Healthy diet during pregnancy—navigating the double-edged sword

See corresponding article on page 540.

The latest US Dietary Guidelines for Americans (2020–2025) highlight pregnancy and lactation as special stages of life for women, and underscore that nutrition plays a vital role before, during, and after pregnancy in order to support the health of mother and child during gestation, lactation, and beyond (1). The specific dietary recommendations for pregnant and lactating women include nutrient-dense foods such as vegetables, fruits, whole grains, seafood, eggs, low-fat dairy products, and poultry.

In this issue of The American Journal of Clinical Nutrition, the study by Lin et al. (2) draws our attention to an unfortunate aspect of maintaining a healthy diet during pregnancy, that is, the recommended maternal dietary intake could be a source of exposure to harmful environmental metals. They studied 1196 women from Project Viva at a mean of 11.3 weeks of gestation. These women completed a validated FFQ (135 food items) as well as measurements of 11 erythrocyte metals [arsenic (As), barium, cadmium, cesium, copper, mercury (Hg), magnesium, manganese, lead, selenium, and zinc]. A diet-wide association study of metals was used to systematically evaluate multiple dietary components (e.g., food items, food groups, and diet scores/patterns) and their associations with the 11 metals. This study confirmed several previously reported diet–metal associations, including seafood with Hg and white rice with As. In addition, it identified fresh fruits, green vegetables, and eggs as new potential sources of exposure to As and Hg. Pregnant women who were classified as having a healthier dietary pattern as measured by higher scores on the Alternative Healthy Eating Index during pregnancy, or adhered more strongly to the Mediterranean and Prudent diets, also had higher concentrations of erythrocyte As and Hg, whereas those classified as having an unhealthy Western dietary pattern had lower concentrations of erythrocyte Hg.

The study findings by Lin et al. are important, given that the developing fetus is highly sensitive and vulnerable to environmental perturbations, including exposure to poor maternal nutrition and environmental chemicals (3); moreover, pregnant mothers are also at risk of poor health outcomes related to the same exposures. This study will also stimulate many new inquiries that warrant additional investigations. Although Project Viva is a prospective cohort study, this analysis was cross-sectional. Prospective analyses with repeated assessments of dietary intake and biomarkers of exposure are needed to better understand the temporal and dose–response relation between dietary intake and metal exposure at different stages of pregnancy. This study examined 11 metals. However, foods can also be sources of other environmental contaminants. For example, a study of 6 European countries (4) assessed the consumption of 7 food groups and the blood concentrations of environmental contaminants, and found that fish consumption was related not only to Hg and As but also to higher exposure to polychlorinated biphenyls (PCBs) and per- and polyfluoroalkyl substances (PFAS), whereas fruit consumption was a source of exposure to organophosphate pesticides. Therefore, future studies should extend their analyses beyond metals. The Lin et al. study did not link their findings with health outcomes, which is important for risk and benefit assessment. There is a growing appreciation about the impact of the complex interplay of nutrients and toxic chemicals on health outcomes. For example, in a previous Project Viva analysis, Oken et al. (5) showed the nutritional benefits as well as the risks of maternal fish intake for child cognition at age 3 y. A preconception cohort study in Chinese women suggested that adequate B-vitamin status could counteract the adverse reproductive effect of 1,1,1-trichloro-2,2,bis(p-chlorophenyl)ethane (DDT) exposure (6). And a study in the Boston Birth Cohort (a primarily urban, low-income, minority population in Boston, MA) showed the protective effect of adequate maternal folate concentrations against intergenerational metabolic risk associated with lead exposure (7). Looking ahead, future studies need to further integrate dietary and chemical exposure assessments, along with longitudinal evaluation of pertinent health outcomes (8). The Lin et al. study was conducted in a New England–based, predominantly white, highly educated, affluent population. Future studies need to include diverse populations with regards to race/ethnicity (especially understudied, underrepresented, and underreported populations), socioeconomic status, geographic location, lifestyle, and nutritional status to deepen our understanding of risks from exposures and diet–chemical exposure–health outcome associations within and across population subgroups.

From a clinical care perspective, the Lin et al. study findings highlight the need to consider both the benefits and risks of food exposures when providing dietary advice to pregnant women. This study does not diminish the importance of eating a healthy diet before, during, and after pregnancy, nor does it contradict the 2020 US dietary recommendations for pregnant women. Rather, this study raises our awareness that what are deemed healthy foods could be contaminated by environmental chemicals and, thus, could lead to unintended adverse exposures. However, to what degree these exposures could affect maternal and fetal health will depend on a multitude of factors, including but not limited to type, dose, timing of exposure, individual susceptibility (including genetic determinants of metal metabolism), coexposure to other chemicals, pre-existing medical conditions, and baseline nutritional status. In the absence of systematic research and evidence-based clinical guidelines, pregnant women and health care providers will have to navigate this double-edged sword. Both parties need to have sound knowledge about what constitutes a healthy diet and likely environmental contaminants and should be vigilant about choosing nutritious foods with low risk of contamination. However, some women lack choice of and access to healthier foods. It is also unlikely that any women will be able to eliminate all potential exposures, because many toxic chemicals are ubiquitous in the environment and could come from sources other than foods, including drinking water, soil, and ambient air (9). Biomonitoring (e.g., measuring chemicals in blood or urine samples) offers an objective assessment of individual overall exposure and body burden. The biomonitoring conducted by NHANES (10) and findings from multiple US birth cohort studies have shown a high percentage of detectable toxic metals in US populations. This study and the growing body of literature on dietary–toxic exposure relations again raise the question: should environmental chemical screening be part of regular preconception and prenatal care? Timely recognition of toxic exposures and removal of contamination sources can prevent adverse developmental effects to the fetus and long-term consequences for both child and mother.

Overall, the study by Lin et al. underscores that foods are vessels of essential nutrients as well as toxic environmental contaminants. It also reinforces current efforts to identify and reduce the root sources of environmental pollution and to enhance environmental and food safety standards. More research is warranted to better understand the impact of the complex interplay between nutrients and environmental chemicals on maternal and child health. Screening of prevalent environmental contaminants in mothers before and during pregnancy can inform individual risk assessment and the development of targeted nutritional interventions and environmental abatement. All these efforts would represent an important advancement in public health and will benefit both current and future generations.

Notes

Supported by Maternal and Child Health Bureau of Health Resources and Services Administration grant UJ2MC31074 (to XW) and NIH grants R01HD086013, 2R01HD041702, R01HD098232, R01ES031272, and R01ES031521 (to XW). The content of this editorial is solely the responsibility of the author and should not be construed as the official position or policy of, nor should any endorsements be inferred by, any of the funding agencies.