A systematic review and meta-analysis of the association between fluoride exposure and neurological disorders

Dose-dependent developmental fluoride exposure leads to neurotoxicity and impairs excitatory synapse development

Developmental fluoride exposure has been implicated in cognitive deficits and neurotoxicity, yet the mechanisms underlying these effects remain unclear. Here, we investigated the dose- and time-dependent impacts of sodium fluoride (NaF) on neuronal morphology, viability, oxidative stress, and synaptic function using both in vitro and in vivo mouse models. Cultured primary embryonic mouse cortical neurons were exposed to varying concentrations of NaF (0-200 μg/ml). Acute exposure led to neuronal swelling at higher concentrations (≥ 50 μg/ml), while prolonged exposure reduced neuronal viability. Notably, NaF dose-dependently elevated reactive oxygen species (ROS) production, implicating oxidative stress as a key mechanism of fluoride-induced neurotoxicity. Synaptic development was also impaired, as evidenced by reduced density and co-localization of excitatory synapse markers with prolonged 2 μg/ml NaF exposure. To extend these in vitro findings, pregnant mice were exposed to 50 mg/L NaF in drinking water, and offspring brain functions were evaluated postnatally. Whole-cell patch-clamp recordings in layer V pyramidal neurons in the prefrontal cortex revealed reduced frequency and amplitude of miniature excitatory post-synaptic currents (mEPSCs), indicating impaired synaptic function. Morphological analysis showed decreased dendritic spine density and head diameter. These findings suggest that fluoride exposure during critical period of brain development disrupts synaptic integrity and function through excitatory synapse impairments.

Assessment of Trends in Non-Restorative and Preventative Dental Treatment Pre- and Post-COVID-19: A Health Informatics Pilot Study

BACKGROUND/OBJECTIVES

The implementation of preventive treatments in pediatric dental care has been a priority in recent years. Understanding the factors that influence the timing and frequency of childhood preventive treatments, such as the impacts of the COVID-19 pandemic, are the focus of many health informatics researchers.

METHODS

A retrospective study was approved to assess changes in specific preventive treatments at a pediatric dental school clinic (sealants, fluoride varnish, and silver diamine fluoride) in the three years prior to (2017-2019) and following the COVID-19 pandemic (2020-2022).

RESULTS

A detailed analysis of these data revealed significant and unexpected shifts in these preventive services, with significant increases in dental sealants from pre- to post-pandemic (35.1%, p = 0.012), but corresponding decreases in other preventive treatments, such as the number of fluoride varnish (-37.6%, p = 0.011) and SDF treatments (-24.0%, p = 0.032), among this patient population.

CONCLUSIONS

These data suggest that the selective pursuit of particular preventive dental services and treatments rather than others and understanding these shifts might help health informatics and dental public health researchers understand which factors influenced these decisions and behaviors, such as long-term durability and efficacy (sealants) or changing public perceptions of safety (fluoride and SDF).

Fluoride Exposure and Children's IQ Scores: A Systematic Review and Meta-Analysis

Importance

Previous meta-analyses suggest that fluoride exposure is adversely associated with children's IQ scores. An individual's total fluoride exposure comes primarily from fluoride in drinking water, food, and beverages.

Objective

To perform a systematic review and meta-analysis of epidemiological studies investigating children's IQ scores and prenatal or postnatal fluoride exposure.

Data Sources

BIOSIS, Embase, PsycInfo, PubMed, Scopus, Web of Science, CNKI, and Wanfang, searched through October 2023.

Study Selection

Studies reporting children's IQ scores, fluoride exposure, and effect sizes.

Data Extraction and Synthesis

Data were extracted into the Health Assessment Workplace Collaborative system. Study quality was evaluated using the OHAT risk-of-bias tool. Pooled standardized mean differences (SMDs) and regression coefficients were estimated with random-effects models.

Main Outcomes and Measures

Children's IQ scores.

Results

Of 74 studies included (64 cross-sectional and 10 cohort studies), most were conducted in China (n = 45); other locations included Canada (n = 3), Denmark (n = 1), India (n = 12), Iran (n = 4), Mexico (n = 4), New Zealand (n = 1), Pakistan (n = 2), Spain (n = 1), and Taiwan (n = 1). Fifty-two studies were rated high risk of bias and 22 were rated low risk of bias. Sixty-four studies reported inverse associations between fluoride exposure measures and children's IQ. Analysis of 59 studies with group-level measures of fluoride in drinking water, dental fluorosis, or other measures of fluoride exposure (47 high risk of bias, 12 low risk of bias; n = 20 932 children) showed an inverse association between fluoride exposure and IQ (pooled SMD, -0.45; 95% CI, -0.57 to -0.33; P < .001). In 31 studies reporting fluoride measured in drinking water, a dose-response association was found between exposed and reference groups (SMD, -0.15; 95% CI, -0.20 to -0.11; P < .001), and associations remained inverse when exposed groups were restricted to less than 4 mg/L and less than 2 mg/L; however, the association was null at less than 1.5 mg/L. In analyses restricted to low risk-of-bias studies, the association remained inverse when exposure was restricted to less than 4 mg/L, less than 2 mg/L, and less than 1.5 mg/L fluoride in drinking water. In 20 studies reporting fluoride measured in urine, there was an inverse dose-response association (SMD, -0.15; 95% CI, -0.23 to -0.07; P < .001). Associations remained inverse when exposed groups were restricted to less than 4 mg/L, less than 2 mg/L, and less than 1.5 mg/L fluoride in urine; the associations held in analyses restricted to the low risk-of-bias studies. Analysis of 13 studies with individual-level measures found an IQ score decrease of 1.63 points (95% CI, -2.33 to -0.93; P < .001) per 1-mg/L increase in urinary fluoride. Among low risk-of-bias studies, there was an IQ score decrease of 1.14 points (95% CI, -1.68 to -0.61; P < .001). Associations remained inverse when stratified by risk of bias, sex, age, outcome assessment type, country, exposure timing, and exposure matrix.

Conclusions and Relevance

This systematic review and meta-analysis found inverse associations and a dose-response association between fluoride measurements in urine and drinking water and children's IQ across the large multicountry epidemiological literature. There were limited data and uncertainty in the dose-response association between fluoride exposure and children's IQ when fluoride exposure was estimated by drinking water alone at concentrations less than 1.5 mg/L. These findings may inform future comprehensive public health risk-benefit assessments of fluoride exposures.

Correlation of fluoride intake with haemoglobin level and intelligence quotient in 8-12 year aged children: an observational study from India

BACKGROUND

Fluorosis caused by excess intake of fluoride can affects various soft tissues of the body, such as the gastrointestinal tract, blood, brain tissues and thyroid gland apart from dental fluorosis and skeletal fluorosis. Nonskeletal fluorosis is considered reversible if diagnosed early and treated promptly. Therefore, diagnostic methods that can be easily performed even by primary health care workers and depict any ongoing health problems, should be validated. Dental fluorosis, assessment of fluoride in urine and water are tests that fulfill these requirements. To date, no study has correlated haemoglobin (Hb) with dental fluorosis; moreover, studies focusing on intelligence quotient (IQ) had conflicting results and need further research. Hence, study was conducted to determine any relationship among different fluoride assessment parameters (severity of dental fluorosis, fluoride level in urine and drinking water) with IQ status and hemoglobin level of children aged 8-12 years, affected with or without dental fluorosis.

METHODS

A total of 300 children aged 8-12 years were evaluated for dental fluorosis via Dean's index, IQ level via Raven's coloured progressive matrices test, Hb level, and fluoride content in water and urine.

RESULTS

Water fluoride, age and gender were significantly associated with Hb. Intelligence was significantly related to urinary fluoride levels. Presence or absence of dental fluorosis and its severity were not significantly related to IQ or Hb.

CONCLUSIONS

Excess fluoride intake has adverse effects on hematological parameters and children's cognitive neurodevelopment, which were evaluated by current fluoride exposure markers, i.e., water and urinary fluoride. However, dental fluorosis cannot be used as a definitive assessment marker for these conditions, as it is not significantly correlated with these conditions.

Effects of Quercetin against fluoride-induced neurotoxicity in the medial prefrontal cortex of rats: A stereological, histochemical and behavioral study

BACKGROUND

Exposure to high levels of fluoride leads to brain developmental and functional damage. Motor performance deficits, learning and memory dysfunctions are related to fluoride neurotoxicity in human and rodent studies.

MATERIALS AND METHODS

Here, we evaluated the effects of Quercetin treatment (25 mg/kg) against sodium fluoride-induced neurotoxicity (NaF, 200 ppm) in the medial prefrontal cortex (mPFC) of male adult rats based on oxidative markers, behavioral performances, mRNA expressions, and stereological parameters. After a 4-week experimental period, the brains of rats were collected and used for molecular and histological analysis.

RESULTS

We found that 4 weeks of NaF exposure decreased body weight, working memory, Brain-derived neurotrophic factor (BDNF) mRNA expression, total volume of mPFC, number of neurons and non-neuronal cells in the mPFC, and anti-oxidative markers (CAT, SOD, and GSH-Px), while increased lipid peroxidation, P53 mRNA expression and anxiety. Quercetin treatment could significantly reverse the neurotoxic effect of NaF in the mPFC.

CONCLUSIONS

In summary, Quercetin could decrease the detrimental effects of NaF in the mPFC of adult rats by improving antioxidant potency and consequently decreasing neuronal and non-neuronal apoptosis.

Sex-Specific Mechanisms of Fluoride-Induced Gonadal Injury: A Multi-Omics Investigation into Reproductive Toxicity and Gut Microbiota Disruption

Fluoride, a common agricultural additive used to enhance plant resilience and pest control, poses toxicity risks when exposure surpasses safe thresholds, affecting ecosystems and human health. While its reproductive toxicity is recognized, the sex-specific and cross-generational effects remain underexplored. To address this gap, we employed an integrative approach combining transcriptomics (next-generation sequencing (NGS)), bioinformatic network analysis, gut microbiota sequencing, and in vivo functional assays. ICR mice (F0 generation), both male and female, were exposed to fluoride (100 mg/L in drinking water) for 35 days, continuing through gestation and offspring weaning. Our transcriptomic analysis revealed significant upregulation of autophagy (via the PI3K-AKT-mTOR pathway) and oxidative stress-induced mitochondrial dysfunction in gonadal tissue, with more pronounced effects observed in males. Further integrated analyses of transcriptomic and metabolomic data, supported by in vivo experiments, highlighted oxidative stress, mitochondrial dysfunction, and PI3K-AKT-mTOR pathway activation with stronger effects in males. The principal component analysis confirmed sex-specific transcriptome alterations, with males showing more substantial disruption. Additionally, 16S rRNA sequencing identified significant gut dysbiosis, particularly in males, with an increased Firmicutes/Bacteroidetes ratio and higher abundances of Oscillospirales and Anaerovoracaceae. Moreover, our study identified significant correlations between specific gut microbiota (e.g., Firmicutes, Proteobacteria) and autophagy, oxidative stress, and mitochondrial dysfunction pathways, with notable sex-dependent differences. These findings suggest that gut microbiota may play a critical role in modulating fluoride-induced reproductive toxicity, particularly through their effects on oxidative stress and cellular homeostasis. The breakdown of the gut barrier and elevated serum/gonadal lipopolysaccharide (LPS) levels in fluoride-treated mice further established a link between gut dysbiosis and fluoride-induced reproductive toxicity. These findings underscore the importance of considering sex differences in xenobiotic-induced reproductive and developmental toxicity.

AMPA and NMDA Receptors in Hippocampus of Rats with Fluoride-Induced Cognitive Decline

This experimental study was performed to evaluate the alterations in the expression of a few subunits composing glutamate AMPA (a-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) and NMDA (N-methyl-D-aspartate) receptors in the hippocampal cells of Wistar rats in response to long-term fluoride (F-) exposure. The animals were given water with background 0.4 (control), 5, 20, and 50 ppm F- (as NaF) for 12 months. The cognitive capacities of rats were examined by novel object recognition (NOR), Y-maze test, and Morris water maze tests. RT-qPCR and Western blotting techniques were used to evaluate the expression of different AMPA and NMDA subunits at transcriptional and translational levels, respectively. Long-term F- poisoning disturbed the formation of hippocampus-dependent working spatial and long-term non-spatial memory. The expression of Gria1, Gria2, and Gria3 genes encoding different subunits of AMPA receptors were comparable in hippocampi of control and F--exposed animals, although the levels of both Grin2a and Grin2b mRNA increased. Long-term F- intake enhanced the ratio of phospho-GluA1/total-GluA1 proteins in subcellular fraction enriched with cytosolic proteins, while decreased content of GluA2 but elevated level of GluA3 were observed in subcellular fraction enriched with membrane proteins. Such changes were accompanied by increased phosphorylation of GluN2A and GluN2B subunits, higher ratios of GluN2A/GluN1 and GluN2B/GluN1 proteins in the cytosol, and GluN2A/GluN2B ratio in membranes. These changes indicate the predominance of Ca2+-permeable AMPARs in membranes and a shift between different NMDARs subunits in hippocampal cells of F--exposed rats, which is typical for neurodegeneration and can at least partially underly the observed disturbances in cognitive capacities of animals.

Do missing teeth cause early-onset cognitive impairment? Re-examining the evidence using a quasi-natural experiment

PURPOSE

Multiple studies have reported a positive association between missing teeth and cognitive impairment. While some authors have postulated causal mechanisms, existing designs preclude assessing this.

METHODS

We sought evidence of a causal effect of missing teeth on early-onset cognitive impairment in a natural experiment, using differential exposure to fluoridated water during critical childhood years (ages 5-20 years) in England as the instrument. We coded missing teeth from 0 (≤ 12 missing) to 3 (all missing) and measured the association with cognitive impairment in the English Longitudinal Study of Ageing data (2014-5), covering 4958 persons aged 50-70 years.

RESULTS

We first replicated previous evidence of the strongly positive association of missing teeth with cognitive impairment (β = 0.25 [0.11, 0.39]), after adjusting for socio-demographic covariates, such as age, gender, education, and wealth. Using an instrumental variable design, we found that childhood exposure to water fluoridation was strongly associated with fewer missing teeth, with being exposed to fluoridated water during childhood (16 years) associated with a 0.96 reduction in the missing teeth scale (β = - 0.06 [- 0.10, - 0.02]). However, when using the instrumented measure of missing teeth, predicted by probability of fluoride exposure, we found that missing teeth no longer had an association with cognitive impairment (β = 1.48 [- 1.22, 4.17]), suggesting that previous oral health-cognitive impairment associations had unobserved confounding.

CONCLUSIONS

Our findings are consistent with the possibility that unobserved confounding leads to the oft-observed association between missing teeth and early-onset cognitive impairment, suggesting that the relationship is spurious rather than causal.

Community Water Fluoridation in Focus: A Comprehensive Look at Fluoridation Levels across America

Objective: This study reports on the number and percentage of community water systems (CWSs) meeting fluoride concentration standards set by the U.S. Department of Health and Human Services (DHHS). The study also explored changes in the population exposed to optimally fluoridated water in these systems between 2006 and 2020. Methods: This study analyzed U.S. Centers for Disease Control and Prevention data from 2006 to 2020, tabulating state-specific CWS fluoridation rates, ranking them, and calculating the percent change. Results: In 2020, 72.7% of the US population received CWS water, with 62.9% of those individuals served by a CWS system meeting DHHS fluoridation standards. This compares to 69.2% receiving CWS water in 2006 and 74.6% in 2012. The overall change in those receiving fluoridated water was 1.4%, from 61.5% in 2006 to 62.9% in 2020. State-specific percentages ranged from 8.5% in Hawaii to 100% in Washington DC in 2020 (median: 76.4%). Conclusions: Although endorsed by the American Dental Association, the percentage of individuals receiving fluoridated water did not increase substantially from 2006 to 2020, indicating that there has not been much progress toward meeting the Healthy People 2030 goal that 77.1% of Americans receive water with enough fluoride to prevent tooth decay.

Prolonged exposure to high fluoride levels during adolescence to adulthood elicits molecular, morphological, and functional impairments in the hippocampus

Fluoride is added to water due to its anticariogenic activity. However, due to its natural presence in soils and reservoirs at high levels, it could be a potential environmental toxicant. This study investigated whether prolonged exposure to fluoride from adolescence to adulthood-at concentrations commonly found in artificially fluoridated water and in fluorosis endemic areas-is associated with memory and learning impairments in mice, and assessed the molecular and morphological aspects involved. For this endeavor, 21-days-old mice received 10 or 50 mg/L of fluoride in drinking water for 60 days and the results indicated that the increased plasma fluoride bioavailability was associated with the triggering of short- and long-term memory impairments after high F concentration levels. These changes were associated with modulation of the hippocampal proteomic profile, especially of proteins related to synaptic communication, and a neurodegenerative pattern in the CA3 and DG. From a translational perspective, our data provide evidence of potential molecular targets of fluoride neurotoxicity in the hippocampus at levels much higher than that in artificially fluoridated water and reinforce the safety of exposure to low concentrations of fluoride. In conclusion, prolonged exposure to the optimum fluoride level of artificially fluoridated water was not associated with cognitive impairments, while a higher concentration associated with fluorosis triggered memory and learning deficits, associated with a neuronal density reduction in the hippocampus.

Analysis of Fluoride-Free Content on Twitter: Topic Modeling Study

BACKGROUND

Although social media has the potential to spread misinformation, it can also be a valuable tool for elucidating the social factors that contribute to the onset of negative beliefs. As a result, data mining has become a widely used technique in infodemiology and infoveillance research to combat misinformation effects. On the other hand, there is a lack of studies that specifically aim to investigate misinformation about fluoride on Twitter. Web-based individual concerns on the side effects of fluoridated oral care products and tap water stimulate the emergence and propagation of convictions that boost antifluoridation activism. In this sense, a previous content analysis-driven study demonstrated that the term fluoride-free was frequently associated with antifluoridation interests.

OBJECTIVE

This study aimed to analyze "fluoride-free" tweets regarding their topics and frequency of publication over time.

METHODS

A total of 21,169 tweets published in English between May 2016 and May 2022 that included the keyword "fluoride-free" were retrieved by the Twitter application programming interface. Latent Dirichlet allocation (LDA) topic modeling was applied to identify the salient terms and topics. The similarity between topics was calculated through an intertopic distance map. Moreover, an investigator manually assessed a sample of tweets depicting each of the most representative word groups that determined specific issues. Lastly, additional data visualization was performed regarding the total count of each topic of fluoride-free record and its relevance over time, using Elastic Stack software.

RESULTS

We identified 3 issues by applying the LDA topic modeling: "healthy lifestyle" (topic 1), "consumption of natural/organic oral care products" (topic 2), and "recommendations for using fluoride-free products/measures" (topic 3). Topic 1 was related to users' concerns about leading a healthier lifestyle and the potential impacts of fluoride consumption, including its hypothetical toxicity. Complementarily, topic 2 was associated with users' personal interests and perceptions of consuming natural and organic fluoride-free oral care products, whereas topic 3 was linked to users' recommendations for using fluoride-free products (eg, switching from fluoridated toothpaste to fluoride-free alternatives) and measures (eg, consuming unfluoridated bottled water instead of fluoridated tap water), comprising the propaganda of dental products. Additionally, the count of tweets on fluoride-free content decreased between 2016 and 2019 but increased again from 2020 onward.

CONCLUSIONS

Public concerns toward a healthy lifestyle, including the adoption of natural and organic cosmetics, seem to be the main motivation of the recent increase of "fluoride-free" tweets, which can be boosted by the propagation of fluoride falsehoods on the web. Therefore, public health authorities, health professionals, and legislators should be aware of the spread of fluoride-free content on social media to create and implement strategies against their potential health damage for the population.

Unraveling molecular characteristic of fluoride neurotoxicity on U87 glial-like cells: insights from transcriptomic and proteomic approach

The potential of fluoride (F) as a neurotoxicant in humans is still controversial in the literature. However, recent studies have raised the debate by showing different mechanism of F-induced neurotoxicity, as oxidative stress, energy metabolism and inflammation in the central nervous system (CNS). In the present study, we investigated the mechanistic action of two F concentration (0.095 and 0.22 μg/ml) on gene and protein profile network using a human glial cell in vitro model over 10 days of exposure. A total of 823 genes and 2,084 genes were modulated after exposure to 0.095 and 0.22 μg/ml F, respectively. Among them, 168 were found to be modulated by both concentrations. The number of changes in protein expression induced by F were 20 and 10, respectively. Gene ontology annotations showed that the main terms were related to cellular metabolism, protein modification and cell death regulation pathways, such as the MAP kinase (MAPK) cascade, in a concentration independent manner. Proteomics confirmed the changes in energy metabolism and also provided evidence of F-induced changes in cytoskeleton components of glial cells. Our results not only reveal that F has the potential to modulate gene and protein profiles in human U87 glial-like cells overexposed to F, but also identify a possible role of this ion in cytoskeleton disorganization.

Prolonged fluoride exposure induces spatial-memory deficit and hippocampal dysfunction by inhibiting small heat shock protein 22 in mice

Millions of residents in areas with high-fluoride drinking water supply ingest excessive levels of fluoride for long periods. This study investigated the mechanisms and impacts of lifelong exposure to naturally occurring moderate-high-fluoride drinking water on spatial-memory function by studying mice in controlled experiments. Spatial-memory deficits and disorders of hippocampal neuronal electrical activity were observed in mice exposed to 25-ppm or 50-ppm-fluoride drinking water for 56 weeks, but not in adult or old mice exposed to 50 ppm fluoride for 12 weeks. Ultrastructural analysis showed severely damaged hippocampal mitochondria, evidenced by reduced mitochondrial membrane potential and ATP content. Mitochondrial biogenesis was impaired in fluoride-exposed mice, manifesting as a significantly reduced mtDNA content, mtDNA-encoded subunits mtND6 and mtCO1, and respiratory complex activities. Fluoride reduced expression of Hsp22, a beneficial mediator of mitochondrial homeostasis, and decreased levels of signaling for the PGC-1α/TFAM pathway-which regulates mitochondrial biogenesis-and the NF-κβ/STAT3 pathway-which regulates mitochondrial respiratory chain enzyme activity. Hippocampus-specific Hsp22-overexpression improved fluoride-induced spatial-memory deficits by activating the PGC-1α/TFAM and STAT3 signaling pathways, while Hsp22-silencing aggravated the deficits by inhibiting both pathways. Downregulation of Hsp22 plays a vital role in fluoride-induced spatial-memory deficits by impacting mtDNA-encoding subsets and mitochondrial respiratory chain enzyme activity.

Association between low fluoride exposure and children's intelligence: a meta-analysis relevant to community water fluoridation

OBJECTIVES

Previous meta-analyses have mainly focused on studies conducted in endemic fluorosis areas with relatively high fluoride concentrations. These are impoverished rural communities in China, India, and Iran, and the findings cannot be generalised to developed countries. Therefore, we investigated the association between fluoride concentrations relevant to community water fluoridation and children's cognition measured with IQ scores by synthesising effect sizes reported in observational studies.

METHODS

A previous meta-analysis and the National Toxicology Program database that included a search of multiple databases and the authors' search of PubMed, Google Scholar, and Mendeley provided the data. Cross-sectional and cohort studies examining the association between fluoride and children's cognition and intelligence scores were selected. Two reviewers abstracted data using standard procedures. We performed three meta-analyses to synthesise the effects using the random effects models.

RESULTS

Eight studies of standardized mean difference in IQ scores from non-endemic fluorosis areas found no statistically significant difference between recommended and lower levels of fluoride (standardized mean difference = 0.07; 95% confidence interval: -0.02, 0.17; I² = 0%), and no significant fluctuation in IQ scores across the differences in fluoride concentrations by non-linear modeling with restricted cubic spline (P = 0.21). Meta-analyses of children's and maternal spot urinary fluoride associated pooled regression coefficients (Beta_children = 0.16; 95% confidence interval: -0.40, 0.73; P = 0.57; I² = 0%, Beta_maternal = -0.92; 95% CI: -3.29, 1.46; P = 0.45; I² = 72%) were not statistically significant. Further regression analysis by standardizing absolute mean IQ scores from lower fluoride areas did not show a relationship between F concentration and IQ scores (Model Likelihood-ratio test: P-value = 0.34.) CONCLUSIONS: These meta-analyses show that fluoride exposure relevant to community water fluoridation is not associated with lower IQ scores in children. However, the reported association observed at higher fluoride levels in endemic areas requires further investigation.

Fluoride Exposure and ADHD: A Systematic Review of Epidemiological Studies

Background and objectives: Attention deficit hyperactivity disorder (ADHD) is a childhood-onset neurodevelopmental disorder characterized by two dimensions: inattentiveness and hyperactivity/impulsivity. ADHD may be the result of complex interactions between genetic, biological and environmental factors possibly including fluoride exposure. Materials and methods: A literature search was performed on 31 March 2023 in the following databases: PubMed, Embase and Web of Science. We defined the following inclusion criteria according to the PECOS statement: a healthy child and adolescent population (P), fluoride exposure of any type (E), comparison with low or null exposure (C), ADHD spectrum disorder (O), and ecological, cross-sectional, case-control and cohort studies (S). Results: We found eight eligible records corresponding to seven different studies investigating the effect of fluoride exposure on children and adolescents. One study had a cohort design and one a case-control one, while five were cross-sectional. Only three studies applied validated questionnaires for the purpose of ADHD diagnosis. As regards exposure assessment, levels of fluoride in urine and tap water were, respectively used in three and two studies, while two used both. Three studies reported a positive association with ADHD risk, all assessing exposure through fluoride levels. By using urinary fluoride, conversely, a positive correlation with inattention, internalizing symptoms, cognitive and psychosomatic problems was found in three studies, but no relation was found in the other one. Conclusions: The present review suggests that early exposure to fluoride may have neurotoxic effects on neurodevelopment affecting behavioral, cognitive and psychosomatic symptoms related to ADHD diagnosis. However, due to the heterogeneity of the studies included, current evidence does not allow to conclusively confirm that fluoride exposure is specifically linked to ADHD development.

Fluoride exposure and cognitive neurodevelopment: Systematic review and dose-response meta-analysis

Many uncertainties still surround the possible harmful effect of fluoride exposure on cognitive neurodevelopment in children. The aim of this systematic review and meta-analysis was to characterize this relation through a dose-response approach, by comparing the intelligence quotient (IQ) scores in the highest versus the lowest fluoride exposure category with a random-effects model, within a one-stage dose-response meta-analysis based on a cubic spline random-effects model. Out of 1996 potentially relevant literature records, 33 studies were eligible for this review, 30 of which were also suitable for meta-analysis. The summary mean difference of IQ score, comparing highest versus lowest fluoride categories and considering all types of exposure, was -4.68 (95% confidence interval-CI -6.45; -2.92), with a value of -5.60 (95% CI -7.76; -3.44) for drinking water fluoride and -3.84 (95% CI -7.93; 0.24) for urinary fluoride. Dose-response analysis showed a substantially linear IQ decrease for increasing water fluoride above 1 mg/L, with -3.05 (95% CI -4.06; -2.04) IQ points per 1 mg/L up to 2 mg/L, becoming steeper above such level. A weaker and substantially linear decrease of -2.15 (95% CI -4.48; 0.18) IQ points with increasing urinary fluoride emerged above 0.28 mg/L (approximately reflecting a water fluoride content of 0.7 mg/L). The inverse association between fluoride exposure and IQ was particularly strong in the studies at high risk of bias, while no adverse effect emerged in the only study judged at low risk of bias. Overall, most studies suggested an adverse effect of fluoride exposure on children's IQ, starting at low levels of exposure. However, a major role of residual confounding could not be ruled out, thus indicating the need of additional prospective studies at low risk of bias to conclusively assess the relation between fluoride exposure and cognitive neurodevelopment.

Applications of MXene and its modified materials in skin wound repair

The rapid healing and repair of skin wounds has been receiving much clinical attention. Covering the wound with wound dressing to promote wound healing is currently the main treatment for skin wound repair. However, the performance of wound dressing prepared by a single material is limited and cannot meet the requirements of complex conditions for wound healing. MXene is a new two-dimensional material with electrical conductivity, antibacterial and photothermal properties and other physical and biological properties, which has a wide range of applications in the field of biomedicine. Based on the pathophysiological process of wound healing and the properties of ideal wound dressing, this review will introduce the preparation and modification methods of MXene, systematically summarize and review the application status and mechanism of MXene in skin wound healing, and provide guidance for subsequent researchers to further apply MXene in the design of skin wound dressing.

Does fluoride exposure impact on the human microbiome?

Fluoride is added to drinking water in some countries to prevent tooth decay (caries). There is no conclusive evidence that community water fluoridation (CWF) at WHO recommended concentrations for caries prevention has any harmful effects. However, research is ongoing regarding potential effects of ingested fluoride on human neurodevelopment and endocrine dysfunction. Simultaneously, research has emerged highlighting the significance of the human microbiome in gastrointestinal and immune health. In this review we evaluate the literature examining the effect of fluoride exposure on the human microbiome. Unfortunately, none of the studies retrieved examined the effects of ingested fluoridated water on the human microbiome. Animal studies generally examined acute fluoride toxicity following ingestion of fluoridated food and water and conclude that fluoride exposure can detrimentally perturb the normal microbiome. These data are difficult to extrapolate to physiologically relevant human exposure dose ranges and the significance to humans living in areas with CWF requires further investigation. Conversely, evidence suggests that the use of fluoride containing oral hygiene products may have beneficial effects on the oral microbiome regarding caries prevention. Overall, while fluoride exposure does appear to impact the human and animal microbiome, the long-term consequences of this requires further study.

Drinking hot beverages from paper cups: Lifetime intake of microplastics

Microplastics (MPs) have been found in many packaged food products such as salt, tea bags, milk, and fish. In a previous study by this group, MPs were found to leach into hot water from the plastic lining of disposable paper cups. No studies were found in the literature quantifying health risks or lifetime intake of MPs. At present, it is not possible to quantify health risks due to MPs because dose-response and toxicity assessments are not available. Therefore, the objective of the current study was to assess the intake of MPs and associated contaminants like fluoride that are released into these hot beverages. MPs in the previous study were quantified in terms of particle counts only and a simple method was adopted in the present study to convert the microplastics count into its respective mass. Chronic daily intake (CDI) and lifetime intake (LTI) of MPs through the ingestion pathway were calculated. CDI and Hazard Quotient (HQ) due to fluoride ingestion were also estimated following USEPA guidelines. Monte Carlo (MC) simulations were used to account for the variability in input variables such as concentration of MPs, body weight, averaging time, exposure duration, exposure frequency and ingestion rate to evaluate the impact on CDI and LTI values. The CDI was used to estimate the LTI of MPs and HQ for fluoride ingestion. MC simulations with 100,000 iterations resulted in an average CDI of 0.03 ± 0.025 mg of microplastic per kg of body weight per day and 7.04 ± 8.8 μg fluoride per kg body weight per day. This study takes us one step closer to estimating the human health risk due to the ingestion of microplastics and other contaminants through food items.

Choline alleviated perinatal fluoride exposure-induced learning and memory impairment through α4β2 nAChRs and α7 nAChRs in offspring mice

Fluoride pollution is widely present in the living environment. As a critical period of brain development, the perinatal period is extremely vulnerable to fluoride. Studies have found that choline can protect the brain's memory and enhance the ability to focus. However, the effect of choline on perinatal fluoride-induced nerve damage remains unclear. Therefore, 32 Kunming newly conceived female mice and their offspring mice were randomly divided into control, NaF, LC + NaF, and HC + NaF groups, and the HE staining, Y-maze test, RT-PCR, western blotting, immunohistochemistry, etc. were used in this study. The results showed that fluoride decreased the brain organ coefficients and brain protein content (p < 0.05, p < 0.01), and caused histomorphological damage in the hippocampus and cortex, which suggested that fluoride affected the development of the brain and damaged the brain. Moreover, the results of the Y-maze test showed that fluoride increased the number of learning days, error reaction time, and total reaction time, and decreased the AchE activity in the brain (p < 0.05, p < 0.01), which indicated that fluoride reduced the learning and memory ability of the mice. Besides, the results showed that fluoride decreased the mRNA and protein expression levels of α4β2 nAChRs and α7 nAChRs in the hippocampus and cortex (p < 0.05, p < 0.01). However, perinatal choline supplementation reversed the aforementioned fluoride-induced changes. In short, these results demonstrated that choline alleviated perinatal fluoride-induced learning and memory impairment, which will provide a rationale for the mitigation and prevention of fluoride-induced brain damage.

The Relationship between Fluoride Exposure and Cognitive Outcomes from Gestation to Adulthood-A Systematic Review

Chronic exposure to high levels of fluoride may cause health concerns, including in cognitive function. This study reviewed the evidence on the association between fluoride exposure and cognitive outcomes in children from gestation up to 18 years old. A literature search was conducted for studies on pregnant women and children below 18, exposed to any source of fluoride, and assessed with a validated cognitive tool. The data were analyzed using a systematic narrative synthesis approach and by subgroup: study design, age of participants, levels of fluoride exposure and methodological quality. Our search retrieved 15,072 articles, of which 46 met the inclusion criteria. Only 6 of the studies had a longitudinal design; the remainder were cross-sectional. The levels of fluoride exposure were ≥2 mg/L in 27 studies and <2 mg/L in 13 studies; 6 studies did not report levels of fluoride exposure. Only 1 of 5 studies graded as excellent quality showed a negative association between fluoride exposure and cognitive outcomes, whereas 30 of 34 poor and fair quality studies reported a negative association. The overall evidence from this review suggests that high fluoride exposure might be associated with negative cognitive outcomes in children. However, more longitudinal studies with high methodological quality are needed on this topic.

Spectrophotometric Fluoride Determination Using St. John's Wort Extract as a Green Chromogenic Complexant for Al(III)

In this study, we applied an innovative approach of green analytical chemistry to develop a novel and eco-friendly chromogenic agent for fluoride determination by making use of the nontoxic Al(III)-flavonoid complex in a natural extract from St. John's wort plant. The initial intensely yellow-colored Al(III)-flavonoid complex formed in the plant extract was converted to a colorless AlF₆ ^3- complex with increasing amounts of fluoride, and color bleaching of the Al-flavonoid chromophore (measured as absorbance decrement) was proportional to fluoride concentration. The developed method gave a linear response within the F^- concentration range of 0.11-1.32 mM with the LOD and LOQ values of 0.026 mM (0.5 mg L^-1) and 0.079 mM (1.5 mg L^-1), respectively. The LOD value for fluoride was below the WHO-permissible limit (1.5 mg L^-1) and the US-EPA-enforceable limit (4 mg L^-1) in water. The possible interference effects of common anions (Cl^-, Br^-, I^-, NO₃ ^-, HCO₃ ^-, SO₄ ^2-, and PO₄ ^3-) and cations (K⁺, NH₄ ⁺, Ag⁺, Ca²⁺, Mg²⁺, Mn²⁺, Fe²⁺, and Fe³⁺) were investigated; the observed interferences from Fe²⁺, Fe³⁺, and PO₄ ^3- were easily eliminated by masking iron with the necessary amount of Na₂EDTA without affecting the blank absorbance of the Al(III)-flavonoid complex, precipitating phosphate with Ag(I) salt, and partly neutralizing alkaline water samples to pH 4 with acetic acid. The developed method was applied to real water samples and also validated against a reference spectroscopic method at the 95% confidence level.

Fluoride exposure duringintrauterine and lactation periods promotes changes in the offspring rats' alveolar bone

The importance of fluoride (F) for oral health is well established in the literature. However, evidence suggests that excessive exposure to this mineral is associated with adverse effects at different life stages and may affect many biological systems, especially mineralized tissues. The purpose of this study was to investigate the effects of F exposure during pregnancy and breastfeeding on the alveolar bone of the offspring since the alveolar bone is one of the supporting components of the dental elements. For this, the progeny rats were divided into three groups: control, 10 mg F/L, and 50 mg F/L for 42 (gestational and lactation periods). Analysis of the quantification of F levels in the alveolar bone by particle-induced gamma emission; Raman spectroscopy to investigate the physicochemical aspects and mineral components; computed microtomography to evaluate the alveolar bone microstructure and analyses were performed to evaluate osteocyte density and collagen quantification using polarized light microscopy. The results showed an increase in F levels in the alveolar bone, promoted changes in the chemical components in the bone of the 50 mg F/L animals (p < 0.001), and had repercussions on the microstructure of the alveolar bone, evidenced in the 10 mg F/L and 50 mg F/L groups (p < 0.001). Furthermore, F was able to modulate the content of organic bone matrix, mainly collagen; thus, this damage possibly reduced the amount of bone tissue and consequently increased the root exposure area of the exposed groups in comparison to a control group (p < 0.001). Our findings reveal that Fcan modulate the physicochemical and microstructural dimensions and reduction of alveolar bone height, increasing the exposed root region of the offspring during the prenatal and postnatal period. These findings suggest that F can modulate alveolar bone mechanical strength and force dissipation functionality.

Intrauterine and Postnatal Exposure to High Levels of Fluoride Is Associated with Motor Impairments, Oxidative Stress, and Morphological Damage in the Cerebellum of Offspring Rats

Fluoride (F) is abundantly present on Earth and plays a beneficial role in human health. However, exposure to high doses of F can be a risk, mainly in endemic fluorosis regions. In light of this, we investigated the effects of F exposure during the intrauterine and postnatal periods of rats, in doses similar to those recommended in drinking water and the levels of F in regions with endemic fluorosis, on the offspring rats’ cerebellum. Pregnant rats were divided into three groups: control (received ultrapure water only), 10 mg F/L, and 50 mg F/L for a period of 42 days (21 days gestation and 21 days lactation). At the end of the lactation period, the male pups were evaluated by behavioral tests, morphological markers, and biochemistry assays. The results pointed out that 50 mg F/L exposure during the intrauterine and lactational period of rats is capable of promoting oxidative stress in the cerebellum with a decrease in Purkinje cell density and myelin basic protein compromise, which could be associated with functional motor impairments. In addition, although 10 mg F/L exposure promoted redox alterations, it did not affect other parameters evaluated, highlighting the safe use of F in low doses.

The Relationships among Microelement Composition of Reindeer Meat (Rangifer tarandus) and Adaptation: A Systematic Review and Meta-Analysis

This systematic review and meta-analysis based on PRISMA statements aimed to summarise the data on the chemical composition of reindeer meat depending on the region of the Rangifer tarandus. We searched SCOPUS, PubMed, Embase, CrossRef, Medline, Cochrane library, eLibrary, and CyberLeninka. A total of 3310 records published between January 1980 and December 2021 were screened. We identified 34 relevant studies conducted in Russia, Norway, the USA, Canada, and Finland for the synthesis. Overall, the consumption of reindeer meat reduces arterial hypertension and atherosclerosis due to many polyunsaturated fatty acids (linoleic, linolenic, arachidonic) and vitamin C, which balances lipid fractions. Venison is an effective means of preventing obesity and adapting to cold due to the content of a complete set of essential trace elements, amino acids, and even L-carnitine. The high content of vitamin C and microelements (iron, zinc, copper) in reindeer meat is likely to increase the body’s antioxidant defence against free radicals and help prevent chronic non-infectious diseases. Thus, venison is an essential component of the adaptation mechanism for the Arctic population.