The effects of soy isoflavones on calcium metabolism in postmenopausal women
Abstract
Several lines of evidence suggest that soy isoflavones and related compounds act as estrogen agonists and have beneficial skeletal effects. However, little is known about the metabolic effects of isoflavones, and specifically, their effects on calcium handling by the body. The aim of this study was to test the effects of soybean isoflavones on calcium metabolism in postmenopausal women. Fifteen postmenopausal women were studied under three different one-month controlled dietary interventions in a randomized, crossover design: soy protein enriched with isoflavones, soy protein void of isoflavones, and a casein-whey control. Dietary calcium was 1106 mg/d, dietary protein was 89 mg/d with 40 g/d coming from isolated protein powders, and dietary isoflavones were ±65 mg/d. Using a combination of metabolic balance methodology and radioisotopic tracer techniques along with kinetic modeling, the following parameters were measured: calcium absorption, calcium excretion, calcium retention, bone formation, and bone resorption. Biochemical markers of bone turnover, hormonal markers, serum isoflavones, and serum lipids were measured. After a 7-day adaptation to the diet, 10 μCi of 45Ca was administered orally. One week later, 10 μCi of 45Ca was administered intravenously. A series of blood samples were obtained following the radioisotopic tracer administration along with a 21-day collection of urine and feces. Total calcium was measured by atomic absorption spectrophotometry and 45Ca was measured by Beta-scintillation counting. The WinSAAM program was used to analyze kinetic modeling parameters and the SAS program was used for all statistical analysis. No significant differences due to the consumption of soy protein enriched with isoflavones compared to soy protein void of isoflavones on calcium absorption, calcium excretion, calcium retention, markers of bone turnover, hormone levels (exception with estrone sulfate), nor serum lipids were evident in the postmenopausal women. Furthermore, dietary treatments had no significant effects on calcium kinetics including bone balance, bone deposition, bone resorption, and transfer rates between compartments of blood, soft tissue, and exchangeable calcium on bone. Soy protein, irregardless of isoflavone content, reduced urinary calcium compared to milk protein by 35%. Soy protein may be reducing urinary calcium through its lower content of sulfur-containing amino acids. Based upon the present study, consumption of soy protein may benefit bone but isoflavones prove to be non-effective in perturbing calcium metabolism or influencing bone metabolism.
Degree
Ph.D.
Advisors
Weaver, Purdue University.
Subject Area
Nutrition|Public health|Womens studies
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