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Soy Isoflavones Do Not Affect Bone Resorption in Postmenopausal Women: A Dose-Response Study Using a Novel Approach with ⁴¹Ca

Foods and Nutrition (J.M.K.C., B.R.M., C.M.W.), PRIME Lab (G.S.J., D.E.), and Statistics (G.P.M., J.R.N.), and Purdue University, West Lafayette, Indiana 47907-2059; Department of Pharmacology (S.B.), University of Alabama, Birmingham, Alabama 53294; and School of Medicine (M.P.), Indiana University, Indianapolis, Indiana 46202

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

Introduction: The purpose of this 3-way crossover study was to identify the effective dose of soy protein isolate enriched with isoflavones for suppressing bone resorption in postmenopausal women using a novel, rapid assessment of antibone resorbing treatments.

Methods: Thirteen postmenopausal women (6 yr since menopause) were predosed with ⁴¹Ca iv. After a 200-d baseline period, subjects were given 43 g soy protein/d that contained 0, 97.5, or 135.5 mg total isoflavones in randomized order. The soy protein isolate powder was incorporated into baked products and beverages. Each 50-d intervention phase was preceded by a 50-d pretreatment phase for comparison. Serum isoflavone levels and biochemical markers were measured at the end of each phase. Twenty-four-hour urine samples were collected approximately every 10 d during each phase for ⁴¹Ca/Ca analysis by accelerator mass spectrometry.

Results: Serum isoflavone levels reflected the amount of isoflavones consumed in a dose-dependent manner. None of the isoflavone levels had a significant effect on biochemical markers of bone turnover, urinary cross-linked N teleopeptides of type I collagen and serum osteocalcin, or bone turnover as assessed by urinary ⁴¹Ca/Ca ratios.

Conclusions: Soy protein with isoflavone doses of up to 135.5 mg/d did not suppress bone resorption in postmenopausal women. This is the first efficacy trial using the novel technique of urinary ⁴¹Ca excretion from prelabeled bone.

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