Soy isoflavones have minimal effect on bone strength
MedWire News: It is "doubtful" that soy isoflavones - taken as dietary supplements -have any clinically relevant impact on fracture risk, US study findings indicate.
The researchers showed that daily supplementation with soy isoflavones for 3 years was modestly beneficial for midshaft femur bone mineral density (BMD) and strength-strain index (SSI), but only when time since last menstrual period (TLMP) and bone turnover increased.
"Isoflavones are structurally similar to estrogen, bind to estrogen receptors, and affect estrogen-regulated gene products. Thus, the estrogen-like effects have sparked considerable interest in the potential skeletal benefits of isoflavones," explain Marta Van Loan (University of California, Davis) and colleagues.
They add that current evidence demonstrating the effect of soy isoflavones on BMD is conflicting, and it is unknown whether the supplements affect bone strength.
The researchers therefore carried out a randomized controlled trial examining the effect of two soy isoflavone doses (80 or 120 mg/day for 3 years) versus placebo on volumetric BMD and bone strength in 171 healthy postmenopausal women aged 46-63 years.
At 6, 12, 24, and 36 months, the team measured cortical BMD, cortical thickness, periosteal circumference, endosteal circumference, and SSI at the midshaft femur, and trabecular BMD, periosteal circumference, and SSI at the distal tibia using peripheral quantitative computed tomography.
As reported in the Journal of Clinical Densitometry, neither treatment had a beneficial or detrimental effect on femur cortical thickness, periosteal circumference, or endosteal circumference, or tibia trabecular BMD or periosteal circumference, compared with placebo.
Both isoflavone treatments exerted a significant negative effect on midshaft femur SSI, but the 80 mg/day treatment became protective as bone turnover - assessed by bone-specific alkaline phosphatase (BAP) - increased. The higher dose showed a similar protective trend with increasing BAP, although this did not reach statistical significance.
In addition, the 120 mg/day dose prevented loss of midshaft femur cortical BMD, relative to placebo, as TLMP increased.
Van Loan and co-authors accept that because the study did not examine fracture it is difficult to assess the clinical relevance of their findings.
"Nonetheless, in light of a growing body of literature suggesting a negligible effect of soy isoflavones on areal BMD, it seems doubtful that isoflavones… would have any clinically relevant impact on fracture risk," they conclude.
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By Laura Dean