Excessive Dietary Intake of Vitamin A Is Associated with Reduced Bone Mineral Density and Increased Risk for Hip Fracture

Age-adjusted rates of hip fracture incidence vary more than sevenfold in Europe; the highest rates are found in northern Europe, particularly Sweden and Norway [1, 2]. Rates are higher among residents of Scandinavia than among comparable persons in North America [2] and, remarkably, are higher among Swedish men than among women in countries such as the United Kingdom, the Netherlands, and Switzerland [1]. Known risk factors [3] cannot explain these observations; this indicates that hitherto unknown environmental factors contribute to the development of osteoporosis. When dietary patterns in Europe were compared, a large variation in vitamin A intake was found: Median intake was up to sixfold higher in Scandinavian countries than in southern Europe [4]. The most prominent features of hypervitaminosis A in laboratory animals are accelerated bone resorption, bone fragility, and spontaneous fractures [5-7]. Anecdotal reports suggest that accidental vitamin A poisoning impairs bone remodeling [6-8], and recent studies have shown that osteoporosis is a toxic effect of long-term therapy with etretinate, a synthetic retinoid [9, 10]. We previously did a large, nested casecontrol study of diet and risk for hip fracture [11] and a cross-sectional study of diet and bone mineral density [12], but we did not analyze dietary intake of retinol in these studies. We therefore decided to investigate whether excessive dietary intake of vitamin A is associated with decreased bone mineral density and increased risk for hip fracture. Methods Bone Mineral Density Study Participants We previously investigated the relation between nutrient intake and bone mineral density in a population-based, cross-sectional study [12]. Briefly, a random sample of women who were living in the county of Uppsala, Sweden, and were 28 to 74 years of age at study entry was selected from the population register. A total of 175 women were included. Diet was assessed by review of four 1-week dietary records for each woman. Outcome Measures Bone mineral density was measured by using dual-energy x-ray absorptiometry (DPX-L, Lunar Co., Madison, Wisconsin) at the lumbar spine, the total body, and three regions of the proximal femur (the neck, the Ward triangle, and the greater trochanter). Nondietary Information Weight and height were measured with a scale and a stadiometer soon after bone density measurements. Participants were asked to complete a questionnaire about major risk factors for osteoporosis [12]. They were asked whether they were former or current smokers and how many cigarettes they smoked each day; how often they had engaged in physical activity in their leisure time (never, occasionally, <1 hour per week, 1 or 2 hours per week, or 2 hours per week) during three periods of life (as a teenager, between 18 and 30 years of age, and in recent years); whether they had diabetes and, if so, what type of treatment they received for it; whether they used cortisone, hormone replacement therapy, or oral contraceptives and, if they did, how long they had used these treatments and when they had used them; whether they were menopausal and their menopausal age; whether they had formerly engaged in athletic activity and, if so, what type of activity they had engaged in and when they had engaged in it; and whether they had had a fracture of the hip, ankle, spine, or lower or upper arm after age 40 years. The question about physical activity had been used and validated in a large European study of hip fracture [13]. No association between potential confounding factors, such as intake of calcium, vitamin D, or alcohol, and site-specific bone mineral density was found in these women [12]. Data Analysis A linear regression model was used in the analyses of bone mineral density. In multivariate modeling, we introduced the variables used in the hip fracture analysis (see below). Body mass index, energy intake, and current and menopausal age were continuous variables, and the following variables were categorical: current and former smoking, any use of cortisone, any use of hormone replacement therapy and oral contraceptives, menopausal status, history of fracture after age 40 years, diabetes mellitus and type of treatment received (oral treatment or insulin treatment compared with no diabetes), and lifetime physical activity (the sum of the scores of physical activity for the three periods of life, expressed as being above or below the median lifetime score). In these linear regression analyses, retinol intake was considered both in original continuous and in categorized form, with cut-off values identical to those used in the hip fracture study. We tested the linear relation between retinol intake and bone mineral density by adding a quadratic term of retinol intake to the multivariate model, which included retinol in continuous form. The P values for the quadratic term were between 0.25 and 0.90 in different models, indicating that there was no obvious nonlinear relation between retinol intake and bone mineral density. The main analysis included retinol intake divided into four (<0.5, 0.51 to 1.0, 1.01 to 1.5, and >1.5 mg/d) or two ( 1.5 and >1.5 mg/d) categories in models that also included the covariates described above. Test results and CIs were computed with the standard method used in regression analysis [14]. The estimated average bone mineral density for each category was computed from the estimated regression models. In multivariate models, the covariate values were set at their means. Hip Fracture Study Study Sample The Swedish Mammography Cohort [15, 16] and our study design have been described in detail elsewhere [11]. Briefly, from 1987 to 1990, all women who were 40 to 76 years of age and were resident in two counties in central Sweden (source population, 90 303) were mailed a validated [16] food-frequency questionnaire; 66 651 (74%) of the women replied. Dietary Questionnaire The questionnaire asked women to report their usual intake of 60 foods during the past 6 months. In addition to requesting information on dietary habits, it also asked for participants' self-reported current weight and height, marital status, parity, and educational level. Case-Patients and Controls Women who had a first hip fracture within 2 to 64 months after entry into the cohort were defined as case-patients in a nested casecontrol analysis of hip fracture. For each case-patient, four controls (individually matched to the case-patient by age and county of residence) were selected from the cohort. Hip fracture was defined as a cervical, trochanteric, or subtrochanteric femoral fracture. By using hospital discharge records from the six hospitals of the two counties included in the study, we identified possible cases of hip fracture. Hospital records for these possible cases were scrutinized, and we excluded women with incorrect diagnoses and women with fractures due to cancer or high-energy trauma. Of the hip fractures included in the study, 62% were located in the cervical portion of the femur. Additional Information A second questionnaire on potentially confounding factors not included in the first questionnaire was mailed to controls and case-patients [11]. The following information was requested: self-reported age at menopause, duration and dates of exposure to postmenopausal hormone replacement therapy and oral cortisone, duration of oral contraceptive use, smoking status (with number of cigarettes) in each decade of life, current use of vitamin or mineral supplementation (with number of tablets taken per week and weeks of use per year), diabetes mellitus with current type of treatment received and age at first appearance, physical activity during leisure time (on a five-point scale) at four different periods of life (as a teenager, at 30 years of age, 5 years earlier, and currently), athletic participation (with type and duration of activity), and previous fractures other than hip fracture after age 40 years. Of those who were eligible, 92.5% of case-patients and 89.1% of controls returned the second questionnaire, and 247 case-patients and 873 controls were included in the analysis. No signs of liver disease were noted in the hospital records of the women in the study. Previous analyses of these data identified several established risk factors for hip fracture. However, no statistically significant differences were seen between case-patients and controls in the use of multivitamin or calcium supplements, alcohol, caffeine, or thiazides [11]. Data Analysis Because of the matched study design, the basic model used in the hip fracture study was conditional logistic regression [17]. To assess risk for hip fracture, we estimated odds ratios and 95% CIs from the model. Retinol intake was analyzed both in continuous form and in four categories (based on the daily intake of 0.5 mg of retinol equivalents recommended by the Food and Agricultural Organization and the World Health Organization [18]): 0.5 mg/d, 0.51 to 1.0 mg/d, 1.01 to 1.5 mg/d, and >1.5 mg/d. The lowest category was considered the reference category. No women in the lowest category had intake less than 0.27 mg/d, which is the basal level sufficient to meet all physiologic needs [18]. The P value for a quadratic term of retinol intake in the multivariate model with retinol intake in continuous form was 0.31, indicating that a linear relation between retinol and hip fracture risk was a reasonable possibility. Thus, tests for trend were performed with retinol intake in both original continuous form and in categorized form by introducing a new variable obtained by assigning the four categories of retinol intake into consecutive integers (0, 1, 2, and 3) in the model. Data were analyzed in a multivariate model that included the following covariates categorized into quartiles on the basis of distribution of controls: body mass index (kg/m2), energy intake, age at menopause, and lifetime physical activity during leisure time