Comparative effect of soy protein, soy isoflavones, and 17beta-estradiol on bone metabolism in adult ovariectomized rats.
This study provided a comprehensive investigation on the effect of soy protein and soy isoflavones on both calcium and bone metabolism in virgin adult rats. The measurements included bone histology, calcium kinetic modeling, calcium balance, bone densitometry, and whole body densitometry. Results confirmed the bone-preserving effect of estrogen but did not support a bone-sparing role of soy isoflavones.


BACKGROUND
Several animal and short-term human studies have indicated that soy protein isolate enriched with isoflavones may be used as an alternative therapy to estrogen replacement therapy. However, none of the previous studies have investigated this estrogenic effect on both calcium and bone metabolism in animals or humans, which is essential in ascertaining the mode of action of isoflavones.


METHODS
This study was designed to determine the effects of soy protein versus isoflavones on calcium and bone metabolism in an ovariectomized rat model. Unmated 6-month-old ovariectomized and sham-operated female Sprague-Dawley rats were randomly assigned to nine groups (16 rats/group) and pair-fed soy- or casein-based diets with or without isoflavones for 8 weeks. A reference group was administered estrogen through subcutaneous implants (20-35 pg/liter plasma). Bone densitometry, histomorphometry, and mechanical testing were used to study bone metabolism and quality. Calcium metabolism was studied using calcium tracer balance and kinetics.


RESULTS
After ovariectomy, estrogen prevented bone loss in trabecular bone and suppressed formation on both trabecular and cortical bone surfaces. Isoflavones given as enriched soy protein isolate or supplements did not prevent trabecular bone loss. Combining isoflavones with estrogen had no additional benefits over estrogen alone. There were no differences in response to isoflavones caused by protein source. None of the treatments significantly affected either total Ca balance or (45)Ca absorption. However, soy protein showed significant effects on reducing urinary loss of Ca in animals, irrespective of isoflavone level, perhaps because of the lower amount of sulfur-containing amino acids in soy protein.


CONCLUSIONS
Estrogen, but not isoflavones at the levels tested, suppressed bone remodeling in both trabecular and cortical bone after ovariectomy.
Cai
David J
DJ
Department of Foods and Nutrition, Purdue University, West Lafayette, Indiana 47907, USA.
Zhao
Yongdong
Y
Glasier
Jennifer
J
Cullen
Diane
D
Barnes
Stephen
S
Turner
Charles H
CH
Wastney
Meryl
M
Weaver
Connie M
CM
D000042:Absorption; D000818:Animals; D001696:Biomechanical Phenomena; D001835:Body Weight; D001842:Bone and Bones / Q000378:metabolism*; D002118:Calcium / Q000378:metabolism; D002364:Caseins / Q000737:chemistry; D003720:Densitometry; D004958:Estradiol / Q000378:metabolism / Q000494:pharmacology*; D004967:Estrogens / Q000378:metabolism; D005260:Female; D005269:Femur / Q000473:pathology; D007529:Isoflavones / Q000737:chemistry* / Q000378:metabolism; D007700:Kinetics; D009929:Organ Size; D010053:Ovary / Q000378:metabolism / Q000502:physiology*; D011863:Radioimmunoassay; D051381:Rats; D017207:Rats, Sprague-Dawley; D030262:Soybean Proteins / Q000737:chemistry*; D013025:Glycine max / Q000737:chemistry; D013977:Tibia / Q000473:pathology; D014599:Uterus / Q000473:pathology
D002364:Caseins; D004967:Estrogens; D007529:Isoflavones; D030262:Soybean Proteins; D004958:Estradiol; D002118:Calcium