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Soy Isoflavones and Fracture Risk After Breast Cancer

10/1/2019 5:02:06 PM
soyBreast cancer patients have a higher risk for osteoporotic fractures than women of their same age who have not had breast cancer. The risk is thought to be attributed to endocrine therapies that are commonly used as adjuvant treatments for hormone-sensitive cancers. By interfering with the action of estrogen, these treatments can have adverse effects on bone health.

Isoflavone-rich soy examined for effects on fracture

Soy foods are rich in isoflavones, which are a class of natural compounds that are selective estrogen receptor modulators (SERMs). SERMs can mimic or compete with estrogen, depending on the target tissue. Isoflavone consumption has previously demonstrated an inverse association with the risk of death from and recurrence of breast cancer. Isoflavones have also been associated with a reduced risk of fracture in healthy postmenopausal women. 
As part of the Shanghai Breast Cancer Survival Study, researchers investigated the association between osteoporotic fractures and soy food consumption in breast cancer survivors. The study prospectively followed a cohort of 5042 women with recently diagnosed breast cancer for 10 years. Information about soy food consumption was collected with a food-frequency questionnaire at baseline and 18 months. Information about bone fractures was collected at years 3, 5, and 10. The primary outcomes were osteoporotic fracture and any fracture during the 10-year study period.

Higher isoflavone consumption linked to reduced fracture risk

In the overall cohort, 13.3% of breast cancer survivors developed a bone fracture and 3.6% had an osteoporotic fracture. The incidence of osteoporotic fractures was 2.9% for premenopausal women and 4.4% for postmenopausal women. These fracture rates were higher than age-matched healthy women in a separate study conducted in a similar geographic area.
Higher soy isoflavone intake was associated with a reduced risk of fracture among premenopausal and perimenopausal but not postmenopausal women. Premenopausal women who consumed more than 56 mg of soy isoflavones per day had a 78% reduced risk of fracture when compared to those who consumed less than 31 mg per day (HR = 0.23; 95% CI, 0.09-0.53).
Aside from soy isoflavone intake, other variables that influenced fracture risk were body mass index, exercise, and tamoxifen use. Being overweight was associated with an increased risk of osteoporotic fracture, exercise was associated with reduced risk, and tamoxifen use was associated with reduced risk (particularly for a long duration of use). Although soy isoflavones and tamoxifen are both SERMs, there was no effect modification by tamoxifen on the soy-fracture association.
This was a large, prospective study that evaluated several risk factors for fracture in breast cancer patients. It was the first study to show that soy isoflavone consumption was associated with a reduced risk of osteoporotic fracture in premenopausal and perimenopausal women survivors of breast cancer.
Zheng N, Hsieh E, Cai H et al. Soy Food Consumption, Exercise, and Body Mass Index and Osteoporotic Fracture Risk Among Breast Cancer Survivors: The Shanghai Breast Cancer Survival Study. JNCI Cancer Spectr. 2019.