Soy Isoflavones & Bone Health

Each month, Silk® soymilk brings you a brief summary of featured research on the benefits of soy in human health. This month we focus on soy isoflavones in bone health.

High Soy Consumption and Reduced Fracture Risk

Zhang et al. Arch Intern Med 2005;165:1890-95.

The first published study of soy consumption and fracture risk, conducted by researchers from Vanderbilt University School of Medicine and the Shanghai Cancer Institute, observed a significantly lower risk of fractures in the highest quintiles of soy intake among 24,403 postmenopausal women enrolled in the Shanghai Women’s Health Study.

Study subjects were women who had not menstruated for at least 12 months (including natural and surgically induced menopause), had never used HRT and had no history of fracture or cancer at baseline. During 4 ½ years of follow-up, 1,770 incident fractures were reported.

After multivariate adjustment, a significant inverse association was found between fracture risk and soy consumption across five quintiles of intake of soy protein or soy isoflavones. The association was strongest among women in early menopause. Relative risk of fracture was reduced by 35 to 37 percent in women in the highest quintile of intake (Table).

Adjusted variables included age, body mass index, hours of exercise per week, cigarette smoking, alcohol consumption, history of diabetes mellitus, level of education, family income, season of recruitment, and intakes of total calories, calcium, non-soy protein, fruits and vegetables.

Risk of Fracture by Quintile of Soy Protein and Soy Isoflavone Intake

Quintile I II III IV V
g/d of soy protein intake <4.98 4.98-7.32 7.33-9.77 9.78-13.26 >13.26
RR 1.00 0.72 0.69 0.64 0.63
mg/d soy isoflavone intake <21.16 21.16-32.39 32.40-44.31 44.32-60.26 >60.26
RR 1.00 0.75 0.67 0.72 0.65

Soy or soy isoflavones may benefit bone by suppressing bone resorption and simultaneously stimulating bone formation. Previous observational studies generally have associated a bone-sparing effect for soy, seen either as improved bone mineral density or reduced urinary excretion of markers of bone loss.

Some, but not all, clinical trials have shown significant protective effects on bone in postmenopausal women supplemented with soy isoflavones and/or soy protein. This prospective cohort study provides the first published evidence for a significant inverse association between soy consumption and risk of fracture in postmenopausal women, particularly among women in the early years following menopause.

Silk soymilk provides between 20 and 40 mg isoflavones per cup and is fortified with calcium carbonate, the preferred calcium fortificant that has been shown to have significantly higher bioavailability than tricalcium phosphate. The calcium in Silk is equally as bioavailable as the calcium in cow’s milk. (Zhao et al. Calcium Bioavailability of calcium carbonate fortified soymilk is equivalent to cows’ milk in young women. J Nutr. 135:2379-82, 2005.)

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