Relationship between dental fluorosis and intelligence quotient of school going children in and around Lucknow district: a cross-sectional study

Abstract

Background: Fluoridation of drinking water, despite being regarded as one of the top ten public health achievements of the twentieth century, has remained a much debated concept. Various studies on animals and aborted human fetuses have confirmed that excessive fluoride intake during infancy and early childhood, causes a number of irreversible structural and functional changes in the CNS leading to memory, learning and intellectual deficits.

Aim: To compare the IQ levels of school children of two different locations, having different fluoride levels in water, and to establish a relationship between fluoride levels, prevalence of fluorosis and its effect on IQ levels.

Materials and Methods: A cross–sectional study was conducted among 429 children aged 6 – 12 years, selected by stratified random sampling from two different areas with different levels of fluoride in drinking water in and around Lucknow district. Dental fluorosis was measured using Dean’s Fluorosis Index. Intelligence Quotient was measured using Raven’s Coloured Progressive Matrices (1998 edition).

Results: Majority of the fluorosis free children (76.3%) had an IQ grade 2 (definitely above the average). Majority of the children suffering from very mild and mild dental fluorosis were found to have IQ grade 3 (Intellectually average). Children with moderate cases of dental fluorosis were found to have IQ grade 4 (Definitely below average). Only 5 children with severe fluorosis were included in the study and they all were found to have an IQ grade 5. Hence, a trend of increase in the IQ grade (decrease in intellectual capacity) was observed indicating a strong correlation between fluorosis grade and IQ grade.

Conclusion: Findings of this study suggest that the overall IQ of the children exposed to high fluoride levels in drinking water and hence suffering from dental fluorosis were significantly lower than those of the low fluoride area.

EXCERPTS:

DISCUSSION

The present study was a cross-sectional study conducted on a stratified random sample of 429 school going children selected from 2 different areas in Lucknow and Unnao with different water fluoride concentrations in their drinking water supply. In the present study, for the estimation of fluoride in drinking water, IS 3025 (Part 60): 2008 (Ion Selective Electrode Method), originally developed by Frant & Ross has been used [16]. This method has been chosen for the reason that it is an automatic analysing system & can be used for estimation of both ionic & non ionic forms of fluoride in contrast to other methods which cannot distinguish between organic and inorganic fluoride [16]. Also, it has tremendous tolerance for extraneous ions like sulfates & phosphates and does calibrations to give final readings in parts per million & is more accurate than the other methods [16].

In this study, on comparison of children at two locations according to IQ grades [Table/Fig-4], majority of the children (74.8%) living in low fluoride area had an IQ grade 2 (definitely above the average in intellectual capacity). None of the children from the low fluoride area had an IQ grade 4 and 5 (definitely below average and intellectually impaired). On the other hand, majority of children (58.1%) from high fluoride area fall under IQ grade 3 (intellectually average). None of the children from high fluoride area had an IQ grade 1 (intellectually superior). This difference in IQ grades of children amongst the two areas was found to be statistically significant (p<0.001). Sudhir et al., has also reported only 7.8% children with IQ grade 2 and 3 (definitely above the average and intellectually average) in the high fluoride area as compared to 29.9% children with IQ grade 2 and 3 in the low fluoride area [24].

In this study, on analysing the association between different grades of dental fluorosis and IQ grades in the overall study population [Table/Fig-6], it was observed that majority of the fluorosis free children (76.3%) had an IQ grade 2. Only 6 out of 429 children with IQ grade 1 were found in our study, and they were all belonging to the fluorosis free population. No IQ grade 5 children were reported amongst normal population. Majority of the children (82.3%) suffering from very mild dental fluorosis were found to have IQ grade 3. Most of the mild fluorosis index children (65.9%) were also found to have IQ grade 3 followed by IQ grade 4 (15.9%). None of the children with moderate fluorosis were found to have IQ grade 1 and 2. Only five children suffering from severe fluorosis were observed in our study sample and each one of them had an IQ grade 5. Thus it is clearly evident that with increase in the grade of fluorosis, a trend of increase in the IQ grade (decrease in intellectual capacity) was observed indicating a strong correlation between fluorosis grade and IQ grade (Spearman’s p=0.766). Sudhir et al., and Shivaprakash et al., in their study have also demonstrated that intellectual capacity of children decreases with increase in the fluorosis grade [4,24].

Possible mechanisms for the neurotoxic effects of fluoride may be explained by several animal studies. Controlled animal studies have shown that rats exposed to drinking water with high concentration of fluoride at weaning were found to have elevated fluoride levels in 6 of 7 brain regions and plasma fluoride levels 7 to 42 times higher than those found in control animals [14]. These were associated with deterioration of learning and memory ability, decreased thickness of post-synaptic density, increased width of synaptic cleft and high cholinesterase activity in brain tissues [30,31].

Once absorbed in the blood through diet, fluoride forms lipid soluble complexes which cross the blood brain barrier to accumulate in the cerebral tissues [32,33]. The penetrated fluoride complexes adversely affect the CNS development by different neurotoxic mechanisms, such as free radical generation, inhibition of anti-oxidants and mitochondrial energy enzymes and inhibition of glutamate transporters [13]. Studies in aborted human foetuses have shown disruption of neurotransmitters, receptors in nerve cells, increased density of neurons & undifferentiated neuroblasts [34]. There has also been evidence of reduction in number of mitochondria, rough endoplasmic reticulum (RER) and free ribosomes. The structural and functional changes in the CNS, specifically in the foetal period and the first 8 years of life, may lead to learning and intellectual deficits and cognitive dysfunction [14,32,34,35].

On comparison of IQ grades between the two genders [Table/Fig-7], no correlation was found (p=0.724) in the present study. These findings were further supported by the studies conducted by Li et al.,, Zhao et al., Sudhir et al., and Seraj et al., [10,24,29,36]. Nonetheless, in the research of Xiang et al., a significant relationship between IQ and gender was found in one of the two study villages [26]. However, in psychological literatures, gender is mentioned as a non-related factor to IQ [37].

Statistically no significant difference in the IQ of children from different age groups was found [Table/Fig-8]. Thus no correlation between age and IQ was established in the present study that included 6-11-year-old children with 429 subjects. Similar findings were reported by Li et al., and Sudhir et al., [24,29].

A host of many genetic, socio-economical and geographical factors have been known to influence the overall neuro-behavioural development of an individual [19]. Therefore, we have recruited samples from a homogenous rural population of Unnao and Tiwariganj, thus diminishing the effect of some environmental and inherited factors, yet it is obvious that complete exclusion of such factors is impossible.

Certain trace elements in water such as Arsenic and Lead as well as Iodine deficiency, may also affect the intellectual ability of children. Many researchers such as Amador et al., and Wang et al., have shown in their studies that children intelligence can be affected by high levels of Arsenic [25,38]. However, Amador et al., have revealed in their study that in comparison to fluoride, the effect attributable to Arsenic in the IQ reduction is smaller [25]. They have also confirmed through their study that observed deficits in IQ scores cannot be attributed to Lead exposure. Susheela et al., in their study revealed that since fluoride interferes with the activity of thyroid gland and secretion of its hormones, elevated fluoride uptake may cause iodine deficiency in fluorosed individuals, even when they reside in non-iodine deficient areas, hereby affecting the developing brain [39]. Nevertheless, further researches are required to investigate the effect of other environmental or geological contaminants on Intelligence.

LIMTATIONS

In the present study, assessment of the effect of fluoride concentration in drinking water on child’s IQ has been done, but it is possible that some other trace elements in drinking water like Arsenic, Lead, Iodine etc. may also have neurological side effects. Thus, further researches are required to investigate the effect of other environmental or geological contaminants on Intelligence.
In the present study, samples from a homogenous rural population of Unnao and Tiwariganj have been recruited to diminish the effect of some environmental and inherited factors, yet it is obvious that complete exclusion of such factors is impossible. Although, children who had a change in the source of drinking water since birth, were excluded in the present study, we could not completely exclude the influence of recall bias. Moreover, fluoride level in a particular source

CONCLUSION

In the light of all the facts discussed above, the present study that encompasses 429 subjects falling within the age group of 6-11 years, concludes that fluorosis and IQ levels were unaffected by age and gender, however, they were both affected by drinking water fluoride concentrations as suggested by the significant deficit in IQ of children living in the high fluoride area as compared to those living in low fluoride area. The data from this research may support the hypothesis that excess fluoride in drinking water has toxic effects on the nervous system. Millions of children around the world are exposed to high concentrations of fluoride in water and are potentially at risk. Therefore, a close monitoring of fluoride levels in local water supplies from areas with endemic fluorosis and implementing public health measures, such as defluoridation, to reduce the fluoride exposure levels in high fluoride regions seem necessary.

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ABSTRACT ONLINE AT https://pubmed.ncbi.nlm.nih.gov/26673535/

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