The Soy Isoflavones for Reducing Bone Loss (SIRBL) Study: a 3-y randomized controlled trial in postmenopausal women1234

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Background: Our previous study indicated that soy protein with isoflavones lessened lumbar spine bone loss in midlife women.

Objective: We examined the efficacy of isoflavones (extracted from soy protein) on bone mineral density (BMD) in nonosteoporotic postmenopausal women. We hypothesized that isoflavone tablets would spare BMD, with biological (age, body weight, serum 25-hydroxyvitamin D) and lifestyle (physical activity, dietary intake) factors modulating BMD loss.

Design: Our double-blind, randomized controlled trial (36 mo) included healthy postmenopausal women (aged 45.8–65.0 y) with intent-to-treat (n = 224) and compliant (n = 208) analyses. Treatment groups consisted of a placebo control group and 2 soy isoflavone groups (80 compared with 120 mg/d); women received 500 mg calcium and 600 IU vitamin D3. Outcomes included lumbar spine, total proximal femur, femoral neck, and whole-body BMD.

Results: Analysis of variance for intent-to-treat and compliant (≥80%) models, respectively, showed no treatment effect for spine (P = 0.46, P = 0.21), femur (P = 0.86, P = 0.46), neck (P = 0.17, P = 0.14), or whole-body (P = 0.86, P = 0.78) BMD. From baseline to 36 mo, BMD declined regardless of treatment. In intent-to-treat and compliant models, respectively, BMD decreases were as follows: spine (−2.08%, −1.99%), femur (−1.43%, −1.38%), neck (−2.56%, −2.51%), and whole body (−1.66%, −1.62%). Regression analysis (compliant model) indicated that age, whole-body fat mass, and bone resorption were common predictors of BMD change. After adjustment for these factors, 120 mg (compared with placebo) was protective (P = 0.024) for neck BMD. We observed no treatment effect on adverse events, endometrial thickness, or bone markers.

Conclusion: Our results do not show a bone-sparing effect of extracted soy isoflavones, except for a modest effect at the femoral neck. This trial was registered at clinicaltrials.gov as NCT00043745.

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1

From the Nutrition and Wellness Research Center, Department of Food Science and Human Nutrition (DLA, LNH, JWS, and KBH) and the Department of Statistics (KJK and CTP), Iowa State University, Ames, IA; the US Department of Agriculture, Agricultural Research Service, Western Human Nutrition Research Center, University of California, Davis, CA (MDVL); and the Department of Food Science and Nutrition, University of Minnesota, St Paul, MN (MSK).

2

The content and views expressed in this article are the sole responsibility of the authors and do not necessarily represent the official views of the funding agencies. Funding agencies did not play a role in data collection, management, analysis, or interpretation, or in manuscript preparation or review.

3

Supported mainly by a grant (RO1 AR046922) from the National Institute of Arthritis and Musculoskeletal and Skin Diseases; also supported by the Nutrition and Wellness Research Center, Iowa State University; USDA/ARS, Western Human Nutrition Research Center, Clinical and Translational Science Center, Clinical Research Center, University of California (1M01RR19975-01); and National Center for Medical Research (UL1 RR024146). Archer Daniels Midland Company (Decatur, IL) donated soy isoflavone tablets (Novasoy), and GlaxoSmithKline (Moon Township, PA) donated calcium and vitamin D supplements (Os-Cal).

4

Address correspondence to DL Alekel, Nutrition and Wellness Research Center, Iowa State University, Research Park, Building 6, 2325 North Loop Drive, Suite 6100, Ames, IA 50010-8281. E-mail: [email protected].