Community water fluoridation, bone mineral density, and fractures: prospective study of effects in older women.

Abstract

Objective To determine whether fluoridation influences bone mineral density and fractures in older women.

Design Multicentre prospective study on risk factors for osteoporosis and fractures.

Setting Four community based centres in the United States.

Participants 9704 ambulatory women without bilateral hip replacements enrolled during 1986-8; 7129 provided information on exposure to fluoride.

Main outcome measures Bone mineral density of the lumbar spine, proximal femur, radius, and calcaneus plus incident fractures (fractures that occurred during the study) of vertebrae, hip, wrist, and humerus.

Results Women were classified as exposed or not exposed or having unknown exposure to fluoride for each year from 1950 to 1994. Outcomes were compared in women with continuous exposure to fluoridated water for the past 20 years (n = 3218) and women with no exposure during the past 20 years (n = 2563). In women with continuous exposure mean bone mineral density was 2.6% higher at the femoral neck (0.017 g/[cm.sup.2], P [is less than] (0.001), 2.5% higher at the lumbar spine (0.022 g/[cm.sup.2], P [is less than] (0.001), and 1.9% lower at the distal radius (0.007 g/[cm.sup.2], P = 0.002). In women with continuous exposure the multivariable adjusted risk of hip fracture was slightly reduced (risk ratio 0.69, 95% confidence interval 0.50 to 0.96, P = 0.028) as was the risk of vertebral fracture (0.73, 0.55 to 0.97, P = 0.033). There was a non-significant trend toward an increased risk of wrist fracture (1.32, 1.00 to 1.71, P = 0.051) and no difference in risk of humerus fracture (0.85, 0.58 to 1.23, P = 0.378).

Conclusions Long term exposure to fluoridated drinking water does not increase the risk of fracture.

Introduction

In 1945 Grand Rapids, Michigan, increased the fluoride concentration of its water supply to 1.0 ppm and became the first city to implement water fluoridation. After 15 years children in Grand Rapids had a 56% reduction in rates of dental decay.[1] Since 1950 the number of individuals drinking fluoridated water has steadily increased. Today an estimated 300 million people are exposed to fluoridated water, including 5.5 million in the United Kingdom and 144 million in the United States.[2]

While the benefit of fluoridation in the prevention of dental caries has been overwhelmingly substantiated, the effect of fluoridation on bone mineral density and rates of fracture is inconsistent. Ecological studies that compare rates of fracture specific for age and sex between fluoridated and non-fluoridated communities have variously found that exposure to fluoridated water increases the risk of hip fracture[3-5] increases the risk of proximal humerus and distal forearm fracture,[6] has no effect on fracture risk,[7-10] and decreases the risk of hip fracture.[11 12] Ecological studies, however, have a major design flaw--they are based on community level data and cannot control for confounding variables at the individual level.

We determined, on an individual level, whether older women with long term exposure to fluoridated water had different bone mass and rates of fracture compared with women with no exposure. Given our widespread exposure to fluoridation, the impact of fluoride on rates of fracture is an important public health issue. In a previous study in which we found no effect of fluoridation on bone mass or risk of fracture in women we had limited power to look at individual fractures and had a relatively small proportion of women exposed to fluoride for long duration.[13] In the current study we increased power to look at individual fractures by increasing the number of participants, including more women with long term exposure to fluoride, and following the population for a longer period of time.

Methods

Participants--This study was ancillary to the study of osteoporotic fractures, a multicentre study of risk factors for osteoporosis and fractures. During 1986-8 we recruited 9704 white women aged at least 65 years in Portland, Oregon; Minneapolis, Minnesota; Baltimore, Maryland; and the Monongahela Valley (an area with several small communities) in Pennsylvania. Recruitment sources included lists for jury selection and registration of voters, motor vehicle records, and membership records of health plans. Men and black women were excluded as were white women who were unable to walk without assistance and women who had a bilateral hip replacement.

Exposure to fluoridated water--Exposure to fluoridated water was assessed with a questionnaire on residence history. Women were asked to list each address (street, city, state, and postal code), years they lived at that address, and the type of water supply (public, well, spring, etc) for each of their residences from 1950 to 1994. Water system maps and the 1992 fluoridation census[2] were used to link street addresses with water system and fluoridation status. If a …