This will surprise you. It certainly surprised me!

It has been demonstrated that exercise has a small but beneficial effect on the bone mineral density of postmenopausal women. Other studies have shown the benefits of phytoestrogens such as soybean isoflavones for improving bone health. You would think, therefore, that combining these two therapies would provide even greater benefit to the skeleton. Makes good hypothetical sense doesn’t it? Well, not so fast! In this month’s issue of the Journal of Bone and Mineral Research, Canadian researchers report on a study that tested this exact hypothesis. They combined exercise with isoflavone supplementation to determine (among other things) the effect on the bone density of postmenopausal women.

The two-year study included 351 postmenopausal women. Participants engaged in an exercise regimen of weight training and walking in combination with supplementing their diet with high doses (165 mg) of isoflavone containing genistin, daidzin, and glycitin. [Participants also received supplemental calcium (1,200 mg) and vitamin D (800 IU) each day.]

The researchers hypothisized that since prior studies had shown the benefits of exercise and isoflavones to bone health individually, that a combination of these two therapies would result in an additive benefit. To their surprise, the combination of these therapies resulted in decreased total hip bone mineral density (BMD)! “Thus, contrary to our hypothesis, there was a negative interaction between the two therapies for total hip BMD.”

“Our results indicated a beneficial effect of exercise training or isoflavone by itself for preserving BMD at the total hip.” “This beneficial effect was lost when exercise training and isoflavone supplementation were combined (i.e., the group combining the two interventions did not differ over time compared to the control group).”

The researchers were obviously surprised by their findings and offered a possible explanation. “One explanation for the apparent interference of isoflavone supplementation with exercise involves the types of estrogen receptors on bone for the detection and transduction of mechanical strains to increase signals for bone formation (i.e. activation of osteoblasts). Estrogen receptor-ɑ, when activated by exercise, will increase osteoblast proliferation, whereas estrogen receptor-β seems to block the beneficial effects of exercise on bone and has been termed the “anti-mechanostat” (because it downregulates the mechanical strain on bone induced by exercise). Phytoestrogens, such as istoflavones, have a stronger affinity for estrogen receptor-β than for estrogen receptor-ɑ. The activation of estrogen receptor-β by isoflavones may cause a downregulation in the detection of strains from exercise: thereby reducing the effectiveness of exercise loading on bone. Our study is in contrast to a number of animal studies that demonstrated a beneficial effect of consuming isoflavones during exercise training.”

“In summary, isoflavone supplementation combined with exercise training appeared to interact negatively on BMD at the proximal part of the femur.”

I’ve never been a huge fan of soy. Concern over allergies, genetically modified soy, and results from some studies that fail to show benefit to bone have made me less than enthusiastic to recommend supplemental isoflavones to my patients. I’m not saying that soy has no benefits…only that this study did not help improve my opinion. There are many approaches to bone health…I’m not sure that supplementing with soy isoflavones should be one of them.

Chilibeck, P.D., et al. 2013. Effect of exercise training combined with isoflavone supplementation on bone and lipids in postmenopausal women: A randomized clinical trial. Journal of Bone and Mineral Research 28(4):780-793.