These PhD projects represent secondary analyses of data from the ongoing Iowa Fluoride Study (IFS)/Iowa Bone Development Study (IBDS). The aim of this dissertation was to explore the associations between period-specific and cumulative fluoride intakes from birth to age 17, and from birth to age 19 years and bone measures of participants. Also, this dissertation looked into the associations between the clinical presence of dental fluorosis and bone outcomes.

Participants have been participating in the IBDS that grew out of the IFS, which is a longitudinal investigation of dietary and non-dietary fluoride exposures, dental fluorosis and dental caries. IFS participants were recruited during 1992-95 from 8 hospital postpartum wards in Iowa, and detailed questionnaires were sent every 1.5-6 months. Data on intakes from water, other beverages, selected foods, dietary fluoride supplements and dentifrice were collected from the questionnaires, and, in combination with water and beverage fluoride levels, combined fluoride was estimated.

For the first dissertation project, the association between fluoride intake and peripheral quantitative computed tomography (pQCT)-derived bone outcomes at age 17 were assessed. Participants underwent pQCT of the radius and tibia (XCT-2000) at age 17 years. pQCT results of trabecular bone mineral density (BMD) and bone mineral content (BMC), cortical BMD and BMC, and compression and torsion strength were related to fluoride intake through bivariate and multivariable analyses, adjusting for height, weight, years since peak height velocity, average daily time spent in moderate-to-vigorous intensity physical activity, daily calcium intake, and daily protein intake. Pvalues <0.01 were considered statistically significant rather than p<0.05 due to multiple hypothesis tests. The mean daily fluoride intake estimated by area-under-the-curve (AUC) from birth to 17 years was 0.79 mg (SD = 0.32) for males and 0.70 mg (SD = 0.25) for females. Spearman correlation coefficients between daily fluoride intake and pQCT bone measures were weak (for females r= -0.01 to 0.15 for radius bone outcomes and -0.001 to 0.23 for tibia bone outcomes;​ for males r= 0.03 to 0.24 for radius bone outcomes and -0.008 to 0.27 for tibia bone outcomes). In sex-specific linear regression analyses for females, partially-adjusted for height, weight, and years since peak height velocity, statistically significant negative associations were detected between all radial bone outcomes and period-specific fluoride intake from 0-8.5 years. Significant positive associations were detected for females between period-specific fluoride intakes from 14- 17 years and all tibia bone outcomes, and between period-specific fluoride intakes from 14-17 years and all radius and tibia bone outcomes for males. In the fully-adjusted models, which also included physical activity, protein intake and calcium intake, statistically suggestive negative associations were detected for females during the early fluoride intake period from 0 to 8.5 years and radial cortical bone content and torsion bone strength. A statistically suggestive positive association was found between periodspecific fluoride intake from 8.5 to 14 years and torsion bone strength (pSSI) (p<0.05) for females. For males, statistically significant positive associations were detected between fluoride intake for the period from 14 to 17 years and cortical content and torsion strength (pSSI) at the 0.01 level.

The second project examined the associations between period-specific and cumulative fluoride intakes from birth to age 19 years and MDCT-derived bone outcomes at age 19. Age 19 MDCT-derived trabecular and cortical bone micro-architecture scans were acquired at the University of Iowa Comprehensive Lung Imaging Center. MDCT results of the trabecular (volumetric bone mineral density (vBMD), transpose bone mineral density (tBMD), plate trabecula bone mineral density (pBMD), plate width (TSPW), trabecular thickness (Tb.Th), trabecular spacing (Tb.Sp), trabecular network area (Tb.NA)) and cortical (cortical bone porosity (Cb.Poro), cortical thickness (Cb.Th)) bone were related to fluoride intake through bivariate and multivariable analyses, adjusting for height, weight, years since peak height velocity, average daily time spent in moderate-tovigorous intensity physical activity, Healthy Eating Index (HEI) score, calcium intake and protein intake. P-values <0.01 were considered statistically significant rather than p<0.05 due to multiple hypothesis tests. The mean daily fluoride intake estimated by area-under-the-curve (AUC) from birth to 19 years was 0.81 mg (SD = 0.33) for males and 0.69 mg (SD = 0.27) for females. Spearman correlation coefficients between daily fluoride intake and MDCT bone measures were weak (for females r= -0.001 to 0.20 for trabecular bone outcomes and -0.01 to 0.02 for cortical bone outcomes;​ for males r= - 0.003 to 0.16 for trabecular bone outcomes and -0.09 to -0.02 for cortical bone outcomes). In sex-specific partially-adjusted regression analysis adjusted for height, weight, and years since peak height velocity, no statistically significant associations were found for females or males. In the fully-adjusted models, which also included physical activity, HEI score, and protein and calcium intakes, no statistically significant associations were found for either females or males.

The third project explored the associations between dental fluorosis score at age 8 and DXA-derived bone outcomes at age 5. DXA bone assessments of the whole body, proximal femur (hip), and lumbar spine were performed at The University of Iowa in the Clinical Research Center (Hologic QDR-2000 DXA unit). The dental fluorosis score was defined as the proportion of zones with definitive or severe fluorosis per person. In the unadjusted associations between bone outcomes and dental fluorosis score, no statistically significant associations were detected for females at the 0.05 level. For males, a statistically significant negative association was found between hip BMD and dental fluorosis score. Sex-specific partial correlation coefficients were estimated between DXA-derived bone outcomes and dental fluorosis score adjusted for height, weight, physical activity, calcium intake and fluoride intake. No statistically significant associations were found for females. For males, a statistically significant negative association was detected between dental fluorosis score and hip BMD.

The findings of this dissertation show that life-long intakes from combined sources for adolescents and young adults living in fluoridated areas in the United States were weakly associated with bone measures at age 17 and 19. Furthermore, it was shown that bone outcomes can’t be predicted by the score of dental fluorosis.

Fluoride is a mineral that plays an important role in the mineralization of bone and teeth, as well as in dental caries prevention. Numerous professional health organizations endorse the adjusted fluoridation of public water supplies for caries prevention. Results from this dissertation will also help in supporting additional efforts to promote water fluoridation and expand its use, as this dissertation’s outcomes did not demonstrate adverse outcomes related to bone.