Importance: The major factor contributing to the high incidence of osteoporosis in older women is the reduction in estrogen synthesis. Isoflavones have a similar structure to 17-β estradiol and can act as selective estrogen receptor modulators.
Objective: To determine the safety and effect of soy isoflavones on bone during early menopause.
Design: Parallel, double blind, study.
Setting: Secondary care research institution at United Kingdom.
Participants: 200 women within 2 years after the onset of their menopause.
Intervention: Participants were randomised to either 30 g soy protein with 66 mg isoflavone (SPI) or 30 g soy protein alone that was isoflavone free (SP), daily for 6 months.
Outcome measures: The primary outcome of this study was assessing any change in plasma bone turnover markers (βCTX and PINP). The secondary outcome for this study was assessing any change in cardiovascular risk markers and thyroid function.
Results: The area under the curve (AUC) was significantly smaller for plasma βCTX (bone resorption marker) with SPI compared to SP (0.25 vs 0.36 μg/l; P value <0.01). There was no significant difference in AUC of P1NP (46.8 vs 45.4; P value=0.64). However, there was a significant reduction of P1NP concentrations (bone formation marker) between 3 and 6 months with SPI. The AUC for fasting glucose, fasting insulin and hsCRP were significantly lower with SPI compared to SP.
Conclusions: This study suggests that soy may confer a beneficial effect on bone health with a significant decrease in bone turnover markers seen for both resorption and formation after supplementation with 30 g soy protein and isoflavones that was not seen for soy protein alone. The initial reduction of osteoclast function, followed by attenuation of coupled osteoblast function through bone remodelling cycle is analogous to the mode of action of anti-resorptive agents used in post-menopausal osteoporosis. There was also an improvement of cardiovascular risk markers.