A new NIEHS-funded study found that higher fluoride levels in the urine of pregnant women in Mexico were associated with lower measures of intelligence in their children. The study was published Sept. 19 in the journal Environmental Health Perspectives.
Researchers tested urine samples from pregnant women and found that each increase of 0.5 milligrams of fluoride per liter of urine was associated with a decrease of 3.15 points on the General Cognitive Index (GCI) scale and a decrease of 2.50 points on the Intelligence Quotient (IQ) test. The study was the first to compare fluoride levels in pregnant women with multiple measures of their children’s neurodevelopment. The team studied participants in the Early Life Exposures in Mexico to Environmental Toxicants (ELEMENT) study.
“Our study shows that the growing fetal nervous system may be adversely affected by higher levels of fluoride exposure,” said Howard Hu, M.D.,Sc.D., from the University of Toronto.
The study participants' fluoride levels during pregnancy were higher than levels typically seen in the general population of women in the U.S. and Canada. However, Hu cautioned that it is hard to compare levels for pregnant and nonpregnant women because relatively little is known about how pregnancy influences fluoride excretion.
Benefits and unknown costs of fluoridation
Fluoride has been added to water, salt, milk, and dental products in many countries for more than 60 years to prevent cavities and improve bone health, according to the authors. They pointed out that although there are some known side effects to excessive fluoride exposure, like tooth staining, there have been few studies of other potential health effects.
Most previous studies in humans have been in communities with naturally high levels of fluoride in their drinking water, the authors explained. Those studies assessed exposure based on levels of fluoride in drinking water, rather than direct measurements from study participants.
Participants in this study were exposed to fluoridated salt and some naturally occurring fluoride in drinking water, according to the authors.
There is some evidence that ingesting fluoride might affect the developing brain and nervous system, or neurodevelopment. In 2016, the National Toxicology Program (NTP) published a systematic review of animal studies that concluded that there is low-to-moderate evidence of a pattern linking fluoride exposure to decreased learning and memory (see sidebar).
"Relatively little is known with confidence about fluoride’s impact on neurodevelopment," Hu said.
A rigorous study
The researchers followed 299 mother-child pairs from pregnancy through childhood. Fluoride was measured in both the mothers' urine collected during pregnancy and the children’s urine when they were 6-12 years old. They measured child intelligence on two scales — the GCI at age 4 years and IQ for the second time when the children were 6-12 years old.
The authors saw a decrease in child intelligence associated with prenatal fluoride levels even after controlling for a number of potential confounders, yet they did not find the same link to elevated fluoride levels in children when they were older. "This suggests that the prenatal nervous system may be more sensitive to fluoride than the nervous systems of school-aged children," Hu said.
Although the study size was modest, Hu said the study’s rigorous methods and highly statistically significant results gave confidence to the study’s conclusions. The authors noted that this was one of the first studies with this finding, and the results need to be confirmed by additional research.
Bashash M, Thomas D, Hu H, Angeles Martinez-Mier E, Sanchez BN, Basu N, Peterson KE, Ettinger AS, Wright R, Zhang Z, Liu Y, Schnaas L, Mercado-Garcia A, Maria Tellez-Rojo M, Hernandez-Avila M. 2017. Prenatal fluoride exposure and cognitive outcomes in children at 4 and 6-12 years of age in Mexico. Environ Health Perspect 125(9):097017.
(Virginia Guidry, Ph.D., is a technical writer and public information specialist in the NIEHS Office of Communications and Public Liaison and a regular contributor to the Environmental Factor.)