New research reveals that fluoride - while protecting teeth - may harm gut microbiota at high doses, highlighting the need to rethink long-term exposure limits.
Effect of Fluoride on Gut Microbiota: A Systematic Review. Image Credit: LedyX / Shutterstock
In a recent study published in the journal Nutrition Reviews, researchers in the United Kingdom explored the effects of fluoride on gut microbiota.
The Human Microbiome Project has reported that more than two-thirds of the human microbiome resides in the gastrointestinal (GI) tract. Advances in computational methods and molecular sequencing have provided an understanding of how the gut microbiota functions symbiotically with the host and contributes to metabolism, nutrition, intestinal architecture, and immune response.
Further, lifestyle interventions targeting the gut microbiota lead to significant changes in its composition. Fluoride is well known for its role in reversing and preventing dental caries. It is added to water, salt, milk, and dental products to prevent dental caries. However, the effects of fluoride on the gut microbiota are poorly understood.
About the study
In the present study, researchers explored the association of fluoride exposure with changes in gut microbiota composition. First, they performed a literature search on six databases: Web of Science, PubMed, Embase, CINAHL, MEDLINE, and Scopus. Eligible studies were analytical, observational studies, quantitative, and laboratory-based studies. All studies of animals or humans of any sex and age exposed to different doses and forms of fluoride were selected.
Studies involving in vitro models from gut or fecal samples were also included. Following the literature search, studies were deduplicated and screened per eligibility criteria. Abstracts/titles were screened, followed by the review of full texts. Relevant data were extracted from included studies, such as identifiers, authors, publication year, journal, study design, study duration, outcomes, statistical methods, and outcomes.
The quality of the studies was assessed using the mixed methods appraisal tool. The intervention or exposure was fluoride in all sources and forms: systemic (e.g., diet) and topical (e.g., dental products). The outcome was assessing microbiota composition following fluoride exposure, including richness, bacterial taxa prevalence, and their functions.
Findings
The literature search yielded more than 1,000 hits; after deduplication, 590 articles were screened at the abstract/title level, followed by the full-text screening of 63 records. Overall, 49 studies were included in the analyses. Studies included 39 randomized controlled trials (RCTs, including animal studies), six experimental or laboratory-based studies, two case-control studies, one cohort study, and one non-RCT.
Most studies were conducted in Asia (90%), followed by Australia (4%), the United States (4%), and Brazil (2%). Three studies used in vitro models, 42 involved animals, and four had human subjects. Animal models included birds, fish, and rodents. The sample size in animal studies ranged between six and 900 animals. Human studies had 15 to 114 subjects.
Sixteen animal studies and all human studies included fecal microbiota analysis; the remaining animal studies relied on other tissues as biomarkers. Gut microbiota composition was estimated using different sequencing workflows across studies: amplicon 16S rRNA gene sequencing, quantitative polymerase chain reaction (qPCR) along with 16S rRNA sequencing, and real-time qPCR with 16S rRNA-specific primers.
The total sample size in animal and human studies was 3,249 and 217, respectively. Most studies (73%) used sodium fluoride (NaF) as a fluoride source in diet or water as an intervention. Three studies used perfluoroalkyl fluorides (e.g., sodium fluoroacetate and perfluorooctanoic acid), and one investigated polyfluorinated ether sulphonate. The effects of fluoride varied depending on the form of fluoride used, with systemic and topical sources showing different microbiota impacts in some studies. Most animal studies (83%) reported long-term associations, while one human study and some animal studies reported short-term outcomes.
Further, 79% of studies were of high quality, and the risk of selection bias was low. An increase or decrease in alpha diversity indices in fluoride-exposed groups relative to controls indicated that fluoride affects microbial community structure. Nine studies also examined beta diversity differences; two found no differences, while four reported significant differences following fluoride exposure.
Importantly, a biphasic response was observed in some in vitro studies, where low doses of fluoride (0.1 mM NaF) promoted bacterial growth and enzyme activity. In comparison, high doses (up to 100 mM) inhibited growth, particularly of lactobacilli.
One human study observed that low-dose NaF resulted in no significant differences in genus- and phylum-level abundance, but it might promote taxa associated with health, such as Lactobacillus and Faecalibacterium. Conversely, high NaF doses (up to 1200 mg/L in animal studies) increased the abundance of Proteobacteria. Notably, the review identified that doses of ≤ 2 mg/L NaF appeared harmless or even beneficial to the gut microbiota, while doses ≥ 50 mg/L NaF consistently caused disruptions in microbiota composition, including reductions in beneficial species such as Firmicutes and Bacteroidetes and increases in potentially harmful Proteobacteria.
Overall, results from human studies indicated that high fluoride exposure modified the gut microbiota composition by disturbing the balance between beneficial and pathogenic microbes.
However, the limited number of human studies (4 total) highlights the need for further research to confirm these findings. Across animal studies, consistent high-dose fluoride exposure led to disturbances in gut microflora, and the most affected genera or phyla differed across animal types. The findings varied across animal species, fluoride forms, and tissues analyzed, indicating that the effects of fluoride on gut microbiota are complex and species-dependent.
Findings from in vitro models suggested a biphasic response with fluoride-inducing bacterial growth at a low dose (0.1 mM NaF), with a dramatic rise in growth and enzyme synthesis. An increase in dose (to 100 mM) inhibited microbial growth, especially lactobacilli.
In one human study, low and high fluoride doses did not affect the production of short-chain fatty acids (SCFAs). In rodents, 100–150 mg/L NaF for up to six months reduced SCFAs, digestive enzymes, p62 proteins, antioxidative enzymes, and metalloenzymes. Fluoride also decreased gastrotropin, glutathione, and follicle-stimulating hormone levels. Besides, fluoride stimulated the production of pro-inflammatory cytokines, secretory immunoglobulin A, and malondialdehyde.
Conclusions
The findings show that in vitro or in vivo fluoride intervention may modify the gut microbiota and its activities. Low-dose fluoride showed no effects on the gut microbiota. Doses of ≤ 2 mg/L NaF were identified as likely safe or beneficial, whereas doses ≥ 50 mg/L NaF disrupted microbial diversity, altered metabolism, and shifted the balance of specific bacterial taxa.
While the effects at high doses were inconsistent, there were changes in overall microbial diversity, metabolism, and the relative abundance of specific taxa. Shifts in these aspects of the microbiota could result in health implications. Further, variability in outcomes depending on species, form, duration of fluoride exposure, and specific tissues studied emphasizes the need for more standardized research approaches.
As such, further studies are required to understand the impact of long-term, low-dose fluoride exposure on key gut microbial communities.
This systematic review is registered under PROSPERO number CRD42022347357.