Introduction: The effect of oestrogens on the male skeleton has been demonstrated in a number of studies and oestradiol (E2) plays an important role in regulating bone mineral density (BMD). The concentration of bioavailable oestradiol (BE), rather than total oestradiol (TE), is associated with bone loss in elderly men.Oestradiol circulates in the bloodstream in one of three forms: bound to SHBG; bound to albumin; and unbound/free. The albumin-bound plus the free fraction comprise BE, the active fraction available to tissues. Although it has been shown that total and bioavailable testosterone show a marked diurnal variation in men, little appears to be established on the variation of total and BE with time of day.
Methods: We compared TE and BE in elderly men with normal BMD (NBMD) (n=8, mean age 70.5±2.09 years), and elderly men with osteoporosis (OBMD) (n=5 75±2.94years). BE was measured by an ammonium sulphate precipitation technique with 3H-oestradiol as the tracer. Circadian rhythm parameters of TE and BE (midline estimate statistic of rhythm (MESOR), acrophase and amplitude) were analysed using Chronolab 3.0. Students t test for unpaired data was used to determine differences between groups.
Results: The 24-h mean concentration of BE was 12.47±0.4 pmol/L in the OBMD group and was significantly lower than in the NBMD group (17.55±0.4 pmol/L; P<0.001).TE was not significantly different between the two groups. BE in the NBMD group demonstrated a significant circadian rhythm with an early morning peak and nadir at night, the Mesor being 16.46±3.4 pmol/L with an amplitude of 1.99±0.7 and Acrophase at 08:27 am.No concerted circadian rhythm was observed in the OBMD group.
Conclusion: Low circulating BE with a loss of circadian rhythmicity may contribute to the development of osteoporosis in aging men.