When investigating the link between mercury exposure in pregnant Norwegian women and child language and communication skills at 5 years of age, researchers from the Norwegian Institute of Public Health (NIPH) were surprised to find a positive rather than a negative association. .
They also found that maternal prenatal seafood intake showed a positive association with the children’s language capability.
“We didn't find any negative associations of mercury and language outcomes at 5 years when we did linear regressions on the whole study population,” commented first author Kristine Vejrup, Ph.D student at NIPH. “As a matter of fact we found positive associations which indicate that mercury is acting as a proxy for the positive effects of seafood.”
Nevertheless, when subjects were grouped into consumption levels of either below or above 400 grams/ week (g/w), the favourable association was only seen in the women with intakes up to the 400 g/w threshold.
“Upon stratification into low and high seafood intake groups, the positive association of mercury exposure disappeared in the high intake group, indicating that those with a high seafood intake are moving towards the tipping point where the beneficial effects of seafood can be outweighed by the adverse effects of mercury,” wrote the researchers.
The study population consisted of 38,581 women from the Norwegian Mother and Child Cohort Study (MoBa). The scientists took blood samples during week 17 of their pregnancy term from 2,239 of the women to measure mercury levels. The women also completed a food frequency questionnaire (FFQ) to assess seafood intake, and dietary mercury exposure. (Exposure calculated from the FFQ correlated well with seafood intake and with blood levels of mercury).
The researchers calculated linear regressions between mercury blood concentrations, mercury dietary exposure and seafood intake with three measures of language and communication scales.
Additional analysis included stratifying seafood consumption into intakes below and above 400 g/w. (This cut-off point was used as the Norwegian Directorate of Health recommends a fish intake of 350-400 g/w).
The scientists also performed matched sibling analysis to eliminate possible uncontrolled genetic- or family-related confounders.
In the sibling analysis, “We found a really small negative effect of mercury in the 90th percentile on one of our language outcomes. It doesn't have any clinical effects, but it indicates that when you're in the high exposure group then might be some negative impact at 5 years,” explained Vejrup.
“This indicates that prenatal low level mercury exposure still needs our attention,” suggested the researchers.
In the light of the findings, “Our recommendations are to follow the [government] guidelines that are already there; to continue to eat fish, but eat fish that is low in mercury and to avoid mercury-high species like tuna and local Norwegian species of predatory fish. We don't want pregnant women or people in general to stop eating fish and seafood,” Vejrup concluded.
These recommendations were echoed by Dr. Harry Rice, Vice President of Regulatory and Scientific Affairs at GOED (Global Organization for EPA and DHA Omega 3s.
“The current findings corroborate past results demonstrating that the benefits of seafood consumption outweigh any potential risk from mercury exposure. From a public health perspective, women need to be encouraged to increase seafood consumption during pregnancy, not warned to limit consumption because of unfounded fears,” he said.
Source: Environment International
Published online. DOI: 10.1016/j.envint.2017.10.008
Prenatal mercury exposure, maternal seafood consumption and associations with child language at five years
Authors: Kristine Vejrup et al