Should Women of Childbearing Age Eat a Plant-Based Diet?

According to research results published in the American Journal of Obstetrics and Gynecology, consuming a primarily plant-based diet is associated with a lower risk of developing hypertensive disorders of pregnancy.

The prospective cohort study followed 11,459 women older than 18 years and evaluated their diet from the beginning using a validated questionnaire about the frequency and quality of plant-based foods. The participants had taken part in the in the Nurses’ Health Study II (1991–2009). From responses on the questionnaire, the investigators calculated the plant-based diet index (PDI) even among participants with an omnivorous diet. A higher score indicated greater adherence to the PDI.

"We wanted to know how one's diet leading up to pregnancy influences the pregnancy, so we monitored women for virtually their entire reproductive life — almost 20 years — and gained an awareness of their typical diet before pregnancy," study author Jorge E. Chavarro, MD, ScD, told Medscape Spanish Edition. Chavarro is a professor of nutrition, epidemiology, and medicine at Harvard University's Medical School and School of Public Health in Cambridge, Massachusetts. He researches how nutrition and lifestyle influence reproductive health and overall lifelong health in women.

Analysis of the data from the Nurses' Health Study II revealed that as the proportion of animal products in diets decreased and the proportion of plant-based products increased, the risk of women experiencing hypertensive disorders of pregnancy decreased as well. Women in the highest PDI quintile had a significantly lower risk of hypertensive disorders of pregnancy in comparison with those in the lowest quintile (relative risk [RR], 0.76). This association was slightly stronger for pregnancy-related hypertension (RR, 0.77) than for preeclampsia (RR, 0.80).

Women in the highest PDI quintile had a 24% lower risk of hypertensive disorders of pregnancy than those in the lowest quintile; the risk of pregnancy-related hypertension decreased in a linear fashion as PDI increased, while the relationship of PDI to preeclampsia was restricted to women in the quintile with the highest adherence.

"It was clearer for pregnancy-related hypertension than for preeclampsia, but a diet made up primarily of plant-based foods seemed to be protective for both," said Chavarro. He added that in addition to the problems these conditions cause during pregnancy, both increase the risk of subsequently developing other chronic diseases. "Could it be that modifiable lifestyle factors before and during pregnancy may not only help reduce problems during gestation but also prevent women's health problems years later? That was the general motivation for this study."

Mercedes Sotos-Prieto, PhD, a researcher at the Autonomous University of Madrid and an associate professor at Harvard University's School of Public Health, told Medscape that the study's methodology was very robust and that the investigators utilized appropriate statistical techniques for the analysis. She highlighted the fact that they used a validated food frequency questionnaire. She believes the study is also important because of the population group it focused on. "There has always been greater resistance when it comes to the diet of pregnant women, and the same is true for older adults. But we have seen that this type of diet, if it's a quality diet, may be associated with health benefits." She did not participate in the study.

Sotos-Prieto has a doctorate in nutritional epidemiology and public health. She works with large epidemiologic cohorts, such as the cohort of American nurses on which this study was based, and ENRICA, a cohort that is representative of the Spanish population and the population of older adults. She is the author of other studies that, like this one, found an association between a plant-based diet and a lower risk of frailty, both in the study involving American nurses and in a study involving a cohort of individuals aged 60 years or older in Spain (ENRICA-1).

Sotos-Prieto is also principal investigator on a project assessing the risk of cardiovascular disease based on modifiable lifestyles. For this project, the researchers created a tool, the healthy heart test, that can be used to evaluate diet quality "in 5 minutes, because we all know that doctors don't have any time." She thinks this test could be implemented in clinical practice to identify lifestyle behaviors that can be improved, such as by replacing refined cereals with whole grains or increasing legume consumption.

Tomatoes and French Fries

The greatest benefit of a plant-based diet comes from the diet overall, not from any single food item. That said, these studies use a scoring system to reflect which items are healthy and which are not.

Diet was assessed every 4 years, starting in 1991, using a semiquantitative food frequency questionnaire that recorded the consumption of 131 foods and drinks during the previous year. The researchers determined the average frequency with which participants consumed each food. Eighteen food groups were sorted into three categories: healthy plant-based foods (whole grains, fruits, vegetables, nuts, legumes, vegetable oils, tea, and coffee), unhealthy plant-based foods (fruit juices, refined grains, potatoes, sugary drinks, sweets, and desserts), and animal-based foods (dairy, eggs, fish or shellfish, meat, and various foods of animal origin).

Healthy plant-based foods were given positive scores, while less healthy plant-based foods and the animal-based food groups were given negative scores. The consumption of each food group was classified into PDI using quintiles.

Women in the highest PDI quintile had a significantly lower risk for hypertensive disorders of pregnancy compared with women in the lowest quintile. There was a negative dose-response relationship between PDI and risk of the disease. "A vegetarian diet isn't necessarily healthier than a nonvegetarian diet if it's made up of superfluous foods like French fries and soft drinks," said Sotos-Prieto. "The difference lies in the quality of the plant-based foods. That's what makes the difference between a healthy and an unhealthy diet."

Give Up Meat?

Chavarro said that removing meat from his dinner menu 22 years ago was one of the hardest things he ever did. "Now, it's no problem," he said. But he understands that there are people for whom changing the diet by replacing animal products with nonanimal products is difficult. But meat need not be entirely abandoned.

"The women in the highest quintile aren't necessarily vegetarian or vegan, but they consume much fewer animal-based foods than the others," he noted. He added that vegetarian or vegan diets are not incompatible with a healthy pregnancy. "All vegans know how to get vitamin B12 from supplements."

Diet or Weight Loss?

Much of the benefit observed in the study appears to be related to better weight control. The body mass index between dietary assessment and pregnancy accounted for 39% of the relationship between PDI and hypertensive disorders of pregnancy and 48% of the relationship between PDI and pregnancy-related hypertension.

"Part of the association seems to be explained by better weight control over long periods," explained Chavarro. Women who adopted diets with more plant-based foods gained weight more slowly than those who consumed more animal-based foods. "They are different in terms of their weight trajectory over many years. So, part of the association that we observe is related to better long-term weight control. But the other half of the association is attributable to the diet itself and not necessarily to weight." The authors suggest mechanisms of action such as endothelial dysfunction, inflammation, or blood pressure before pregnancy to explain the association.

Sotos-Prieto believes that this point is "extremely relevant." In her opinion, it reveals that controlling weight at the start of pregnancy is important for pregnant women. Weight control may also improve other factors, like gestational diabetes. "I think preventive measures should focus on that. These results show that interventions are needed to increase the likelihood of going into pregnancy with an appropriate weight. And this includes modifying diet."

Generalizable Results?

More than 90% of the participants in the Nurses' Health Study were White, not Hispanic. Can the results be extrapolated to other populations? "The answer: the study needs to be repeated in other populations," said Chavarro, "and that's going to take time. But even without that information, I think we can use this study to inform other populations, regardless of ethnicity."

Sotos-Prieto admitted that this hypothesis has not yet been tested in the Spanish population, but she is the author of a similar study that followed nearly 12,000 Spanish adults for a decade using the same PDI. In this study, every 10-point increase in PDI was associated with a 14% lower risk of mortality from any cause (hazard ratio [HR], 0.86) and a 37% lower risk of death from cardiovascular disease (HR, 0.63). She also believes that the recommendations derived from the study could be generalized to other populations "as long as each country's culture is taken into account, to see how it can be culturally adapted. If it's a population that consumes a lot of refined cereals, for example, make small changes to whole grains."

Weighing the Evidence

The study has strengths and limitations, owing to its methodology, and Chavarro himself recognizes that "in terms of hypertensive disorders of pregnancy specifically, this won't be the last word." But there is a pressing need to find answers.

The American College of Obstetricians and Gynecologists and the World Health Organization encourage women to follow healthy diets before and during pregnancy. But they provide little guidance on what constitutes a healthy diet when it comes to minimizing the risks of adverse pregnancy outcomes. "They are quite ambiguous and vague," said Chavarro.

These new findings suggest that plant-based diets may be one such strategy, particularly because some evidence was found that these diets may be beneficial for women older than 35 years, who are considered a high-risk group.

"There are certainly many ways to eat healthily, but if we think about these pregnancy complications that can have serious consequences for the mother and the fetus, we might consider this as a healthy diet option," Chavarro noted.

But is the evidence robust enough to recommend that patients make changes? "Ideally, there will be more studies," stated Chavarro. "There are two ways to understand the problem. One is not making recommendations until you have three controlled clinical trials, which, even with the willingness and funding to do so, will take 15 to 20 years. But if we have to provide the best available information to those who need it today, I think these are solid results for guiding behavior.

"It's always better if we can make decisions based on solid, incontrovertible information. But it's not always available, and you must learn to live in both worlds and make decisions with uncertainties," he concluded.

Sotos-Prieto and Chavarro have disclosed no relevant financial relationships.

Follow Roxana Tabakman of Medscape Spanish Edition on X: @RoxanaTabakman.

This article was translated from the Medscape Spanish Edition.

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