Soy protein versus milk protein and urinary calcium excretion in postmenopausal women

Elaine Ramona Lipscomb, Purdue University

Abstract

High animal protein intake and low vegetable protein intake increase dietary acid load as indicated by high urinary excretions of sulfate, ammonium, net acid, and calcium. The primary purpose of the present study was to compare dietary soy protein with milk protein as a calcium-conserving agent in postmenopausal women. Fifteen women were fed a controlled diet containing 40g protein powder as casein-whey (milk) or soy in a crossover design. Mean urinary calcium excretion on the soy protein-based diet (80mg ± 42) was 35% lower (p < 0.01) than on the milk protein diet (123mg ± 64). The 18% difference in SAA content of the powders was reflected in greater (p < 0.05) urinary sulfate excretion on the milk protein (4.5mEq ± 1.1) vs. the soy protein-based diet (3.3mEq ± 0.97). Moreover, there was no significant difference in NAE or urinary creatinine. The greater urinary calcium excretion on the milk vs. soy protein-based diet was not accompanied by a greater creatinine clearance. Furthermore, there was no significant difference between the soy and milk protein-based diet in urinary nitrogen excretion or nitrogen balance. There were no differences found in any of the calcitropic hormones (PTH, 1,25 OH2-D, and 25 OH-D) or any of the biochemical markers of bone turnover (NTX, BAP, OC, DPD). However, there was a 37% higher fractional renal tubular reabsorption of calcium and a 27% lower calcium clearance on the soy (0.041 ± .025 and 1.95ml/min, respectively) vs. milk protein-based diet (0.029 ± 0.016 and 2.66ml/min ± 1.8, respectively). In conclusion, soy protein is calcium-conserving as indicated by lower urinary calcium excretion.

Degree

Ph.D.

Advisors

Weaver, Purdue University.

Subject Area

Nutrition

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