Soy Protein Has a Greater Effect on Bone in Postmenopausal Women Not on Hormone Replacement Therapy, as Evidenced by Reducing Bone Resorption and Urinary Calcium Excretion

Author: Bahram H. Arjmandi, Dania A. Khalil, Brenda J. Smith, Edralin A. Lucas, Shanil Juma, Mark E. Payton and Robert A. Wild
Publication: The Journal of Clinical Endocrinology & Metabolism Vol. 88, No. 3 1048-1054.

Other studies suggest that soy protein may reduce the risk of osteoporosis in postmenopausal women. A women spends about on third of her live in the menopausal period, during which she experiences an increased risk for osteoporosis. There are many therapies to help prevent bone loss including hormone replacement therapy (HRT) and treatment with calcitonin or bisphosphates. Women are increasingly looking for natural therapies such as plant based products. Soy isoflavones have an estrogen activity and may influence bone cells. Some studies have already demonstrated that soy protein and soy isoflavones improve bone formation and suppress bone resorption.

The objective of this study was to examine if soy protein and its isoflavones exerts beneficial effects on biomarkers of bone metabolism in postmenopausal women and to determine if the effect of soy protein was different for women on HRT and those not on HRT. In total 71 women were randomly assigned to either soy protein group or dairy protein group and were given 40 g protein during 3 months. Soy protein as well as dairy protein increased serum IGF-I that is correlated with bone formation. Soy protein had a more pronounced effect than dairy protein on IGF-I levels. The function of IGF-I is still unclear but IGF-I has been linked to increased collagen production in vitro by the osteoblasts and increased bone mass in pre-menopausal, peri-menopausal and post-menopausal women. Urinary excretion of deoxypyridinoline, which is a biomarker of bone resorption, was significantly reduced by soy protein and not by the dairy protein. The actual loss of calcium through the kidneys was only increased in the dairy protein group. Soy protein seems to have a calcium preserving effect on bones and help to prevent osteoporosis. This effect could be explained by the lower levels of sulphur-containing amino acids in soy protein. Other studies have shown that soy isoflavones stimulate the activity of osteoblasts (which are responsible for bone building) by the activation of estrogen receptors. The results of the women who took HRT were compared to those not on HRT. These result indicate that soy protein had the greatest impact on IGF-I in women not on HRT. That soy protein is not effective in reducing bone resorption in women on HRT has been illustrated in a study by Alekel et al (Isoflavone-rich soy protein isolate attenuates bone loss in the lumbar sine of peri-menopausal women. Am. J. Clin Nutr. 2000 Sep; 72:844-852).

The obtained results suggest that soy protein may have a positive influence on prevention of osteoporosis and calcium homeostasis in postmenopausal women, especially those women who are not on HRT.