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Background: Seafood provides nutrients that are important for optimal development of the unborn child. However, seafood is also a source of contaminants including mercury (Hg) and methylmercury (MeHg) that may have adverse effects on neurodevelopment of the fetus. Humans are predominantly exposed to MeHg through seafood consumption, however, levels of MeHg vary considerably between species. Objectives: Investigate, in a randomized controlled trial (RCT) during pregnancy, if an increased intake of Atlantic cod (Gadus morhua), a fish species with relatively low levels of MeHg contamination, influences total hair Hg (THHg) concentrations in humans. Methods: Pregnant women (n = 137) were enrolled in the RCT “Mommy’s Food” (2016–2017), which was designed to increase iodine status. Participants were randomly assigned to intervention (400 g of cod fillets per week) or control (continued habitual diet) groups for 16 weeks (gestational week 20–36). THHg concentrations were measured at baseline and post-intervention using thermal decomposition, amalgamation, and atomic absorption spectrophotometry (US EPA method 7473). The trial is registered in ClinicalTrials.gov, NCT02610959. Results: Post-intervention, the intervention group had median (inter-quartile range) THHg concentrations of 554 (392–805) µg/kg, and the control group 485 (341–740) µg/kg (p = 0.186). When adjusting for baseline THHg concentrations, there was a significant difference between the groups in those participants with baseline THHg concentrations below 534 µg/kg. Post-intervention, 8% of the study population exceeded the US EPA reference dose in hair (1,000 µg/kg) (intervention group: n = 6, control group: n = 4). Conclusion: THHg concentrations were generally low in both study groups of pregnant women, despite the relatively high seafood intake. While the intervention with 400 g of cod per week slightly increased THHg concentrations, it did not lead to an increase in number of subjects exceeding the US EPA reference dose; a dose level at which no adverse effects are expected to occur over a period of lifetime exposure.

Background: The toxic effects of prenatal methylmercury (MeHg) include neurological abnormalities and developmental delay of which infants and children are particular susceptible. Studies on the effects of low and moderate exposure show conflicting results. Seafood is the main dietary source of MeHg, but also contributes with nutrients regarded as beneficial for development. Objectives: To measure the change in total hair mercury concentration (THHg) after an intervention of lunch meals with fatty fish or meat in Norwegian preschool children, and to examine the associations between THHg and cognitive function. Methods: Children (n = 232) 4–6 years old were randomized to lunch meals with fatty fish (n = 114) or meat (n = 118) three times a week for 16 weeks. THHg was determined using a Direct Mercury Analyzer, and cognitive function was assessed by the Wechsler Preschool and Primary Intelligence Scale-III (WPPSI-III) pre- and post-intervention. Linear mixed effect models were used to analyze changes in THHg and WPPSI-III scores. Results: The mean (SD) THHg pre-intervention was 0.373 (0.204) mg kg−1. Children in the fish group had an increase in THHg (change 0.162, 95% CI 0.111, 0.213 mg kg−1), whereas children in the meat group had decreased THHg (−0.053, 95% CI −0.103, −0.002 mg kg−1). There were no notable associations between THHg and the WPPSI-III raw scores at baseline or after 16 weeks of the fish/meat intervention. Conclusions: Lunch meals including fatty fish led to a significant increase in THHg, but the values remain below the point of departures used for risk assessment by the EFSA, WHO and US-EPA. We observed no associations between THHg and cognitive function. Keywords: Hair mercury, Methylmercury, Fish consumption, Cognitive development, Preschool children