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Soy Protein Has a Greater Effect on Bone in Postmenopausal Women Not on Hormone Replacement Therapy, as Evidenced by Reducing Bone Resorption and Urinary Calcium Excretion

Departments of Nutritional Sciences (B.H.A., D.A.K., B.J.S., E.A.L., S.J.) and Statistics (M.E.P.), Oklahoma State University, Stillwater, Oklahoma 74078; and Department of Obstetrics and Gynecology (R.A.W.), University of Oklahoma Health Sciences Center, Oklahoma City, Oklahoma 73190

Address all correspondence and requests for reprints to: Bahram H. Arjmandi, M.D., Department of Nutritional Sciences, 425 Human Environmental Sciences, Oklahoma State University, Stillwater, Oklahoma 74078-6141. E-mail: arjmand{at}okstate.edu.

Recent reports suggest that soy protein may reduce the risk of osteoporosis in peri- and postmenopausal women. The objective of this study was to examine whether soy supplementation exerts beneficial effects on serum and urinary biomarkers of bone metabolism in postmenopausal women, regardless of whether or not they are on hormone replacement therapy (HRT). A total of 71 women were randomly assigned to either soy protein (SP) or milk-based protein (MBP), 40 g daily for 3 months, in a double-blind parallel design. Forty-two women completed the study (20 on SP and 22 on MBP). Overall, both protein supplements positively influenced serum IGF-I, known to correlate with bone formation. However, SP had a more pronounced effect on IGF-I than MBP. Urinary deoxypyridinoline (Dpd) excretion, a specific biomarker of bone resorption, was significantly reduced by SP, but not by MBP when all women were included. Furthermore, women on MBP experienced a 33% increase in urinary calcium excretion, whereas SP did not have such an effect. To evaluate whether SP affects women differently on the basis of their HRT status, data from women on HRT (n = 22) and those not on HRT (n = 20) were analyzed separately. The subanalysis of the data indicated that SP had the greatest impact on serum IGF-I (an increase of 97%) in the women not on HRT. The changes in urinary Dpd due to SP were only observed in women not on HRT, indicating that the overall decrease in Dpd occurred with SP in the absence of HRT. These results indicate that soy protein may positively influence bone and calcium homeostasis in postmenopausal women, particularly those not on HRT.

This work was supported by Grant AR982-006 from the Oklahoma Center for the Advancement of Science and Technology.

Abbreviations: BSAP, Bone-specific alkaline phosphatase; Dpd, deoxypyridinoline; E₂, 17ß-estradiol; HRT, hormone replacement therapy; MBP, milk-based protein; NTx, N-telopeptide; SERM, selective estrogen receptor modulator; SP, soy protein.

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