The Association between Fluoride and Bone Mineral Density in US Children and Adolescents: A Pilot Study.

In this cross-sectional study of a nationally representative sample of US children and adolescents aged 8 to 19 years, we found negative associations between water fluoride and plasma fluoride and BMD via linear analysis. In addition, non-linear dose-response associations were found between the concentration of water fluoride and the concentration of plasma fluoride and BMD in the whole population. Specifically, the correlation between the concentration of water fluoride and the BMD was generally negative when the concentration was below 0.81 mg/L but became positive thereafter. Similarly, a negative correlation between the concentration of plasma fluoride and BMD was observed when the concentration was below 0.52, followed by a positive correlation.

Several studies have assessed the correlation between fluoride exposure and BMD; however, the results are inconclusive. Emilie et al. observed a positive correlation between the concentrations of water fluoride (0–1 mg/L) and BMD in a cohort of postmenopausal women [14]. In contrast, a cross-sectional study involving 722 women reported that the concentration of urine fluoride is negatively related to BMD [25]. A cross-sectional survey involving 907 Chinese farmers [15] and a cross-sectional study involving 943 Chinese residents [26] also reported a negative association between the concentration of urine fluoride and BMD. In addition, a study that included 248 women aged 46–65 years revealed no significant association between the concentration of water fluoride and lumbar BMD [17]. Taken together, disparities in research design, population, fluoride levels, fluoride exposure indicators, and BMD sites may help to partially explain why various studies have yielded conflicting results.

The effect of fluoride on bone metabolism is complex. Fluoride is known to exhibit a bidirectional effect on bone production and resorption. It can enhance osteogenic activity, leading to osteosclerosis, but it can also promote bone resorption, resulting in osteoporosis [12,27]. Many studies have confirmed that fluoride can promote the proliferation and differentiation of osteoblasts and increase bone mass [28,29,30,31]. In terms of osteoclasts, researchers found an inverted U-shaped correlation between fluoride levels and osteoclast formation in a mouse model [32]. These mechanisms partially explain the observation in this study that an increase in fluoride concentration leads to enhanced osteoclastogenesis, resulting in decreased BMD. Furthermore, upon surpassing a certain threshold, higher fluoride levels were found to suppress osteoclast formation while promoting the proliferation and differentiation of osteoblasts, ultimately leading to increased BMD.

In the current study, we found that the association between fluoride and BMD is influenced by sex. We observed a gradual decrease in BMD among males as the concentration of fluoride in water increased from its lowest value (Figure 2b), whereas there was an initial increase in BMD followed by a subsequent decrease in females (Figure 2c). Previous studies have shown that estrogen has a significant effect on bone metabolism by promoting bone formation and reducing bone resorption [33,34]. Based on the above hypothesis, we speculate that the positive association between BMD and water fluoride observed in females with lower levels of water fluoride is attributable to the influence of estrogen. At low fluoride concentrations, the potential protective effect of estrogen on bone may counterbalance the potentially deleterious impact of fluoride on BMD. With increasing fluoride exposure, these protective effects may become overwhelmed, resulting in a subsequent decline in BMD.

Multiple biomarkers, including urine, plasma, nails, hair, saliva, and dental fluorosis, have been employed for the assessment of fluoride exposure [35,36]. In the present study, we examined the correlation between plasma fluoride concentration and BMD, yielding results consistent with the relationship observed between water fluoride concentration and BMD. However, no significant association was found between urine fluoride levels and BMD. These findings suggest that spot urine samples may not be reliable indicators of fluoride exposure in individuals. Indeed, various factors such as collection timing, hydration level, and bladder retention duration can influence an individual’s spot urine sample fluoride concentration [37]. Additionally, inter-individual variations in urinary flow and creatinine excretion rates contribute to differences in spot urine fluoride levels among individuals [38]. Generally, water and blood concentrations of fluoride remain relatively stable over time, providing a more accurate reflection of chronic fluoride exposure effects on bone health. Conversely, spot urine fluoride levels are influenced by various factors, potentially explaining the lack of a significant association between urine fluoride and BMD in this study. Furthermore, it is worth noting that subjects might have been instructed to consume water during the urine collection process in order to expedite urine production, potentially introducing additional variability between measured urinary fluoride levels and actual fluoride exposure levels.

In this study, we investigated the association between fluoride concentration and BMD while considering potential confounding factors including sex, age, race, BMI, PIR, milk product consumption, and physical activity. It is important to recognize that these covariates may influence both BMD and plasma or urine fluoride levels. For example, racial differences in fluoride metabolism could result in varying urine fluoride levels despite similar exposure [39]. Obesity might alter fluoride metabolism through changes in renal function [40], while dietary calcium from milk products can reduce fluoride absorption [41]. Furthermore, physical activity may potentially increase fluoride absorption and retention, thereby raising systemic fluoride levels [42]. Therefore, more research is required to confirm the relationship between these factors and plasma or urine fluoride levels.

The strengths of this study include its nationally representative sample, large sample size, different measurements of fluoride exposure, and the exploration of both linear and non-linear correlations between fluoride exposure and BMD. However, there are several limitations. (1) This study was a cross-sectional study, and a causal relationship between fluoride exposure and BMD could not be determined. (2) The participants included in this study were US children and adolescents. The association between fluoride exposure and BMD may vary among different ethnic groups or regions. (3) Since this study solely utilized TBLH BMD as the dependent variable, it is crucial to conduct further research to explore the effects of fluoride exposure on BMD in different regions. (4) Fluoride in water is not the only source of human intake of fluoride; other possible sources include the use of topical dental products, fluoride drops, and fluoride tablets. (5) Due to the unavailability of serum calcium measurements for individuals aged below 12 years, we were unable to incorporate serum calcium into the covariate analysis. (6) Self-reported levels of milk consumption were utilized as a dietary indicator; however, it should be noted that this measure may not accurately reflect dietary calcium, vitamin D, and protein levels. Additionally, the recall period of 30 days might introduce inconsistencies with actual consumption patterns.

This study suggested that fluoride concentrations in water and plasma were inversely associated with TBLH BMD in a nationally representative sample of US children and adolescents aged 8–19 years, with their dose-response relationships exhibiting a non-linear trend. However, no significant linear or non-linear association was found between urine fluoride levels and TBLH BMD, possibly due to the use of spot urine collection, which may not provide stable markers of fluoride exposure. These findings provide valuable insights into the intricate relationship between fluoride exposure and skeletal health in children and adolescents.