Effect of Fluoride on Gut Microbiota: A Systematic Review

In the paper

“Three of the studies used doses of 25, 50, 100, and 150 mg L–1 NaF for 70 days10 and 12 weeks.54,65 Fluoride led to significantly lower levels of Actinobacteria, Bacteroidetes, and Verrucomicrobia and significantly higher levels of Firmicutes, whereas Turicibacter, Lachnospiraceae, Roseburia, and Clostridium sensu stricto were negatively associated with fluoride exposure.”

In the paper

“The sample size in the case of animal studies ranged from 6 to 900 animals.”

In the paper

“On the contrary, 1 study used 15, 45, and 75 mg L–1 NaF for 3 months and found that low fluoride exposure upregulated the abundance of Lactobacillus and Lachnospiraceae; bacilli, Lactobacillus, and Lactobacillaceae were also dominant in the 45 mg L–1 NaF group; and Bifidobacteriales, Bacteroidetes, and Actinobacteria were dominant taxa in the 75 mg L–1 NaF group.62”

In the paper

“A total of 1004 articles were identified from the database search.”

In the paper

“One of the ex vivo studies of human feces, which used a fermenter, indicated a low dose (1-2 mg L–1) of NaF did not show a significant difference in the abundance at phylum and genus levels but may promote taxa associated with health, including Faecalibacterium and Lactobacillus.”

In the paper

“All 49 articles reported on studies of the effect of different forms and doses of fluoride on the gut microbiome.”

In the paper

“Amplicon 16S rRNA gene sequencing was used in 44 studies (92%), 2 studies41,46 also included quantitative polymerase chain reaction to validate 16S rRNA gene sequencing findings.”

In the paper

“An additional 10 articles were identified from reference lists and citation chaining of included studies.”

In the paper

“The majority (90%) of the studies were conducted in Asia (China, Turkey, India, and Japan),10,14,31–33,35–43,46–52,54–68,70–76 4% in the United States,45,53 4% in Australia,34,69 and 2% in Brazil (Table 2).44”

In the paper

“The included studies used different animal models, such as rodents, birds, and fish (Table 2).”

In the paper

“These studies were 39 randomized controlled trials,10,31–67 1 nonrandomized controlled trial,68 6 laboratory-based or experimental studies,14,69–73 1 cohort study,74 and 2 case-control studies.75,76”

In the paper

“After deduplication 590 records were screened at the title and abstract level, and 63 articles were included in the full-text screening based on the preset inclusion and exclusion criteria.”

In the paper

“The remaining 26 animal studies used other tissues as biomarkers (Table 2).”

In the paper

“Of the Actinobacteria, Bifidobacterium was also influenced by fluoride intervention, with an increased abundance indicated in 5 studies31,47,50,51,75 and a decrease in 3 studies when animals were exposed to 50-200 mg L–1 NaF.”

In the paper

“Forty-nine studies were included (n = 42 in animals, 4 of humans, 3 in vitro studies) after screening for title, abstract, and full text using Covidence to check against eligibility criteria.”

• Also stated in: § Characteristics of Included Studies · plain

  “Forty-two10,31,33–70,76 studies (88%) were conducted with animals, 3 were in vitro (6%),71–73 and 4 studies (8%)14,74–76 included humans as the participants (Table 2).”

In the paper

“In the case of human studies, it ranged from 15 to 114 participants.”

In the paper

“The patients had significant variations in microbiota composition and abundance compared with healthy participants. This was characterized by an increase in the relative abundance of Verrucomicrobiota, Desulfobacterota, Acidobacteriota, and Proteobacteria, and a significant decline in the relative abundance of Firmicutes and Bacteroidetes at the phylum level.”

In the paper

“Zhou et al76 investigated the changes in the gut microbiome of children with dental fluorosis and compared them with a mouse model established by administering 100 mg L–1 NaF in water. This study, therefore, was considered both an animal and human study.”

What the sources say

• ?Not matched

  Couldn’t be matched to a paper in Crossref, so it can’t be checked.

  cites Zhou et al76

In the paper

“Shi et al66 detected that exposure to perfluorooctanoic acid increased Bacteroidetes and decreased Firmicutes populations significantly.”

In the paper

“Rarer gut phyla, such as Proteroidetes,49 Cyanobacteria,50 Proteobacteria,31 Atoprostipes, Lachnospiraceae,61 Erysipelotrichaceae,63 and Tenericutes (synonym Tenericuteota)36 increased upon high fluoride (25-150 mg L–1 NaF) exposure, but populations of Saccharibacteria31,49 and Verrucomicrobia (synonym Verrucomicrobiota)36,49,56 decreased.”

In the paper

“Of the 73 remaining articles, 49 were eligible for and included in this review.”

In the paper

“Feng et al67 found that 5.7 mg L–1 polyfluorinated ether sulphonate decreased the abundance of lactobacilli but increased that of Staphylococcus.67”

In the paper

“The animal studies examined the effects of high fluoride doses in water and diet (50-1200 mg L–1 NaF) for long-term (1-6 months) and short-term (6 hours to 7 days) exposure, with all showing a significant disturbance in the Firmicutes to Bacteroidota ratio.”

In the paper

“After deduplication 590 records were screened at the title and abstract level, and 63 articles were included in the full-text screening based on the preset inclusion and exclusion criteria.”

In the paper

“However, in 1 study of mice, by Yasuda et al,53 applicable fluoride concentrations of 4 mg L–1 and 4 mg L–1 plus gavage resulted in the treated groups having a reduction in acidogenic oral bacteria, but there was no impact on the gut microbiome.”

In the paper

“Amplicon 16S rRNA gene sequencing was used in 44 studies (92%), 2 studies41,46 also included quantitative polymerase chain reaction to validate 16S rRNA gene sequencing findings.”

What the sources say

• ?Not checked — no full text of the sourcecites [41] Luo, 2016, Dietary high fluorine alters intestinal microbiota in broiler chickens, Biol Trace Elem Res source
• ?Not checked — no full text of the sourcecites [46] Sun, 2020, Probiotic Lactobacillus johnsonii BS15 prevents memory dysfunction induced by chronic high-fluorine intake through modulating intestinal environment and improving gut development, Probiotics Antimicrob Proteins source

In the paper

“All human14,74–76 and 16 (40%)33,36,40,45–47,52–54,56,57,60,65–67,76 animal studies included an analysis of microbiota on fecal samples.”

In the paper

“Six studies50,51,55,58,67,71 used real-time–quantitative polymerase chain reaction using 16S rRNA-specific primers to quantify bacterial species.”

What the sources say

• Not checked yet

  Checked before — its earlier verdict is held back until a new check runs.

  cites [50] Xin, 2021, Preventive effects of Lactobacillus johnsonii on the renal injury of mice induced by high fluoride exposure: insights from colonic microbiota and co-occurrence network analysis, Ecotoxicol Environ Saf source
• Not checked yet

  Checked before — its earlier verdict is held back until a new check runs.

  cites [51] Xin, 2021, Probiotic alleviate fluoride-induced memory impairment by reconstructing gut microbiota in mice, Ecotoxicol Environ Saf source
• ?Not checked — no full text of the sourcecites [55] Yu, 2021, Effects of fluorine on intestinal structural integrity and microbiota composition of common carp, Biol Trace Elem Res source
• ?Not checked — no full text of the sourcecites [58] Huang, 2024, Fluoride exposure-induced gut microbiota alteration mediates colonic ferroptosis through N6-methyladenosine (m6A) mediated silencing of SLC7A11, Ecotoxicol Environ Saf source
• ?Not checked — no full text of the sourcecites [67] Feng, 2024, Maternal F-53B exposure during pregnancy and lactation induced glucolipid metabolism disorders and adverse pregnancy outcomes by disturbing gut microbiota in mice, Sci Total Environ source
• ?Not checked — no full text of the sourcecites [71] Ma, 2014, Effects of fluoride on bacterial growth and its gene/protein expression, Chemosphere source

In the paper

“Overall, all studies concluded that fluoride in suitable amounts (per the WHO recommendation for each species type) boosts gut microbial growth (probiotics) but, in excess, discourages microbial growth.”

In the paper

“Fluoride intervention increased the relative abundance of Firmicuteota and decreased the Bacteroidota population, leading to the elevated Firmicuteota to Bacteroidota ratio.”

In the paper

“Some studies also showed an increase in the abundance of Firmicutes and a decrease in Bacteroidetes.10,32,37”

In the paper

“Overall, when comparing specific bacterial phyla and genera within the gut microbiota of animals treated with fluoride relative to the control, the most consistent findings were a lower abundance of Firmicutes and Verrucomicrobia31,35,42–44,46,47,49,50 followed by an increase in abundance of Bacteroidetes, Actinobacteria, Streptococcus, Staphylococcus, Desulfobacterota, and Proteobacteria31,36,42,”

In the paper

“This decrease was correlated with the decreased ratio of Firmicutes to Bacteroidetes and increased abundance of Proteobacteria, Acidobacteriota, and Verrucomicrobiota.75”

In the paper

“Many genera were implicated across the control and treated groups, including the lower abundance of lactobacilli, Faecalibacterium, and Bifidobacterium31,32,36,37,40,46,49,50,52 (important in maintaining intestinal function), as well as higher abundance of E coli.42”

In the paper

“Although effects at high doses are inconsistent, impacts could be seen in overall microbial diversity change, change in relative abundance of specific taxa, and changes in the metabolism of present microbiota.”

In the paper

“Most of the discussed studies also reported that fluoride may induce obesity and colitis along with IBD.”

In the paper

“The mechanisms linking changes in bacterial composition to metabolic dysfunction have not been confirmed, meaning the exact process by which fluoride intervention modulates the gut microbiota and alters SCFAs remains unknown.”

In the paper

“On the contrary, in ducks, an increased richness was observed for Bacteroidetes and Firmicutes, but lower richness for Verrucomicrobia and Proteobacteria upon intake of 750 mg kg–1 NaF for 28 days.38”

In the paper

“This systematic review shows the possible relationship between fluoride and gut microbiota composition and its associated outcomes at a concentration of ≥ 50 mg L–1 NaF in animals for ≥ 1 month and ≥ 10 mg L–1 NaF in humans for 24 hours.”

In the paper

“Findings showed that at high doses of fluoride (given as NaF, 1200 mg kg–1 and 800 mg kg–1 in diet) the abundance of Lactobacillus and Bifidobacterium was lower and that of Escherichia-Shigella, Gammaproteobacteria, Enterobacteriaceae, Streptococcus, and Staphylococcus was higher compared with controls after both short- (16 hours) and long-term (21-59 days) exposure.42,43”

In the paper

“After exposure for 30 days, there was an increase in the levels of MDA, SOD, and catalase but a decrease in GSH.”

What the sources say

• ?Not checked — no full text of the sourcecites [54] Zhong, 2022, Effect of fluoride in drinking water on fecal microbial community in rats, Biol Trace Elem Res source

In the paper

“After 60- and 90-day exposure, levels of reactive oxygen species, MDA, and myeloperoxidase were enhanced. However, the levels of catalase, GSH, acid phosphatase, and lysozyme were lowered and were associated with numbers of Firmicutes54”

What the sources say

• ?Not checked — no full text of the sourcecites [54] Zhong, 2022, Effect of fluoride in drinking water on fecal microbial community in rats, Biol Trace Elem Res source

In the paper

“In silkworm39,68,70 and Bufo gargarizans tadpole49 animal models, increased exposure to a high dose of fluoride (200 mg L–1 and 5-50 mg L–1 NaF, respectively) led to a decrease in Enterococcus and Bacillus in silkworms and Firmicutes, Synergistetes (synonym Synergistota), and Bacteroides in Bufo gargarizans but an increase in Staphylococcus in silkworm and in Deltaproteobacteria, Endomicrobia, and”

In the paper

“Most of the included studies were animal based and used very high doses of fluoride (25 mg L–1 NaF in water) compared with the safe intake recommended by WHO (0.8–1.7 mg L–1 NaF in water) in humans.”

In the paper

“Findings indicated a biphasic response with fluoride-inducing bacterium growth at a low dose (0.1 mM NaF)71 and a dramatic increase in bacterium growth and enzyme production, especially of enolase.”

In the paper

“However, there are a limited number of human studies exploring the effects of fluoride on the gut microbiome, making it difficult to draw conclusions.”

In the paper

“Specific bacteria shown to be more abundant in health compared with disease states include Bifidobacterium and Lactobacillus spp.12”

What the sources say

• Not checked yet

  Checked before — its earlier verdict is held back until a new check runs.

  cites [12] Mishra, 2014, Fluoride uptake and net primary productivity of selected crops, Oen J Soil Sci source

In the paper

“Fluoride is well recognized for its role in preventing and reversing dental caries.13”

What the sources say

• Not checked yet

  Checked before — its earlier verdict is held back until a new check runs.

  cites [13] Štepec, 2019, Fluoride in human health and nutrition, Acta Chim Slov source

In the paper

“A literature search was undertaken in July 2022 and a rerun of the search was conducted in July 2023 and then in 2024, using the following 6 electronic databases: MEDLINE, CINAHL, Embase, Scopus, PubMed, and Web of Science.”

In the paper

“There were no restrictions based on the date of publication.”

In the paper

“The methodological quality of the included studies was assessed based on the Mixed Methods Appraisal Tool using criteria specific to observational studies (including cohort studies and case-control studies), quantitative studies (including randomized and nonrandomized controlled studies), and laboratory-based studies.”

In the paper

“The full details of the processes including the eligibility criteria, search strategy, extraction process, and data analysis were prespecified and documented in a protocol that was registered in the PROSPERO database (registration no. CRD42022347357).”

In the paper

“A search of the reference lists of relevant articles was also performed to identify other potentially relevant sources.”

In the paper

“Results from all the 49 studies are presented narratively because they were unable to be statistically pooled.”

In the paper

“Although fluoride has been widely used as a preventive agent for dental caries, the effects of fluoride on the gut microbial communities still need to be understood.16”

What the sources say

• ?Not checked — no full text of the sourcecites [16] Jones, 2005, The effective use of fluorides in public health, Bull World Health Organ

In the paper

“Full text of seemingly relevant articles was then screened by 2 reviewers independently (M.Y. and M.P.S.) to determine their eligibility for inclusion in the final review.”

In the paper

“Fluoride can affect microbial community abundance, which may enhance its ability to prevent the growth of harmful bacteria and fungi or stimulate the growth of useful bacteria that reside in the gastrointestinal tract and vice versa, as summarized by Moran et al17”

What the sources say

• ?Not matched

  Couldn’t be matched to a paper in Crossref, so it can’t be checked.

  cites Moran et al17

In the paper

“This disturbance can lead to IBD, colitis, and even cancers, due to lack of microbial metabolites (eg, SCFAs) that control the metabolic pathways.”

In the paper

“Data extraction was conducted by 1 reviewer (M.Y.) and cross-checked by another reviewer (E.A.K.).”

In the paper

“Manual searching was also done to exhaust all possibilities and to reduce the risk of bias, as follows: (1) reference lists from the included studies were searched to identify any relevant study; (2) performing citation tracking in which all the articles that cited each of the included articles were tracked; and (3) related articles in Google Scholar and Science Direct were searched for to prevent”

In the paper

“The abstract and title of each paper were screened independently for eligibility by 2 individuals (M.Y. and M.P.S.) according to the eligibility requirements.”

In the paper

“This systemic review was conducted according to the recommendations of the Preferred Reporting Items for Systematic Reviews and Meta-analysis (PRISMA) statement and the guidelines of the Cochrane Handbook for Systematic Reviews of Interventions.25,26”

What the sources say

• Not checked yetcites [25] Page, 2021, The PRISMA 2020 statement: an updated guideline for reporting systematic reviews, Int J Surg source
• ?Not checked — no full text of the sourcecites [26] Higgins, 2019 source

In the paper

“Nevertheless, it is mostly accepted that greater diversity, richness, and stability, and a higher relative abundance of species associated with the production of short-chain fatty acids (SCFAs) (eg, Faecalibacterium prausnitziii, Bacteroides spp, Roseburia spp, Bifidobacterium spp, and Lactobacillus spp) are hallmarks of a microbial community that is associated with better health outcomes.11”

What the sources say

• Not checked yetcites [11] So, 2018, Dietary fiber intervention on gut microbiota composition in healthy adults: a systematic review and meta-analysis, Am J Clin Nutr source

In the paper

“The Mixed Methods Appraisal Tool was used to assess the quality of the included studies.30”

What the sources say

• Not checked yetcites [30] Hong, 2018, The Mixed Methods Appraisal Tool (MMAT) version 2018 for information professionals and researchers, Educ Inf source

In the paper

“The increased expression of proinflammatory cytokines and antimicrobial peptides followed by a decrease in antioxidative enzymes and tight junction proteins, for example, can be described by a lower abundance of lactobacilli, Bifidobacterium, Clostridium, and Faecalibacterium, which was responsible for decreased SCFAs levels—specifically, butyrate content that maintains balance in the production”

In the paper

“Tryptophan metabolites are important for regulating host physiology and an aberrant decrease in their production is associated with IBD and colorectal cancer.82”

What the sources say

• Not checked yetcites [82] Agus, 2021, Gut microbiota-derived metabolites as central regulators in metabolic disorders, Gut source

In the paper

“An abnormal increase or decrease in the expression of these genes can result in several diseases, such as familial tumoral calcinosis.80,81”

What the sources say

• Not checked yet

  Checked before — its earlier verdict is held back until a new check runs.

  cites [80] Kielak, 2016, The ecology of Acidobacteria: moving beyond genes and genomes, Front Microbiol source
• Not checked yet

  Checked before — its earlier verdict is held back until a new check runs.

  cites [81] Sun, 2018, Functional metabolomics discover pentose and glucuronate interconversion pathways as promising targets for Yang Huang syndrome treatment with Yinchenhao Tang, RSC Adv source

In the paper

“The changes also were associated with a reduction in levels of glycoproteins and antioxidant enzymes such as SOD, catalase, and GSH, which may cause colorectal cancer and is linked with impairment in the integrity of the intestinal barrier.10,37,49,54”

What the sources say

• ?Not checked — no full text of the sourcecites [10] Zhu, 2022, Fluoride exposure cause colon microbiota dysbiosis by destroyed microenvironment and disturbed antimicrobial peptides expression in colon, Environ Pollut source
• ?Not checked — no full text of the sourcecites [37] Fu, 2022, Exercise alleviated intestinal damage and microbial disturbances in mice exposed to fluoride, Chemosphere source
• ?Not checked — no full text of the sourcecites [49] Wang, 2019, The effect of fluorine exposure on morphological indicators and intestinal microbial community in Bufo gargarizans tadpoles, Ecol Indicat source

In the paper

“These changes resulted in increased expression of proinflammatory cytokines, such as IL-17A and IL-22,10,37 and mucosal sIgA antibody,34,36 that further led to the production of antimicrobial peptides such as CRAMP, β-defensin-1, and β-defensin-3, consequently leading to inflammatory diseases like IBD.10”

What the sources say

• ?Not checked — no full text of the sourcecites [10] Zhu, 2022, Fluoride exposure cause colon microbiota dysbiosis by destroyed microenvironment and disturbed antimicrobial peptides expression in colon, Environ Pollut source
• ?Not checked — no full text of the sourcecites [37] Fu, 2022, Exercise alleviated intestinal damage and microbial disturbances in mice exposed to fluoride, Chemosphere source
• Not checked yet

  Checked before — its earlier verdict is held back until a new check runs.

  cites [34] Dionizio, 2021, Intestinal changes associated with fluoride exposure in rats: integrative morphological, proteomic and microbiome analyses, Chemosphere source

In the paper

“This systematic review disentangles the effects of fluoride on gut microbiota and its associated functions with good reliability, due to high-quality assessment scores for most studies and specificity due to the focus on gut microbiota within the inclusion criteria.”

In the paper

“Additionally, a decrease in the mRNA levels of BDNF, cAMP-responsive element, and stem cell factor were observed, which are an important modulators of neuroplasticity, thus leading to neurodevelopmental disorders.50”

What the sources say

• Not checked yet

  Checked before — its earlier verdict is held back until a new check runs.

  cites [50] Xin, 2021, Preventive effects of Lactobacillus johnsonii on the renal injury of mice induced by high fluoride exposure: insights from colonic microbiota and co-occurrence network analysis, Ecotoxicol Environ Saf source

In the paper

“However, the effect patterns in different studies varied, which might be due to different exposure levels of fluoride, biomarkers used, and the use of diverse animal models with different genetic makeup, sensitivity, and other characteristics such as age, sex, and weight.”

In the paper

“High doses of NaF (range, 25-200 mg L–1) with long-term exposure (1-6 months) most often were administered in studies using rat models, and these demonstrated that excessive fluoride exposure impacts gut microbiota composition, although variations in specific phyla and genera abundances were observed in different studies using the same dose and duration of fluoride.”

In the paper

“which could be due to the disturbance in the Firmicutes to Bacteroidetes ratio”

In the paper

“Additionally, including all eligible studies led to high inconsistency among the reported outcomes, thus preventing pooling of results.”

In the paper

“Overall, doses of fluoride higher than 25 mg L–1 NaF in animals and 2 mg L–1 NaF in humans could disturb microbial homeostasis, which could lead to the upregulation of proinflammatory metabolites expression and downregulation of antioxidative enzymes and junction proteins, thus triggering immune responses and intestinal integrity.”