Maternal Urinary Fluoride Levels of a Large Pregnancy Cohort in the United States: Findings from the ADORE Study.

Introduction

Fluoridation of public water systems (PWS) in the US is considered one of the greatest public health achievements of the 20th century because it reduced the risk of dental caries in young children.^1,2 However, a Cochrane review concluded that the studies on this topic are poor quality, and most were conducted in the 1970s, indicating that more contemporary research on water fluoridation is warranted.³ At this time, the US Department of Health and Human Services¹ (DHHS) recommends a water fluoridation level of 0.7mg/L, as this is considered optimal for preventing dental caries while reducing risk of dental fluorosis.² Water fluoridation is locally mandated in the US; however, some local water fluoridation policies do not always reflect this recommendation. For example, the state of Ohio mandates that PWS maintains water fluoride levels between 0.8 and 1.3mg/L,⁴ while Kansas leaves the decision to each individual PWS.⁵

Fluoride at high exposure levels in early life is known to have a negative effect on neurodevelopment,^6,7 and a recently published monograph from the DHHS found with moderate confidence that higher fluoride exposure (i.e., drinking water that exceeds the 1.5-mg/L recommendation of the World Health Organization) was associated with lower childhood intelligence quotient (IQ).⁷ However, a recent study in Canada associated adverse offspring neurodevelopmental outcomes with maternal urinary fluoride concentration (MUFsg) during pregnancy in a cohort of women consuming water within recommended levels of fluoridation (<0.7mg/L).⁸ Other studies have supported this finding, particularly in cohorts in Mexico^9–11; however, one study, completed in Spain, did find a positive association between pregnancy MUFsg and offspring IQ.¹² Previously, Grandjean et al. developed a safety benchmark dose calculation for MUFsg during pregnancy as related to offspring cognitive outcomes of 0.2mg/L,¹³ but more recently increased the safety benchmark to 0.45mg/L.¹⁴

While MUFsg has been measured in pregnant women in other countries, including Canada,⁸ Poland,¹⁵ India,¹⁶ Spain,¹² and Mexico,^9–11 it has only been reported in pregnant women in the US in two studies conducted in California.^17,18 The primary goal of our study was to measure MUFsg in a large cohort of pregnant women (n=965) who enrolled in a multisite randomized controlled trial in the midwestern US. Since water is the primary source of ingested fluoride, and the only source of intentional fluoride ingestion that is regulated,¹ we also obtained the PWS fluoridation levels for women in the cohort and explored the relationship between MUFsg and PWS fluoridation.

Methods

Results

Participant demographic and social information is shown in Table 1 for the cohort as a whole and by PWS fluoridation <0.7mg/L and >0.7mg/L. Baseline and third trimester MUFsg and fluoride intake from tap water can be found in Table 2. The median PWS water fluoridation level (median: 0.9mg/L; Q1, Q3: 0.8, 0.9) for all participants was above the US Department of Health and Human Services recommendation (Table 1) and ranged from 0.09 to 1.1mg/L.¹ Baseline MUFsg (median: 1.0mg/L; Q1, Q3: 0.6, 1.5) was weakly correlated to water fluoridation level (r_s=0.30; p<0.01; n=928) and fluoride intake estimated from self-reported tap water intake (r_s=0.29; p<0.01; n=728). For women who provided two urine samples (n=307), there was a weak but statistically significant positive correlation between baseline MUFsg obtained prior to 20 wk gestation and MUFsg obtained in the third trimester (r_s=0.43; p<0.01). Table S1 displays demographic and social information, baseline and third trimester MUFsg and specific gravity, water fluoridation level, and fluoride intake between participants providing third trimester urine samples and participants who did not.

MUFsg levels and fluoride intake levels were higher in those consuming water and matched to a PWS with fluoride concentration >0.7mg/L than those consuming water and matched to a PWS with fluoride concentration <0.7mg/L. Table 2 contains participants’ median MUFsg and fluoride intake levels by water fluoridation level of <0.7mg/L and >0.7mg/L. The median MUFsg (1.0mg/L) of those living in areas with PWS fluoridation level >0.7mg/L was significantly higher than the median MUFsg (0.8mg/L) of women living in areas with PWS fluoridation <0.7mg/L.

We explored the association between demographic, social, and health variables identified from the literature and MUFsg, using a linear regression model. MUFsg was associated with the site of study enrollment, maternal smoking status, and self-reporting as non-Hispanic/Latina black/African American. When compared to those enrolled at KUMC, MUFsg was higher in those enrolled at UC (p<0.001) or OSU (p<0.001). In contrast, when compared to participants who self-reported non-Hispanic/Latina white, MUFsg was lower in participants who self-reported non-Hispanic/Latina black/African American. Finally, MUFsg was lower in those who reported not smoking compared to those who reported smoking (p=0.03). Furthermore, water fluoridation level (p=0.07), higher maternal education (p=0.06 and 0.08), and higher income (p=0.06) were moderately associated with MUFsg, though the associations did not reach statistical significance. Results from this linear regression model can be viewed in Table 3.

Discussion

The median baseline MUFsg (1.0mg/L) was correlated with PWS fluoridation, and PWS fluoridation was above the US Health and Human Services recommendation of 0.7mg/L for the PWS serving 87% of participants.¹ The US Department of Health and Human Services recommends but does not mandate that PWS fluoridate to 0.7mg/L. Municipalities can choose at which level to fluoridate their water, and Ohio fluoridates between 0.8 and 1.3mg/L.⁴ Because two of the sites in this multi-center study enrolled women in Ohio, this is consistent with the difference in MUFsg and water fluoridation levels between women enrolled at the Kansas site compared to those enrolled at the Ohio sites.

Previously, MUFsg and community water fluoridation levels were measured in a small US cohort of pregnant women (n=48) living in California, with a mean MUFsg of 0.63±0.35mg/L and water fluoride level of 0.50±0.33mg/L.¹⁷ A larger study (n=491) conducted in the US, also in California, reported a median MUFsg of 0.65mg/L in the first trimester and 0.8mg/L in the third trimester, but community water fluoridation levels were not reported.¹⁸ The median MUFsg of 1.0mg/L (mean: 1.2mg/L) in the second trimester and 1.1mg/L (mean: 1.4mg/L) in the third trimester in our study were slightly higher than those reported in the California cohorts.^17,18 The mandate that water be fluoridated to 0.8–1.3mg/L in Ohio⁴ may explain why our median MUFsg is higher than the California cohorts in both early and late pregnancy.

In addition to the studies completed in the US, MUFsg has been measured in large cohorts of pregnant women in Canada,⁸ Poland,¹⁵ India,¹⁶ Spain,¹² and Mexico.^9–11 Except for the study in India, which was conducted in an area of high environmental exposure to fluoride,¹⁶ the other studies were conducted in areas of presumed low to moderate fluoride exposure. Unlike most of these studies and like our study, the study in Canada⁸ measured both MUFsg and water fluoridation level. However, unlike our study, 40% of women in the Canadian cohort lived in regions where the water was not fluoridated. Accordingly, they report a lower mean water fluoridation level (0.31mg/L) and lower mean MUFsg (0.51mg/L) for the whole cohort.⁸ For women living in areas that added fluoride to the drinking water, the mean water fluoridation level was 0.59mg/L and mean MUFsg was 0.69mg/L, both lower values than observed in our cohort.⁸

Among the five studies^8–12 that determined the association between neurodevelopment and fetal fluoride exposure in areas with water fluoride levels <0.7mg/L, four^8–11 found worse neurodevelopmental outcomes with higher fetal exposure to fluoride. Some of the studies report an association of maternal MUFsg and water fluoride levels to lower scores on the Wechsler Primary and Preschool Scale of Intelligence-III (WPPSI) at 3–4 years of age.⁸ The Canadian cohort related higher maternal MUFsg to lower WPPSI scores in 3- to 4-year-old boys but not girls.⁸ All participants lived in areas where water fluoride levels were below 0.7mg/L.⁸ Similar findings of lower IQ with higher MUFsg have been reported for Mexican participants living in regions with fluoride levels similar to Canada.^9–11 In contrast, a single study conducted in Spain found maternal MUFsg was positively related to childhood cognition.¹² We did not measure neurodevelopmental outcomes in the offspring of participants. Because we reported higher MUFsg in a cohort of US women, our results suggest a need for additional studies in the US to confirm our findings and for further research to assess the cognitive outcomes of children in relation to maternal MUFsg or water fluoridation levels in several US states.

We found differences in MUFsg by study site, maternal smoking status, and for non-Hispanic/Latina black or African American participants. The differences related to study site are most likely due to the different water fluoridation levels recommendations between states, with Ohio recommending 0.8–1.3mg/L compared to the DHHS recommendation of 0.7mg/L.^1,4 Moreover, our results are in alignment with other research in finding higher urinary fluoride in individuals who smoke tobacco. Finally, the literature describing the association between race and ethnicity and urinary fluoride is mixed, with some studies showing that individuals who are non-Hispanic black or African American have higher urinary fluoride than their non-Hispanic white counterparts,²⁹ while others find the opposite. Our results are similar to the Maternal and Developmental Risks from Environmental and Social Stressors (MADRES)¹⁸ cohort, which found that non-Hispanic black or African American participants had lower MUFsg than non-Hispanic white participants.

The large number of pregnancies studied, the ethnic diversity among participants, and PWS fluoridation levels in five states are all strengths of our study. A possible limitation is that we measured MUFsg by the direct method described by Martínez-Mier et al.²¹ as opposed to the microdiffusion method.²¹ The microdiffusion method is required to obtain a valid assessment of MUFsg in urine samples with high protein, as protein can artificially increase MUFsg.²¹ The Canadian study used microdiffusion.⁸ However, in the absence of preeclampsia, urinary protein is only slightly elevated, and this increase occurs in the second half of pregnancy, whereas our baseline timepoint was in the first half.³⁰ A somewhat higher MUFsg such as we observed in our third trimester samples compared to baseline has been reported previously²² and may be related to a physiological increase in urinary protein later in pregnancy. Alternatively, the increase could be due to the fact that we did not use the microdiffusion method.²¹

Although we did find a significant correlation between PWS fluoridation and individual spot MUFsg, MUFsg and PWS fluoride are presented and discussed for the population or by subgroups of PWS fluoride. Our findings cannot be used to assess the fluoride exposure of individuals. The primary purpose of the study was to measure MUFsg while a secondary purpose was to determine if MUFsg was associated with PWS fluoride. A limitation is that we made the assumption that PWS fluoride was consistent over the period between 2016, when patients were enrolled, and 2021, when we contacted state agencies, as the agencies only provided the most recent year of data available. The assessment of fluoride intake from tap water was limited in that some of the dietary intake assessments distinguished between tap water and bottled water while others only reported water. The assessments also did not ask participants about the source of tap water (municipal vs. private well water). Additionally, some assessments reported bottled water intake but did not specify brand; therefore, we were unable to determine if the bottled water consumed was fluoridated. In addition, due to missing data, we were only able to estimate fluoride intake for 728 participants and so were not able to include this variable in our final model. These limitations may have contributed to our inability to relate water intake to MUFsg.

We do not attempt to account for sources of fluoride intake other than water, e.g., black tea, supplements, dental products, etc. Future studies could address individual intake by taking water samples from the home and directly measuring fluoride concentration along with collecting information on black tea intake and use of fluoridated dental products or supplements.

Despite these limitations, our results relate PWS fluoridation to MUFsg concentrations that exceed the safety benchmark of 0.45mg/L¹⁴ in one region of the US, a finding that suggests the need for additional study of pregnant women in the US that are designed to assess MUFsg and sources of fluoride exposure in pregnancy.

Supplementary Material