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.


METHODS
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.
Spence
Lisa A
LA
Department of Foods and Nutrition, Purdue University, West Lafayette, IN, USA.
Lipscomb
Elaine R
ER
Cadogan
Jo
J
Martin
Berdine
B
Wastney
Meryl E
ME
Peacock
Munro
M
Weaver
Connie M
CM
D015992:Body Mass Index; D001842:Bone and Bones / Q000187:drug effects / Q000378:metabolism; D002118:Calcium / Q000378:metabolism*; D018592:Cross-Over Studies; D005260:Female; D006801:Humans; D007529:Isoflavones / Q000008:administration & dosage / Q000097:blood / Q000494:pharmacology*; D008875:Middle Aged; D017698:Postmenopause / Q000378:metabolism*; D030262:Soybean Proteins / Q000008:administration & dosage / Q000494:pharmacology*
D007529:Isoflavones; D030262:Soybean Proteins; D002118:Calcium