ArticlesFluoride in drinking water and risk of hip fracture in the UK: a case-control study
Introduction
Fluoride has been added to drinking-water supplies in some areas of the UK since 1955 in an attempt to reduce the frequency of dental caries.1 Although this benefit of fluoridation of water supplies is widely accepted, concerns remain about possible adverse health effects, particularly on bone.2 About 50% of ingested fluoride is taken up by bone, and fluoridation of water can increase normal dietary intake by about 50%.3 Fluoride affects bone in at least two ways-fluoride ions can replace hydroxyl ions in the hydroxyapatite lattice, and increased fluoride concentrations in plasma directly increase osteoblastic differentiation and activity. Such changes could have an important effect on risk of fracture.
Trials have shown that high doses of sodium fluoride substantially increase bone density at axial sites, but this effect was not associated with lower rates of spinal fractures.4, 5 However, at peripheral sites, bone density was decreased, which suggests that fluoride might increase the risk of some fractures. This effect has only been seen when intake has been substantially higher than would be expected from fluoridation of water, however, and the implications of lower exposure for risk of fracture are not certain.
Most epidemiological evidence comes from ecological studies of hip fracture,6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16 but the results of such investigations have not been consistent. Some studies have suggested a positive association between the concentration of fluoride in water and incidence of fractures,11, 12, 13, 14, 16 but others have found no association7, 8, 10, 15 or even an inverse relation.6, 9 Only two studies have related the risk of hip fracture to fluoride ingestion in individuals rather than populations,17, 18 and for one of these only a brief preliminary analysis has been published.18
The limited scope in ecological studies to adjust for the effects of potential confounding variables (such as physical activity, body build, cigarette smoking, dietary calcium intake, and reproductive variables) makes interpretation of their conflicting findings difficult. To address this uncertainty, we did a population-based case-control study in which the exposure of patients with a hip fracture to fluoride in drinking water was compared with that of controls after adjustment for possible confounding by other risk factors. We did this investigation in the county of Cleveland in northeast England. The Hartlepool area of Cleveland had always been supplied with water naturally high in fluoride (> 1 ppm), whereas the rest of the county received water with a low fluoride concentration.
Section snippets
Methods
The study population consisted of residents of Cleveland aged 50 years and older. Virtually all cases of hip fracture in the county are treated at three hospitals—Hartlepool General Hospital, North Tees General Hospital (Stockton), and Middlesbrough General Hospital. By regularly searching ward admission books, we identified all members of the study population who were admitted to these hospitals during a 17-month period with newly diagnosed fractures of the femoral neck that were through or
Results
914 cases were identified during the study period, of whom 514 (56%) had an interview. There were 1196 controls, of whom 527 (44%) were interviewed (table 1). The main reason for cases not completing interviews was a low score on the mental test (295 patients)-such low scores were more common at older ages. The main reasons for the incomplete response from controls were because general practitioners advised that they should not be approached (108); they could not be contacted at the address
Discussion
We found no evidence of any increase in the risk of hip fracture from fluoride in drinking water at concentrations of about 1 ppm. The association was examined in two ways. First, in the subset of people who underwent interview, we looked at risk of fracture in relation to estimates of exposure to fluoride in water during different periods of life. This analysis had the advantage that it allowed for possible confounding by various known and suspected causes of fracture. Furthermore, since
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