Soy & Osteoporosis

A worldwide problem, osteoporosis is the most prevalent metabolic bone disease in developed countries including the United States.¹ In the Western world, women are disproportionably affected by osteoporosis, with fracture rates approximately twice as high as those of men.^2-4

For women, the risk of many chronic diseases increases with age; an increase especially notable in the case of osteoporosis. Around the fourth decade of life, bone breakdown exceeds bone formation, leading to a loss in bone density. This imbalance often worsens considerably around the time of menopause because of the marked reduction in the production of estrogen – the main female reproductive hormone.

Since overall diet is known to influence bone health, there is considerable interest in the possible skeletal benefits of soyfoods. Initial enthusiasm for research in this area was largely driven by the fact that soybeans are a unique dietary source of isoflavones.⁵ Isoflavones are naturally present constituents of soybeans that, although much different from the hormone estrogen, may possess some estrogen-like properties. Estrogen therapy has been shown to reduce bone loss and fracture risk in postmenopausal women.⁶ Early epidemiologic studies generally showed that higher Asian soy intake was positively associated with higher bone mineral density (BMD.)⁷

Two prospective Asian epidemiologic studies recently lent support to the hypothesis that soyfoods are protective against osteoporosis. Importantly, both of these studies included fractures as an end point.^8,9 In both, relative risk among high-soy consuming postmenopausal women was lower by approximately one-third in comparison to women who consumed relatively little soy. One study included approximately 24,000 postmenopausal women from Shanghai who were followed for almost five years.⁸ During this time nearly 1800 fractures occurred. In the other study, during the seven year follow-up period there were 692 hip fractures among approximately 35,000 women from Singapore.⁹

Despite these encouraging epidemiologic observations, the results from clinical trials of isoflavone-rich products are much more mixed. At least 20 such trials evaluating BMD in postmenopausal women have been conducted, with some studies showing dramatic benefits. Most notable is a three-year Italian study which found spinal bone loss decreased by approximately 12 percent in the placebo group whereas in the group given the isoflavone genistein, BMD increased by 9 percent.¹⁰ However, in contrast, several recent large intervention studies have failed to show isoflavones have any skeletal benefits ^11-13

It is unclear why the results from the epidemiologic and intervention studies differ. Is it because the former assess the health effects of traditional soyfoods, whereas the latter generally examine the effects of soy supplements? Or perhaps it is because epidemiologic studies identify long-term users of soy. In any event, although no firm conclusions about the direct skeletal benefits of soyfoods can be made at this time, soyfoods like fortified soymilk certainly play an important role in a bone-healthy diet, simply due to their high-quality protein and abundant calcium content.

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References

1. Wasnich RD. Vertebral fracture epidemiology. Bone 1996;18 (3 suppl):179S-183S.
2. Chalmers J, Ho KC. Geographic variations in senile osteoporosis. The association with physical activity. J Bone Joint Surg [Br] 1970;52:667-675.
3. Melton LJ, III. Epidemiology of hip fractures: implications of the exponential increase with age. Bone 1996;18:121S-5S.
4. Melton LJ, 3rd. Epidemiology worldwide. Endocrinol Metab Clin North Am 2003;32:1-13.
5. Franke AA, Custer LJ, Wang W, Shi CY. HPLC analysis of isoflavonoids and other phenolic agents from foods and from human fluids. Proc Soc Exp Biol Med 1998;217:263-73.
6. Atmaca A, Kleerekoper M, Bayraktar M, Kucuk O. Soy isoflavones in the management of postmenopausal osteoporosis. Menopause 2008;15:748-57.
7. Messina M, Ho S, Alekel DL. Skeletal benefits of soy isoflavones: a review of the clinical trial and epidemiologic data. Curr Opin Clin Nutr Metab Care 2004;7:649-658.
8. Zhang X, Shu XO, Li H, Yang G, Li Q, Gao YT, et al. Prospective cohort study of soy food consumption and risk of bone fracture among postmenopausal women. Arch Intern Med 2005;165:1890-5.
9. Koh WP, Wu AH, Wang R, Ang LW, Heng D, Yuan JM, et al. Gender-specific associations between soy and risk of hip fracture in the Singapore Chinese Health Study. Am J Epidemiol 2009;170:901-9.
10. Marini H, Bitto A, Altavilla D, Burnett BP, Polito F, Di Stefano V, et al. Breast safety and efficacy of genistein aglycone for postmenopausal bone loss: a follow-up study. J Clin Endocrinol Metab 2008;93:4787-96.
11. Brink E, Coxam V, Robins S, Wahala K, Cassidy A, Branca F. Long-term consumption of isoflavone-enriched foods does not affect bone mineral density, bone metabolism, or hormonal status in early postmenopausal women: a randomized, double-blind, placebo controlled study. Am J Clin Nutr 2008;87:761-70.
12. Kenny AM, Mangano KM, Abourizk RH, Bruno RS, Anamani DE, Kleppinger A, et al. Soy proteins and isoflavones affect bone mineral density in older women: a randomized controlled trial. Am J Clin Nutr 2009;90:234-42.
13. Vupadhyayula PM, Gallagher JC, Templin T, Logsdon SM, Smith LM. Effects of soy protein isolate on bone mineral density and physical performance indices in postmenopausal women-a 2-year randomized, double-blind, placebo-controlled trial. Menopause 2009.