnuts in particular are beneficial.
• Include a portion of vegetables in every main meal.
• Choose whole grain over refined cereals.
• Consume two to three servings of fruit daily (but avoid fruit juices).
• Minimize red and processed meats; consume fish or poultry instead.

While adopting a Mediterranean diet in the second trimester or beyond still could have benefits, Assaf-Balut recommends switching as early as possible. The best bet, she says: Begin even before the pregnancy starts. Then both mom and baby will have the maximum amount of time to reap the benefits of the Mediterranean way.

— Jamie Santa Cruz is a health and medical writer in Parker, Colorado.


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