The effect of soy protein and soy isoflavones on calcium metabolism in postmenopausal women: a randomized crossover study

Background: Evidence suggests that soy isoflavones act as estrogen agonists and have beneficial skeletal effects, but the effects on calcium metabolism in humans are not known. Objective: This study tested whether soybean isoflavones, soy protein, or both alter calcium metabolism in postmenopausal women. Design: Calcium metabolism in 15 postmenopausal women was studied by using metabolic balance and kinetic modeling in a randomized, crossover design of three 1-mo controlled dietary interventions: soy protein isolate enriched with isoflavones (soy-plus diet), soy protein isolate devoid of isoflavones (soy-minus diet), and a casein-whey protein isolate (control diet). Results: There was no significant difference between the diets in net acid excretion (P = 0.12). Urinary calcium excretion was significantly (P < 0.01) less with consumption of either of the soy diets (soy-plus diet: 85 +/- 34 mg/d; soy-minus diet: 80 +/- 34 mg/d) than with consumption of the control diet (121 63 mg/d), but fractional calcium absorption was unaffected by treatment. Endogenous fecal calcium was significantly (P < 0.01) greater with consumption of the soy-minus diet than with consumption of the other diets. Total fecal calcium excretion, bone deposition and resorption, and calcium retention were not significantly affected by the dietary regimens. Conclusions: The lower urinary calcium seen with the consumption of an isolated soy protein than with that of an isolated milk protein was not associated with improved calcium retention. This finding reinforces the importance of evaluating all aspects of calcium metabolism. Soy isoflavones did not significantly affect calcium metabolism.