2025 Journal Article

Early pregnancy blood heavy metal(loid)s and low sea-fish consumption in relation to risk of gestational diabetes mellitus among Shanghai Birth cohort (SBC) women

Q: What is the link between mercury exposure during pregnancy and gestational diabetes?

Based on findings from the large Shanghai Birth Cohort, a prospective study, higher blood mercury levels in early pregnancy were associated with an increased risk of developing gestational diabetes mellitus (GDM). These higher mercury levels were also observed in women with preexisting diabetes mellitus (PDM). This conclusion is supported by several key statistical findings from the research: Higher Mercury Levels - Women diagnosed with GDM and PDM had higher geometric mean blood mercury (Hg) concentrations (1.88 µg/L and 2.18 µg/L, respectively) compared to women with normal glucose levels (1.74 µg/L). Increased GDM Risk - After adjusting for other factors, women in the group with the highest one-fifth (quintile) of mercury exposure had a 1.66-fold higher odds of developing GDM compared to those in the lowest quintile. Increased PDM Risk - The link was even more pronounced for preexisting diabetes. Women in the highest quintile of mercury exposure had a 7.10-fold higher odds of having PDM compared to those in the lowest quintile. However, the researchers note that since mercury was measured after the PDM was diagnosed, this finding shows a strong association but cannot be interpreted as a causal link. Blood Glucose Impact - Higher blood mercury levels were also directly associated with higher plasma glucose levels measured at all three points (fasting, 1-hour, and 2-hour) during the oral glucose tolerance test (OGTT).

Q: How can mercury increase the risk of gestational diabetes?

The study suggests that the link between mercury exposure and gestational diabetes is biologically plausible, based on mercury's known effects on the body's cells and metabolism. The potential mechanisms can be understood in three steps: 1. Oxidative Stress - Mercury is a toxic metal known to cause oxidative stress and systemic inflammation. It does this by damaging the active sites of the body's natural antioxidant systems (specifically, sulfhydryl groups), leading to cellular damage. 2. Pancreatic Cell Damage - This inflammation and stress can directly impact the pancreas. It may inhibit the release of insulin from the pancreas's crucial β-cells and potentially cause dysfunction or damage to these cells. 3. Metabolic Disruption - This damage to the pancreas's ability to produce and release insulin can lead to insulin resistance and broader disturbances in glucose metabolism, which are the defining characteristics of diabetes. These findings are consistent with other research that has linked mercury exposure to an increased risk of type 2 diabetes in general populations.

Q: What are the common sources of mercury exposure mentioned in the study?

While the study does not catalog all possible sources, it identifies fish consumption as a primary dietary source of mercury for the participants. The research established a direct relationship between the frequency of fish consumption and blood mercury levels. A higher intake of both sea-fish and river-fish was associated with higher mercury concentrations in the blood. To illustrate this, the study found that among mothers in the highest quintile for blood mercury, 12.73% consumed sea-fish more than three times per week. In contrast, only 2.25% of mothers in the lowest mercury quintile ate sea-fish that frequently.

Q: If fish is a source of mercury, should pregnant women avoid it?

No. The study's findings do not support avoiding fish. In fact, the research identified a crucial counteracting effect, where higher sea-fish consumption appeared to protect against the risks associated with mercury. This critical nuance is one of the study's most significant findings. The positive association between mercury and an increased risk of GDM and PDM was statistically significant only among women who ate sea-fish less than once per week. For women who consumed sea-fish more frequently, the increased risk from mercury was not statistically significant. The researchers suggest this is because fish are a principal source of beneficial nutrients like DHA and EPA (long-chain n-3 polyunsaturated fatty acids), which are known to reduce various health risks. For this specific population, the beneficial effects of these key nutrients appear to outweigh the potential harm from the relatively low levels of mercury exposure found in their diet.

Q: What is the key takeaway from this research for diet during pregnancy?

This study provides important insights into managing dietary risks during pregnancy, highlighting a balance between contaminant exposure and nutritional benefits. The key public health implications can be summarized in two points: Mercury as a Risk Factor - The research confirms that mercury exposure is a potential risk factor for gestational diabetes, even at the relatively low levels observed in this study, which are common in many general populations. The Importance of Fish Choice - The study's conclusion strongly supports the dietary recommendation for pregnant women to consume fish. The counteracting effect of higher sea-fish consumption suggests that the nutritional benefits can outweigh the risks. The findings reinforce advice to choose "recommended fish"—those known to be high in beneficial nutrients like long-chain n-3 polyunsaturated fatty acids but low in mercury—to potentially help protect against GDM.

Does exposure to mercury and other heavy metals during early pregnancy increase a woman's risk of developing gestational diabetes, and does eating fish make this better or worse?

DOI: 10.1016/j.envint.2025.109959 PubMed ID: 41314136

College of Health researcher(s)

Sarah Rothenberg

College unit(s)

Methylmercury Lab

Environmental and Occupational Health

Public Health

School of Nutrition and Public Health

Center for Global Health

Abstract

Background

Gestational diabetes mellitus (GDM) is a common metabolic complication during pregnancy which increase both maternal and fetal adverse outcomes. Selected heavy metal(loid)s may lead to insulin resistance and increase risk of GDM. We sought to investigate associations between maternal heavy metal(loid)s exposure, sea-fish consumption and GDM risk.

Methods

In 2174 women, including 686 with GDM, 48 with preexisting diabetes mellitus (PDM), and 1440 euglycemic pregnancies from the Shanghai Birth Cohort, we measured maternal whole blood concentrations of 13 metal(loid)s including mercury (Hg), arsenic, cadmium, lead, magnesium, calcium, manganese, iron, copper, zinc, selenium, rubidium, and strontium at early pregnancy. Logistic regressions were used to evaluate the associations of prenatal blood metal(loid)s with GDM and PDM, and linear regressions for plasma glucose (fasting, 1-hour, and 2-hour) during 75 g-oral glucose tolerance test (OGTT), with stratification by prenatal fish consumption and adjusting for pertinent covariates.

Results

Early pregnancy blood Hg concentrations were higher in women with GDM and PDM versus euglycemic (geometric mean was 1.88, and 2.18 versus 1.74 μg/L). Compared with the lowest quintile of Hg, the highest Hg quintile was associated with 1.66-fold higher odds of GDM (95 % CI: 1.17, 2.37, P < 0.01) and 7.10-fold higher odds (95 % CI: 1.82, 47.06, P < 0.05) of PDM. Consistently, blood Hg was also positively associated with higher plasma glucose at fasting, 1-hour, and 2-hour during OGTT. No significant associations were found between other measured metal(loid)s and GDM or PDM. The positive Hg-associations with GDM, PDM， and plasma glucoses were stronger among women with low fish intake (<1 time/week) and not statistically significant among higher sea-fish intake (interaction test P < 0.05).

Conclusions

In this prospective birth cohort, despite relatively low maternal Hg exposure, maternal Hg during early pregnancy was positively associated with increased risk of GDM and PDM, while higher sea-fish consumption showed counteracting effect.

Li, Y., Zhang, T., Ma, J., Ma, R., Liu, J., Xu, J., Rothenberg, S.E., Yan, C., Zhang, J., Luo, Z., Wang, X., Ouyang, F. (2025) Early pregnancy blood heavy metal(loid)s and low sea-fish consumption in relation to risk of gestational diabetes mellitus among Shanghai Birth cohort (SBC) womenEnvironment International206

FAQ: Mercury Exposure in Pregnancy and Gestational Diabetes

What is the link between mercury exposure during pregnancy and gestational diabetes?

Based on findings from the large Shanghai Birth Cohort, a prospective study, higher blood mercury levels in early pregnancy were associated with an increased risk of developing gestational diabetes mellitus (GDM). These higher mercury levels were also observed in women with preexisting diabetes mellitus (PDM).

This conclusion is supported by several key statistical findings from the research:

Higher Mercury Levels: Women diagnosed with GDM and PDM had higher geometric mean blood mercury (Hg) concentrations (1.88 µg/L and 2.18 µg/L, respectively) compared to women with normal glucose levels (1.74 µg/L).
Increased GDM Risk: After adjusting for other factors, women in the group with the highest one-fifth (quintile) of mercury exposure had a 1.66-fold higher odds of developing GDM compared to those in the lowest quintile.
Increased PDM Risk: The link was even more pronounced for preexisting diabetes. Women in the highest quintile of mercury exposure had a 7.10-fold higher odds of having PDM compared to those in the lowest quintile. However, the researchers note that since mercury was measured after the PDM was diagnosed, this finding shows a strong association but cannot be interpreted as a causal link.
Blood Glucose Impact: Higher blood mercury levels were also directly associated with higher plasma glucose levels measured at all three points (fasting, 1-hour, and 2-hour) during the oral glucose tolerance test (OGTT).

How can mercury increase the risk of gestational diabetes?

The study suggests that the link between mercury exposure and gestational diabetes is biologically plausible, based on mercury's known effects on the body's cells and metabolism. The potential mechanisms can be understood in three steps:

Oxidative Stress: Mercury is a toxic metal known to cause oxidative stress and systemic inflammation. It does this by damaging the active sites of the body's natural antioxidant systems (specifically, sulfhydryl groups), leading to cellular damage.
Pancreatic Cell Damage: This inflammation and stress can directly impact the pancreas. It may inhibit the release of insulin from the pancreas's crucial β-cells and potentially cause dysfunction or damage to these cells.
Metabolic Disruption: This damage to the pancreas's ability to produce and release insulin can lead to insulin resistance and broader disturbances in glucose metabolism, which are the defining characteristics of diabetes. These findings are consistent with other research that has linked mercury exposure to an increased risk of type 2 diabetes in general populations.

What are the common sources of mercury exposure mentioned in the study?

While the study does not catalog all possible sources, it identifies fish consumption as a primary dietary source of mercury for the participants.

The research established a direct relationship between the frequency of fish consumption and blood mercury levels. A higher intake of both sea-fish and river-fish was associated with higher mercury concentrations in the blood. To illustrate this, the study found that among mothers in the highest quintile for blood mercury, 12.73% consumed sea-fish more than three times per week. In contrast, only 2.25% of mothers in the lowest mercury quintile ate sea-fish that frequently.

If fish is a source of mercury, should pregnant women avoid it?

No. The study's findings do not support avoiding fish. In fact, the research identified a crucial counteracting effect, where higher sea-fish consumption appeared to protect against the risks associated with mercury.

This critical nuance is one of the study's most significant findings. The positive association between mercury and an increased risk of GDM and PDM was statistically significant only among women who ate sea-fish less than once per week. For women who consumed sea-fish more frequently, the increased risk from mercury was not statistically significant.

The researchers suggest this is because fish are a principal source of beneficial nutrients like DHA and EPA (long-chain n-3 polyunsaturated fatty acids), which are known to reduce various health risks. For this specific population, the beneficial effects of these key nutrients appear to outweigh the potential harm from the relatively low levels of mercury exposure found in their diet.

What is the key takeaway from this research for diet during pregnancy?

This study provides important insights into managing dietary risks during pregnancy, highlighting a balance between contaminant exposure and nutritional benefits. The key public health implications can be summarized in two points:

Mercury as a Risk Factor: The research confirms that mercury exposure is a potential risk factor for gestational diabetes, even at the relatively low levels observed in this study, which are common in many general populations.
The Importance of Fish Choice: The study's conclusion strongly supports the dietary recommendation for pregnant women to consume fish. The counteracting effect of higher sea-fish consumption suggests that the nutritional benefits can outweigh the risks. The findings reinforce advice to choose "recommended fish"—those known to be high in beneficial nutrients like long-chain n-3 polyunsaturated fatty acids but low in mercury—to potentially help protect against GDM.