Soy Protein has a Greater Effect on Bone in Postmenopausal Women not on Hormone Replacement Therapy

Menopausal women have a higher risk for osteoporosis because of decreasing estrogen levels. Many of these women take hormone replacement therapy to prevent osteoporosis. Although hormone replacement therapy slows down bone resorption, its long-term use does not decrease fracture incidence. Menopausal women are more looking for natural therapies such as consumption foods, herbs or supplements. Soy isoflavones may act as weak estrogens and may have an effect on bone tissue. Recent studies suggest that soy proteins and isoflavones may reduce the risk of osteoporosis in menopausal women. Picherit et all showed in their study "Daidzein is more efficient than genistein in preventing ovariectomy induced bone loss in rats" (J. Nutr. 130:1675-1681) and "Soybean isoflavones dose dependently reduce bone turnover but do not reverse established osteopenia in adult ovariectomized rats" (J. Nutr. 131:723-728) that soy isoflavones may prevent bone loss. Other studies show that soy protein intake in postmenopausal women is associated with lower calcium loss. The objective of this study was to investigate if the consumption of soy proteins could influence bone metabolism in postmenopausal women. The study involved women who were on hormone replacement therapy and women who were not.

A total of 71 women were divided in two groups: the soy protein group (received soy protein as protein source) and the dairy group (received milk based protein). Serum and urinary markers of bone turnover were measured. Both groups showed a positive influence on serum insulin-like growth factor one (IGF-I) which is an indication for bone formation. There were significant differences on the biomarkers of bone formation:

• Compared to the dairy group, the soy protein group as a more pronounced effect on IGF-I. The role of IGF-I on bone formation is still unclear but there are indications that IGF-I may stimulate the formation of collagen.
• Urinary deoxypyridinoline excretion was significantly reduced in the soy protein group only. Urinary deoxypyridinoline excretion is a biomarker of bone resorption.
• Only the women who consumed dairy protein showed an increased urinary calcium excretion. The reduction of urinary calcium loss could be explained by low levels of sulphur containing amino acids in soy protein.

When investigating women who took hormone replacement therapy and those who did not take hormone replacement therapy if was shown that the effect of soy protein was the highest in women who took not hormones. Soy protein significantly reduced urinary deoxypyridinoline levels in women not treated with hormone replacement therapy.

The obtained results indicate that soy protein may have a positive effect on bone health, particularly in women who are not on hormone replacement therapy. Soy protein appeared to decrease bone loss by suppressing bone resorption and enhancing the rate of bone formation.
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Bahram H. Arjmandi, Dania A. Khalil, Brenda J. Smith, Edralin A. Lucas, Shanil Juma, Mark E. Payton and Robert A. Wild (The Journal of Clinical Endocrinology and Metabolism Vol. 88, No. 3 1048-1054)