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OBJECTIVE: The objective of this study was to evaluate the relations between bone mineral density (BMD) and lead in blood, tibia, and patella and to investigate how BMD modifies these lead biomarkers in older women. DESIGN: In this study, we used cross-sectional analysis. PARTICIPANTS: We studied 112 women, 50-70 years of age, including both whites and African Americans, residing in Baltimore, Maryland.

MEASUREMENTS: We measured lumbar spine BMD, blood and bone lead by dual energy X-ray absorptiometry, anodic stripping voltammetry, and (109)Cd-induced K-shell X-ray fluorescence, respectively. We measured vitamin D receptor and apolipoprotein E (APOE) genotypes using standard methods. RESULTS: Mean (+/- SD) BMD and lead levels in blood, tibia, and patella were 1.02+/-0.16 g/cm(2), 3.3+/-2.2 microg/dL, 19.7+/-13.2 microg/g, and 5.7+/-15.3 microg/g, respectively. In adjusted analysis, higher BMD was associated with higher tibia lead levels (p=0.03). BMD was not associated with lead levels in blood or patella. There was evidence of significant effect modification by BMD on relations of physical activity with blood lead levels and by APOE genotype on relations of BMD with tibia lead levels. There was no evidence that BMD modified relations between tibia lead or patella lead and blood lead levels.

CONCLUSIONS: We believe that BMD represents the capacity of bone that can store lead, by substitution for calcium, and thus the findings may have relevance for effect-size estimates in persons with higher BMD.

RELEVANCE TO CLINICAL PRACTICE: The results have implications for changes in lead kinetics with aging, and thus the related risk of health effects associated with substantial early- and midlife lead exposure in older persons.

This study has several strengths. First, there was diversity by race/ethnicity and the inclusion of important genotypes, allowing us to evaluate relations of these important predictor variables with both lead biomarkers and BMD. Our sample size was > 50% larger than the only other study of the relation between tibia lead and BMD (Potula et al. 2006). Finally, ours was the first study, to our knowledge, to evaluate associations between BMD and patella lead levels. The limitations ot the study include the cross-sectional design, which does not allow causal inferences. second, the lead biomarkers were obtained from different visits (ranging from 18 to 36 months apart), and BMD was measured months after the measurements of other biomarkers. Therefore, the associations we found may not reflect actual cross-sectional associations of covariates and lead biomarkers, particularly for blood, BMD, and covariates that change over time. However, we do not think this problem is likely to be very severe for tibia and patella lead levels because residence time of lead in tibia is almost 3 decades (Rabinowitz et al. 1976) and in patella is 3-5 years (Chettle 1995; Todd and Chettle 1994). Third, we measured the lead biomarkers and BMD only one time, and mainly in women who had already passed menopause. The BMD and lead biomarker levels may have already significantly changed during this period, altering associations that may have existed during the premenopausal and perimenopausal periods. Finally, the various biomarkers and questionnaire-based variables were each measured with different measurement errors, and thus some of the contrasting associations could be explained by possible residual confounding.