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Antiresorptive Effects of Phytoestrogen Supplements Compared with Estradiol or Risedronate in Postmenopausal Women Using ⁴¹Ca Methodology

Departments of Foods and Nutrition (C.M.W., B.R.M., S.R., M.E.B.) and Statistics (G.P.M., J.R.N., L.D.M.) and Purdue Rare Isotope Measurement Laboratory (G.S.J.), Purdue University, West Lafayette, Indiana 47907; Department of Pharmacology (S.B.), University of Alabama, Birmingham, Alabama 35294; and Department of Medicine (M.P.), Indiana University, School of Medicine, Indianapolis, Indiana 47405

Address all correspondence and requests for reprints to: Connie M. Weaver, Ph.D., Department of Foods and Nutrition, Purdue University, 700 West State Street, West Lafayette, Indiana 47907-2059. E-mail: weavercm{at}purdue.edu.

Introduction: Reduction of ovarian estrogen secretion at menopause increases net bone resorption and leads to bone loss. Isoflavones have been reported to protect bone from estrogen deficiency, but their modest effects on bone resorption have been difficult to measure with traditional analytical methods.

Methods: In this randomized-order, crossover, blinded trial in 11 healthy postmenopausal women, we compared four commercial sources of isoflavones from soy cotyledon, soy germ, kudzu, and red clover and a positive control of oral 1 mg estradiol combined with 2.5 mg medroxyprogesterone or 5 mg/d oral risedronate (Actonel®) for their antiresorptive effects on bone using novel ⁴¹Ca methodology.

Results: Risedronate and estrogen plus progesterone decreased net bone resorption measured by urinary ⁴¹Ca by 22 and 24%, respectively (P < 0.0001). Despite serum isoflavone profiles indicating bioavailability of the phytoestrogens, only soy isoflavones from the cotyledon and germ significantly decreased net bone resorption by 9% (P = 0.0002) and 5% (P = 0.03), respectively. Calcium absorption and biochemical markers of bone turnover were not influenced by interventions.

Conclusions: Dietary supplements containing genistein-like isoflavones demonstrated a significant but modest ability to suppress net bone resorption in postmenopausal women at the doses supplied in this study over a 50-d intervention period.