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We add it to drinking water for our teeth — but is fluoride hurting us?

Three new studies released today link fluoride exposure to ADHD and thyroid problems — and point to drinking water as the major source of exposure.

Two studies — one from Canada and one Mexico — released today point to potential health problems from fluoride, which, in a majority of U.S. communities, is purposefully added to drinking water to protect people's teeth.


The Canada study found that adults who are iodine deficient and have higher levels of fluoride in their system have a greater risk of an underactive thyroid. The Mexico study found mothers with higher fluoride exposure during pregnancy were more likely to have children with symptoms of ADHD. Both studies were published in the journal Environmental International.

A third study, published in Environmental Health Perspectives, found that among 1,566 pregnant women in Canada, fluoride levels in urine were almost two times higher for women who lived in regions where the element was added to their drinking water compared to pregnant women in regions with non-fluoridated water.

The studies call into question the practice of purposely adding fluoride to water or salt, which is done to prevent cavities and, to a lesser extent, osteoporosis. Many cities in the U.S. and Canada add fluoride to public drinking water and in Mexico it's added to some salt. Approximately 66 percent of people in the U.S. receive drinking water with added fluoride, according to the Centers for Disease Control and Prevention (CDC).

About 80 percent of fluoride exposure comes from water and beverages such as tea, which can leach fluoride from soil. Other sources include grapes and shellfish.

"I have grave concerns about the health effects of fluoride exposure," Ashley Malin, lead author of the Canada thyroid study and a researcher at the Department of Environmental Medicine and Public Health, Icahn School of Medicine at Mount Sinai, told EHN. "And not just from my study but the other studies that have come out in recent years."

Fluoride, iodine and thyroids

Malin and colleagues had massive amounts of information from the Canadian Health Measure study. They looked at fluoride levels in the urine of nearly 7 million Canadians, as well as iodine deficiency and thyroid gland activity.

They found Canadians who were deficient in iodine—a mineral crucial for proper functioning of the thyroid — and who had high amounts of fluoride in their urine also had higher levels of thyroid stimulating hormones. Elevated levels of these hormones are a marker for a suppressed thyroid gland – commonly referred to as hypothyroidism, a condition that can cause a host of problems including fatigue, disrupted heart rates, and altered metabolism.

Small increases in thyroid stimulating hormones can be problematic, Malin said.

"Someone doesn't need to have full blown hypothyroidism to have an elevation in [thyroid stimulating hormones]. Research is showing more and more that subclinical elevations are associated with bad health effects," Malin said.

Iodine helps flush fluoride from the body so a deficiency leaves the body with more fluoride, which has been shown to interfere with certain enzymes important for thyroid function. This could explain why only iodine deficient Canadians seemed sensitive to fluoride impacts.

Malin said 18 percent of the nearly 7 million people they studied were iodine deficient. "We're talking about potentially [more than] a million people at risk of an underactive thyroid due to fluoride exposure."

But there are major health benefits of fluoride in water. According to the CDC, drinking fluoridated water reduces cavities (also called tooth decay) by about 25 percent in children and adults. The agency named water fluoridation one of its "Ten Great Public Health Achievements" of the 20th Century.

Dr. Manish Arora, a dentist and vice chairman of the Department of Environmental Medicine and Public Health at the Icahn School of Medicine at Mount Sinai, told EHN via email that it "is important to balance these results with what we know about the benefits of water fluoridation as well."

"There have been tremendous gains in children's oral health worldwide over the past decades that, at least in part, can be attributed to the beneficial effects of fluoride," said Arora, who was not involved in any of the studies released today but is collaborating with some of the researchers on other projects.

While the new study doesn't prove fluoride impacts thyroid function, previous studies have linked the element to reduction thyroid hormones, and to elevated thyroid stimulating hormones and increased likelihood of hypothyroidism and diabetes in adults.

Behavior impacts

In the other study published today, researchers looked at 213 Mexican mother-children pairs and examined mothers' urine fluoride levels during pregnancy and assessed children for attention-deficit/hyperactivity disorder (ADHD) symptoms at ages 6 to 12. They found mothers with higher levels of fluoride during pregnancy were more likely to have children with ADHD symptoms, especially inattention.

It's not clear from this study why fluoride may impact child's behavior, but it could be driving thyroid hormone insufficiency in pregnant mothers (which can lead to problems in their unborn), or altering children's levels of dopamine, which moves signals from nerve cells to the brain and is vital for behavior development.

Christine Till, an associate professor and researcher at York University, told EHN one of her main concerns is that pregnant women are susceptible to iodine deficiency, which, according to the study from Canada, could leave the mothers-to-be with thyroid problems.

Also, fluoride easily crosses the placenta from mother to her unborn. The study is not the first to find a fluoride-behavioral link: A previous study linked the element to ADHD in U.S. children.

Dr. Howard Hu, co-author of the Mexico study and an epidemiological researcher at the University of Washington, told EHN the research from Canada on fluoride levels in pregnant women "makes the results of this study from Mexico even more applicable to what might be going on in North America."

To add or not to add

The evidence that fluoride may have negative impacts on health is building, Hu said, adding that one of the "most awkward features of this debate" is that it pits one branch of public health vs another.

Arora said "as a dentist and environmental health scientist, I feel this is an opportune moment in our professions to have an honest discussion."

"A question that is becoming increasingly important – is fluoridation of water supplies the best way to deliver the oral health benefits of fluoride?" Arora said. "For me, there is no 'one size fits all' answer to this. Socioeconomics, background risk and other aspects of the community have to be considered, but now is the time to have the scientific debate."

In a statement, the American Dental Association told EHN their National Fluoridation Advisory Committee would review the new studies, adding that "public health policy is based on a collective weight of scientific evidence, not the results of a single (or few) studies. The ADA remains committed to fluoridation of public water supplies as the single most effective public health measure to help prevent tooth decay."

Hu echoed Arora and said the answer in moving forward with fluoride is more nuanced than being pro- or anti-fluoride.

"Clearly this warrants additional research and consideration with how policies related to fluoride may need to be rethought," Hu said. "And not simply 'do we use fluoride or not,' but can we figure out a way to preserve the benefits while minimizing the potential adverse effects."

Till said she is "certain the safety of fluoride ingestion has not been proven."

"The problem is that it's an uncontrolled dose – everyone is exposed to different levels. It may be prudent for pregnant women to reduce ingesting fluoride during pregnancy."

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