In the early 1990s, the National Institutes of Health (NIH) issued a large grant to University of Iowa researchers to investigate the relationship between total daily fluoride intake (from all sources) and several health outcomes of interest, namely: tooth decay, dental fluorosis, and bone health.
Known as the “Iowa Fluoride Study,” the researchers monitored the fluoride intake of over 600 Iowan children from birth to adolescence, while conducting periodic exams of the children’s dental and bone health. Thus far, the study — which remains an ongoing one — paints a picture of fluoride’s risk/benefits that is at stark odds with the quasi-mythical narrative that U.S. health authorities have long peddled.
1. Effect of Total Fluoride Ingestion on Teeth
In 2009, the Iowa researchers published the long-awaited data on the effect of total fluoride exposure on tooth health after 9 years of the children’s life. Much to the disappointment of fluoridation advocates, fluoride intake was found to be significantly associated with dental fluorosis, but not tooth decay.
The lack of effect between fluoride intake and tooth decay can be visually seen in a figure that the authors published, which shows that children with no cavities have ingested almost identical amounts of fluoride at each year of life as children with cavities. According to the authors:
“These findings suggest that achieving a caries-free status may have relatively little to do with fluoride intake, while fluorosis is clearly more dependent on fluoride intake.” (Warren 2009)
Based on these findings, the Iowa researchers note that the long-held idea of an “optimal fluoride intake” is “problematic” and “perhaps it is time that the term optimal fluoride intake be dropped from common usage.”
Not exactly the kind of statements you’d hear from your dentist.
2. Effect of Water Fluoride Level on Teeth
Another unflattering result from the Iowa study is the repeated finding in several published analyses that tooth decay is not significantly related to the fluoride level in the children’s water supply, but dental fluorosis is.
The authors first reported this in 2006 at the annual conference of the International Association of Dental Research, where they reported the results of their first two dental exams (the first at age 5 and the second at age 9). To quote:
“This study assessed the relationship between dental caries and fluorosis at varying fluoride levels in drinking water. 420 study subjects received dental examinations at age 5 on primary teeth and at age 9 on early-erupting permanent teeth… Conclusions: Fluorosis prevalence increased significantly with higher water fluoride levels; however, caries prevalence did not decline significantly.” (Hong 2006a)
In 2011, the Iowa team published the results of a third dental exam, which was conducted when the children reached the age of 13. As with the first two dental exams, the authors reported that the water fluoride level did not have a significant effect on tooth decay, including overt tooth decay (“cavitated caries”) and early decay (“non-cavitated caries”). To quote:
“Greater toothbrushing frequently was significantly associated with fewer new non-cavitated caries, while gender, exam variable, and composite water level were not significantly associated with new non-cavitated caries. . . . Gender, SES, tooth brushing frequency, and composite water fluoride level were not significantly associated with new cavitated caries.” (Chankanka 2011a)
In a separate study, also published in 2011, the authors looked at whether the fluoride level in the children’s water between the ages of 5 and 9 had any bearing on the development of new tooth decay during these years. While fluoride in water during these years was associated with a slight reduction in new decay among the girls, it was associated with a slight increase in tooth decay among the boys. (Chankanka 2011a)
Consistent with these results, the Iowa team reported that the subset of children in its study who drink bottled water (which has low levels of fluoride) do not have an increased rate of tooth decay. As the researchers noted:
“Presumably, such reduced exposure to fluoride [from drinking bottled water] would result in increased caries occurrence. However, the present study did not find any significant differences in caries prevalence or incidence between bottled water users and those who did not use much bottled water.” (Broffit 2007)
While the researchers cautioned that the number of children in the bottled water analysis was small, their findings are consistent with the previous findings of a large-scale Australian study.
3. Effect of Fluoride Exposure During Infancy
When dental fluorosis is present on a child’s front teeth, it can cause significant embarrassment and anxiety for a child. The Centers for Disease Control has defined even “mild” forms of fluorosis as “cosmetically objectionable” when present on the front teeth. (Griffin 2002). It was significant, therefore, when the Iowa team reported that infants who consumed fluoridated water had a significantly greater risk of developing fluorosis on their front teeth. (Hong 2006b; Marshall 2004).
In the past, some dental researchers had claimed that fluoride exposure during infancy was unlikely to cause dental fluorosis on the front teeth. (Evans & Darwell 1995). The Iowa team found, however, that infant exposure was a stronger predictor of front-tooth fluorosis than exposures from ages 1 to 4. (Hong 2006b) Based on these findings, the Iowa researchers stated that an effective way of reducing fluorosis risk would be to encourage parents to breast-feed their child instead of using fluoridated formula. To quote:
“Our results suggest that the fluoride contribution of water used to reconstitute formulas increases risk of fluorosis and could be an area for intervention… Supporting long-term lactation could be an important strategy to decrease fluorosis risk of primary teeth and early developing permanent teeth.” (Marshall 2004).
4. Effect of Water Fluoride Level on Bone
In 2009, the Iowa team published their first analyses on the association between fluoride intake and bone health. To determine fluoride’s impact on developing bone tissue, the researchers periodically examined the bone mineral content and bone density of various bones in the children’s skeleton. The 2009 study reported on the results of the exams from birth through age 11.
Although the authors did not find a statistically significant relationship, their data shows that – at all ages – the highest-exposed girls had lower bone mineral content and density than girls from the lowest exposed group. At 8.5 years of age, the highest-exposed girls had 6.4% less bone mineral content in their hips (p = 0.01) and 4% less bone mineral content in their whole body (p = 0.02). At 11 years of age, the highest-exposed girls continued to have lower mineral content, with 5.7% less mineral content in their hip and 4.3% less mineral content in their whole body (p = 0.02).
By contrast, fluoride intake was generally associated with higher mineral content and density in the boys, although the association was not as strong as the negative association detected in girls. The spine was the bone site that showed the greatest increase in bone mineral content and density in the boys. At age 11, for example, the spines of the boys with the highest fluoride exposure had 4.5% more bone mineral content (p = 0.07) and 4.4% more bone mineral density (p = 0.05).
Importantly, the pattern of bone mineral changes identified by the Iowa researchers is roughly similar to the pattern seen with high-fluoride exposures. Specifically, the trabecular-rich spine was the bone site in boys with the strongest upward trend in mineral content, and was the only bone site in girls that did not have a declining trend in mineral content.
These findings suggest that fluoride’s differential effect on bone density can be discerned among highly exposed children in fluoridated communities, which is particularly significant when considering that reductions in cortical bone density are a key mechanism by which fluoride can increase fracture rates.
The findings of the Iowa Fluoride Study have helped to clarify our understanding of the relationship between total fluoride intake, bones, and teeth. To the surprise of many fluoridation advocates, the study has found that fluoride intake has (1) little relationship to cavity prevention, (2) a significant relationship to dental fluorosis, and (3) a potentially far more significant relationship to bone health than the authors have thus far acknowledged.
Broffitt B, et al. (2007). An investigation of bottled water use and caries in the mixed dentition. Journal of Public Health Dentistry 67(3):151-8.
Chankanka O, et al. (2011a). Longitudinal associations between children’s dental caries and risk factors. Journal of Public Health Dentistry71(4):289-300.
Chankanka O, et al. (2011b). Mixed dentition cavitated caries incidence and dietary intake frequencies. Pediatric Dentistry 33(3):233-40.
Evans RW, Darvell BW. (1995). Refining the estimate of the critical period for susceptibility to enamel fluorosis in human maxillary central incisors. Journal of Public Health Dentistry 55(4):238-49.
Griffin SO, et al. (2002). Esthetically objectionable fluorosis attributable to water fluoridation. Community Dentistry & Oral Epidemiology 30:199-209.
Hong L, Levy S, Warren J, Broffit B. (2006a). Dental caries and fluorosis in relation to water fluoride levels. ADEA/AADR/CADR Conference, Orlando Florida, March 8-11, 2006. http://iadr.confex.com/iadr/2006Orld/techprogram/abstract_73811.htm
Hong L, Levy SM, et al. (2006b). Timing of fluoride intake in relation to development of fluorosis on maxillary central incisors. Community Dentistry and Oral Epidemiology 34:299-309.
Marshall TA, et al. (2004). Associations between Intakes of Fluoride from Beverages during Infancy and Dental Fluorosis of Primary Teeth. Journal of the American College of Nutrition 23:108-16.
Warren J, et al. (2009). Considerations on optimal fluoride intake using dental fluorosis and dental caries outcomes: A longitudinal study. Journal of Public Health Dentistry 69:111-15.