Context: The key role of serum estradiol (E2) for bone health in men is well established. The effect of serum sex steroids on bone microstructure, measured by high-resolution peripheral quantitative computed tomography, remains unknown in elderly men. Objective: The objective of the study was to examine the associations between serum sex steroids and bone microstructural parameters in older men. Methods: Trabecular and cortical bone microstructure at the tibia was measured by high-resolution peripheral quantitative computed tomography in 440 men (mean 80 y of age) participating in the population-based Osteoporotic Fractures in Men Sweden cohort. Serum levels of E2 and T were analyzed with mass spectrometry and free E2 and free T levels were calculated using law-of-mass-action equations. Results: Age-adjusted models demonstrated that E2 and free E2 but not T or free T associated significantly inversely with cortical porosity. The associations between E2 and free E2 and cortical porosity remained significant after further adjustment for height, weight, physical activity, calcium intake, and smoking. Models including both serum E2 and T demonstrated that E2 (standardized beta = -.12, P < .05) but not T associated independently with cortical porosity. A similar independent association was found for free E2 (standardized beta = -.12, P < .05) but not free T. Free E2 associated significantly with trabecular bone volume fraction in the age-adjusted models, but this association did not remain significant after further adjustment. Conclusions: Serum E2 levels associated inversely with cortical porosity in 80-year-old men. We propose that low serum E2 may reduce cortical bone strength, at least partly, by increasing cortical porosity and thereby increase fracture risk in older men.
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