Iodine Status Modifies the Association between Fluoride Exposure in Pregnancy and Preschool Boys’ Intelligence

Abstract

In animal studies, the combination of in utero fluoride exposure and low iodine has greater negative effects on offspring learning and memory than either alone, but this has not been studied in children. We evaluated whether the maternal urinary iodine concentration (MUIC) modifies the association between maternal urinary fluoride (MUF) and boys’ and girls’ intelligence. We used data from 366 mother–child dyads in the Maternal–Infant Research on Environmental Chemicals Study. We corrected trimester-specific MUF and MUIC for creatinine, and averaged them to yield our exposure variables (MUFCRE, mg/g; MUICCRE, µg/g). We assessed children’s full-scale intelligence (FSIQ) at 3 to 4 years. Using multiple linear regression, we estimated a three-way interaction between MUFCRE, MUICCRE, and child sex on FSIQ, controlling for covariates. The MUICCRE by MUFCRE interaction was significant for boys (p = 0.042), but not girls (p = 0.190). For boys whose mothers had low iodine, a 0.5 mg/g increase in MUFCRE was associated with a 4.65-point lower FSIQ score (95% CI: −7.67, −1.62). For boys whose mothers had adequate iodine, a 0.5 mg/g increase in MUFCRE was associated with a 2.95-point lower FSIQ score (95% CI: −4.77, −1.13). These results suggest adequate iodine intake during pregnancy may minimize fluoride’s neurotoxicity in boys.

Cohort profile update: The Canadian Maternal–Infant Research on Environmental Chemicals Child Development study (MIREC‐CD PLUS)

“…A 1‐mg/L increase in maternal urinary fluoride concentrations, averaged across three trimesters, was associated with a 4.49 point lower Full Scale IQ (FSIQ) score (95% CI, −8.38 to −0.60) in boys 51 . A stronger association with IQ was seen for prenatal (maternal) versus concurrent (childhood) fluoride exposure, and among those mothers with low iodine status (<200 μg/g) 50,52 . In addition, increasing fluoride levels in tap water during the first 6 months of the child's life were associated with lower Performance IQ in both formula and breastfed infants, even after controlling for prenatal fluoride exposure 53 .…”

“…Several investigations have examined the associations between prenatal exposure to a number of chemicals and child IQ. Prenatal concentrations of certain flame retardants (polybrominated biphenyl ethers), 47 organophate pesticides, 48 lead 49 and fluoride 50–52 all showed associations with lower IQ scores at approximately 3 years of age, mainly in boys. A 1‐mg/L increase in maternal urinary fluoride concentrations, averaged across three trimesters, was associated with a 4.49 point lower Full Scale IQ (FSIQ) score (95% CI, −8.38 to −0.60) in boys 51 .…”

“…Prenatal concentrations of certain flame retardants (polybrominated biphenyl ethers), 47 organophate pesticides, 48 iodine status (<200 μg/g). 50,52 In addition, increasing fluoride levels in tap water during the first 6 months of the child's life were associated with lower Performance IQ in both formula and breastfed infants, even after controlling for prenatal fluoride exposure. 53 In a latent profile analysis of 27 different gestational exposures, the profile high in monoethylphthalate and smoking showed inverse associations with all IQ outcomes while the profile high in persistent organic pollutants showed inverse association only with Verbal IQ.…”

“…lead49 and fluoride[50][51][52] all showed associations with lower IQ scores at approximately 3 years of age, mainly in boys. A 1-mg/L increase in maternal urinary fluoride concentrations, averaged across three trimesters, was associated with a 4.49 point lower Full Scale IQ (FSIQ) score (95% CI, −8.38 to −0.60) in boys 51.…”

A Scoping Review of Iodine and Fluoride in Pregnancy in Relation to Maternal Thyroid Function and Offspring Neurodevelopment

“…One Canadian cohort study ( n = 366) measured both maternal UIC and UFC and neurodevelopment in offspring [ 153 ]. I status modified the relationship between maternal UFC and full-scale IQ (3–4 y of age by Wechsler Preschool and Primary Scales of Intelligence-III) in boys [ 153 ]. An interaction between UIC, UFC, and sex was observed with boys but girls remained unaffected [ 153 ].…”