The research team, led by Shaheryar Shafqat and colleagues in the Division of Epidemiology, Biostatistics, and Environmental Health, drew on data from the National Health and Nutrition Examination Survey, or NHANES, covering the 2013 to 2016 survey cycles. Their analytical sample comprised 4,470 participants aged 6 to 19 years, each of whom had measurements of both plasma fluoride, the concentration of the ion circulating in blood, and fluoride in household tap water. Mean plasma fluoride in the sample was 0.40 micromoles per liter, while mean tap-water fluoride was 0.50 milligrams per liter, a level consistent with the ranges typically maintained by community water fluoridation programs.

The central methodological innovation of the study lies in how the authors confronted the Achilles heel of all observational fluoride research: confounding. Children who drink more tap water may differ systematically from those who drink bottled beverages; families with certain diets, incomes or health behaviors may both consume more fluoride and differ in their cardiovascular or metabolic profiles. Any naive comparison between fluoride levels and health markers could therefore produce spurious signals. To break this tangle, the team employed an instrumental variable analysis, a technique borrowed from economics that has become increasingly fashionable in environmental epidemiology.

The logic of the instrumental variable approach is elegant. Rather than relating health outcomes directly to measured plasma fluoride, which may be entangled with unmeasured lifestyle factors, the researchers used household tap-water fluoride as an instrument, a variable that plausibly influences plasma fluoride but has no independent pathway to the health outcomes except through that exposure. Because fluoride concentrations in tap water are largely determined by geography, water sourcing and municipal treatment decisions rather than by individual choices, they offer a form of natural randomization. The analysis proceeded in two stages using survey-weighted two-stage least squares regression: first, plasma fluoride was modeled as a function of tap-water fluoride, and second, each biomarker was regressed on the predicted plasma fluoride derived from that first stage.

The models were adjusted for a comprehensive set of potential confounders, including age, sex, body mass index, race and ethnicity, the income-to-poverty ratio, health insurance status, food security, birthweight and whether the child was born in the United States. The biomarkers examined spanned the major physiological systems: cardiovascular measures such as systolic blood pressure, metabolic markers, serum electrolytes, hormones including testosterone, and indicators of renal and hepatic function. This breadth is what makes the study a genuine systems-level portrait rather than a narrow look at a single organ or disease.

The results were striking in places. Each 1 micromole per liter increase in predicted plasma fluoride was associated with a decrease in systolic blood pressure of 7.91 millimeters of mercury, with a 95 percent confidence interval stretching from 15.15 down to 0.68, meaning the effect was statistically distinguishable from zero but with considerable uncertainty. Testosterone told a different story: higher predicted plasma fluoride was linked to a geometric mean ratio of 0.49, indicating testosterone levels roughly halved across the modeled exposure increment, with a confidence interval of 0.30 to 0.80. Meanwhile, uric acid rose by 0.92 milligrams per deciliter and serum sodium climbed by 3.41 millimoles per liter per unit increase in predicted plasma fluoride.

Not every system responded. Estimates for estimated glomerular filtration rate, the standard gauge of kidney filtering capacity, pointed in a lower direction but were too imprecise to support firm conclusions, and the majority of the biomarkers examined, including most measures of hepatic function and metabolic health, showed no detectable association with fluoride levels. The authors are careful to frame these findings as associations attributable to drinking-water fluoride exposure rather than as proof of clinical harm, and the wide confidence intervals on several estimates underscore that the true magnitudes remain uncertain.

One of the most intriguing aspects of the analysis concerns effect modification. When the researchers tested whether the fluoride-biomarker relationships differed across demographic strata, they found significant interactions for multiple biomarkers, most frequently modified by sex and by age, and in some cases by race and ethnicity. Notably, the interactions were significant without indicating a consistent direction, meaning the study could not determine whether fluoride’s effects are stronger in boys or girls, in younger children or older adolescents, only that the relationships are unlikely to be uniform across the pediatric population. This heterogeneity is a call to action for future research, since pooled average effects may mask subgroup vulnerabilities.

The findings land in a charged policy context. Community water fluoridation remains one of the most debated interventions in public health, praised by dental organizations for reducing cavities and scrutinized by critics concerned about developmental effects. This study does not settle that debate, and its authors did not set out to. What it does provide is a methodological template: by exploiting variation in tap-water fluoride that is essentially outside individual control, researchers can estimate the systemic biology of fluoride exposure with far less contamination from the socioeconomic and behavioral confounders that have plagued earlier work. The team acknowledged the children and families who participated in NHANES, the National Center for Health Statistics at the Centers for Disease Control and Prevention, the U.S. Geological Survey for groundwater fluoride data, and the CDC Division of Oral Health for fluoridation information, and the research received no external funding.

For now, the takeaway is one of measured scientific progress rather than alarm. Plasma fluoride attributable to tap water was associated in this large national sample with lower systolic blood pressure and testosterone, and higher serum sodium and uric acid, alongside meaningful heterogeneity across demographic subgroups. Whether these biochemical shifts translate into lasting health consequences, whether they persist into adulthood, and what mechanisms underlie them are questions the instrumental variable framework is now well positioned to pursue. As biomonitoring programs continue to measure fluoride in blood and water simultaneously, the era of guessing what fluoridation does beyond the enamel may finally be giving way to one of rigorous, population-scale answers.

Subject of Research: Associations between plasma fluoride from drinking water and systemic biomarkers in U.S. children and adolescents

Article Title: Plasma fluoride and systemic biomarkers in U.S. children: an instrumental variable analysis using household tap water fluoride

Article References: Shafqat, S., Yu, X., Mou, X., Levy, M., Zhang, H., & Naser, A. M. (2026). Plasma fluoride and systemic biomarkers in U.S. children: an instrumental variable analysis using household tap water fluoride. Environmental Health. https://doi.org/10.1186/s12940-026-01338-z

Image Credits: AI Generated

DOI: 10.1186/s12940-026-01338-z

Keywords: fluoride, water fluoridation, NHANES, instrumental variable, plasma fluoride, biomarkers, children's health, environmental health, blood pressure, testosterone, uric acid, epidemiology