From Promise to Paradox: GLP-1 Agonists May Offset Their Own Benefits Through Gut Microbiome Dysbiosis
With thanks to Romilly Hodges, CNS, for her contributions to this article.
I’m not here to fan the flames of controversy, but I do believe in asking smart, systems-oriented questions—especially when we’re talking about chemicals that may be added to something as essential as drinking water. Fluoride is one of those topics that sits right at the intersection of medicine, environment, public health, and ethics. As a functional medicine practitioner, I think we owe it to our patients to look past slogans and soundbites and ask: what does the evidence really say, and what does it mean for this unique individual sitting in front of me? ~ DrKF
Fluoride in Public Health: Controversy, Consensus, and Clinical Questions
Fluoride is both praised and questioned, depending on who you ask. The American Academy of Pediatrics, American Dental Association and the Centers for Disease Control and Prevention credit fluoridated water with dramatically lowering tooth decay rates in the U.S. since its introduction in the mid-20th century. But over the past few decades, a growing number of clinicians, environmental researchers, and even mainstream scientists and journals have raised concerns about potential unintended consequences—particularly related to neurodevelopment and cumulative exposure.
This divergence in opinion is, of course, partly philosophical: conventional medicine and public health authorities usually evaluate benefit in terms of population averages, while functional medicine considers the individual and their unique presentation. In this context, mass fluoridation can feel like too blunt an instrument. Let’s tease this out a little more…
Common Sources of Fluoride Exposure and Why They Matter
Fluoride is naturally present in soil and groundwater, but most exposure today comes from human-introduced sources. In the U.S., approximately 73% of those on community water systems receive fluoridated municipal water and the recommended concentration is 0.7 mg/L. But drinking water isn’t the only route for exposure. There’s also:
- Dental products: Toothpaste (usually 1,000–1,500 ppm fluoride), mouth rinses, and fluoride gels. It is estimated that children who accidentally swallow toothpaste and other oral products while brushing can ingest between 3 mg – 0.8 g fluoride, enough to cause fluorosis (more on that below). Dental products do come with warnings about using pea-sized amounts and storing away from children for this reason.
- Professional applications: Fluoride varnishes used in pediatric and adult dental care. These contain 5% sodium fluoride, i.e., 22,600 ppm fluoride. The effect of fluoride varnish applications on plasma fluoride levels has been examined and deemed safe, although only in relation to acute toxic doses of fluoride (not in relation to cumulative chronic impacts, which have not been assessed).
- Dietary exposure: Foods prepared with fluoridated water will contain fluoride in amounts dependent on the quantity of water used. Many foods also naturally contain fluoride, to varying degrees. Tea and coffee plants, for example, take up fluoride from the soil more readily than other plants leading to higher amounts in their respective brewed beverages. Here are some select foods and their estimated fluoride content:
- 1 c brewed black tea 0.07 – 1.5 mg
- 1 c brewed coffee 0.22 mg
- 3 oz canned shrimp 0.17 mg
- 1/4 c raisins 0.08 mg
- 1/2 c cooked rice 0.04 mg
- 3 oz pork 0.03 mg
- Infant formula 0.2 – 0.3 mg/L
- Supplements: Drops or tablets, particularly for children in non-fluoridated areas. These commonly contain25 mg per dose, with a few products containing 0.5 or 1 mg per dose.
The concern about fluoride isn’t necessarily about isolated exposures (unless they’re at levels associated with acute toxicity – 5 mg/kg or 375 mg for someone who weighs 165 lbs), but when collective levels of intake become “elevated” over time. Even at levels much less than those associated with acute effects. It’s also likely that some individuals are more susceptible to potential negative effects than others, a concept well-recognized in functional medicine.
Does Fluoride Affect IQ? Reviewing the Research on Brain Development
The watershed moment in the conversation around fluoride exposure and neurodevelopment occurred following the publication of a Canadian prospective cohort study analysis – by Green et al. (2019) – in the prestigious JAMA Pediatrics journal. Prior to this study, most investigations were cross sectional (which can’t be used to assess causation) and/or of lower quality (for instance – not properly controlled for confounding factors and without any individual level assessment of exposure). Their findings established temporality – i.e. that fluoride exposure preceded any IQ outcome – and were able to link data of each individual’s exposure level to their own outcomes. These are important building blocks towards understanding causation.
Here’s a quick summary:
Green et al. (2019): This study looked at maternal fluoride levels during pregnancy in 601 mother-child pairs in a prospective Canadian cohort and their association with a child’s IQ scores assessed between 3-4 years of age. They found that for every 1 mg/L increase in a mother’s urinary fluoride level, their boys’ IQ was 4.49 points lower (95% CI -8.38, -0.60). There was no statistically significant difference for girls.
Fast forward a few years, to 2025, and the conversation has reached a new level: In January of this year, the same venerable JAMA Pediatrics published an extensive systematic review and meta analysis of 74 publications (they included those up to October 2023) and reported the following findings:
- 65 of the studies were used in the primary analysis, most of which found an inverse relationship between fluoride exposure and children’s IQ levels.
- In their combined meta-analysis (with 59 studies included, totaling 20,932 children), higher fluoride exposure was significantly associated with lower IQ with a difference between pooled averages of -0.45 points (p<0.001).
- A dose-response relationship was determined (p<0.001), even for fluoride concentrations that were less than 2 mg/L (but not less than 1.5 mg/L).
- In the group of 13 studies that were assessed to have a low risk of bias, IQ was 1.14 points lower for every 1 mg/L increase in urinary fluoride levels (p<0.001)
All of a sudden, these findings were difficult to ignore. Unsurprisingly, there has been significant pushback from the public health community, but the authors have defended their methodology and interpretations rationally and cogently.
Of course there are still limitations. Most of the data in the 2025 meta analysis is observational, which can only tell us about associations, not causation. It is also true that many environmental and social variables can influence IQ. Yes, it’s possible to say that spot urine measurements (one of the most common ways of assessing fluoride exposure) is prone to variability (although many tests that we routinely use are, and using a large dataset such as the meta analysis set can reduce that concern). In sum, we can’t say for sure that certain types of fluoride exposure lead to lower IQ levels.
However, I agree with the authors of the 2025 systematic review and meta analysis by Taylor et al., that there’s a case for considering the precautionary principle —especially during vulnerable periods:
“While research continues, it is worth emphasizing the importance of limiting total fluoride intake during pregnancy, infancy, and early childhood, known critical periods of brain development.” – Taylor et al., 2025
But, and this is a relatively big but – that doesn’t necessarily mean avoiding fluoride altogether…
Fluoride’s Benefits for Dental and Whole-Body Health
Fluoride effectiveness in preventing tooth caries has been well established. And that’s no small thing! Back in the early 1900s, tooth decay (let alone caries) was very common – 90% of 12-year-olds had tooth decay, with an average of 4 decayed teeth per child. Dental health has also been strongly related to social class, which rightly deserves public health attention.
Fluoride’s ability to reduce tooth decay was established in the first half of the 20th century: as water fluoridation became commonplace, dental health improved tremendously. And equitably across all portions of society. This was considered a great improvement to “quality of life.” And so it was.
I’m sure my regular readers/podcast listeners are ahead of me here – in functional medicine, we also consider the systemic impacts of oral health. Oral dysbiosis associated with caries can, via its contribution to systemic inflammation, predispose to many conditions including cardiovascular diseases, inflammatory bowel diseases, and metabolic disorders like obesity and diabetes. So much so that patients undergoing oral surgery are often given antibiotics to prevent infective endocarditis, a serious heart infection. The oral microbiome is also implicated in autoimmune diseases via various mechanisms. I routinely pay attention to oral health in my patients for this reason.
Here are a few resources on drkarafitzgerald.com that address this connection:
- The Mouth: A Mirror of Health or Disease
- Functional Dentistry and the Oral-Systemic Connection with Dr. Mark Burhenne, DDS
- Endodontic Endotoxemia: The Oral-Systemic Connection
It’s for these reasons that I will sometimes recommend topical (rather than systemic) fluoride applications, combined with a supervised rinse and spit to remove, in children who have a history of cavities.
A Functional Medicine Approach to Fluoride: Clinical Best Practices
So, what conclusions can we draw and how do I handle fluoride in practice?
- I think there’s reason to reassess community water fluoridation. The ethics of dosing a population through an essential nutrient (water) are problematic. While there may be benefits for underserved communities with poor oral health, there may be risks to other vulnerable populations like pregnant women, infants, and children. This deserves open investigation and debate.
- Lean into fluoride alternatives where possible. Hydroxyapatite toothpastes are excellent alternatives to fluoride toothpaste (there are randomized, controlled clinical trials demonstrating their effectiveness). A full functional medicine approach will also support healthy bone and teeth mineralization.
- Supervised topical fluoride has a place—especially in high-risk pediatric cases. Rinse-and-spit products, occasional varnishes, and supervised brushing with fluoride toothpaste are still important considerations for children with a history of dental caries and if the other approaches I just mentioned aren’t effective or within financial reach. However, professionals, parents or caregivers need to be the ones applying the fluoride and confirm that the spit and rinse step is followed properly. Fluoride should be treated with the same care we would give medications.
Let me know your thoughts in the comments below.
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