News Author: Laurie Barclay, MD
CME Author: Désirée Lie, MD, MSEd

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Release Date: September 15, 2005; Valid for credit through September 15, 2006

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Sept. 15, 2005 — Soy foods may reduce the risk of fracture in postmenopausal women, according to the results of a prospective cohort study reported in the Sept. 12 issue of the Archives of Internal Medicine.

"Soy consumption has been shown to modulate bone turnover and increase bone mineral density in postmenopausal women," write Xianglan Zhang, MD, MPH, from Vanderbilt University School of Medicine in Nashville, Tennessee, and colleagues. "To our knowledge, no published studies have directly examined the association between soy consumption and risk of fracture.... The Shanghai Women's Health Study, a large cohort study conducted in a population that has a high, yet wide, range of soy food consumption, provides us with a unique opportunity to evaluate this important hypothesis."

Between March 1, 1997, and May 23, 2000, approximately 75,000 Chinese women aged 40 to 70 years were enrolled in the Shanghai Women's Health Study, including 24,403 postmenopausal women with no history of fracture or cancer. Usual soy food intake at baseline and during follow-up was determined through in-person interviews using a validated food frequency questionnaire (FFQ).

During a mean follow-up of 4.5 years (110,243 person-years), there were 1,770 incident fractures. Risk of fracture decreased with increasing soy intake, after adjustment for age, major risk factors for osteoporosis, socioeconomic status, and other dietary factors. Relative risks (RRs), based on quintiles of soy protein intake, were 1.00, 0.72 (95% confidence interval [CI], 0.62 - 0.83), 0.69 (95% CI, 0.59 - 0.80), 0.64 (95% CI, 0.55 - 0.76), and 0.63 (95% CI, 0.53 - 0.76; P < .001 for trend).

The protective effect of soy protein intake was more pronounced for women in early menopause. Multivariate RRs of fracture for extreme quintiles of soy protein intake were 0.52 (95% CI, 0.38 - 0.70) for women within 10 years of menopause, and 0.71 (95% CI, 0.56 - 0.89) for late postmenopausal women. Isoflavone intake showed a similar pattern.

Study limitations include observational design, inability to establish a causal relationship between soy consumption and fracture risk reduction, differences in baseline disease risk factors and other dietary or lifestyle factors in women in different quintiles of soy consumption, possible residual confounding, fracture information based on self-report, insufficient power to investigate the association of soy with site-specific fractures, and lack of generalizability to other populations.

"Soy food consumption may reduce the risk of fracture in postmenopausal women, particularly among those in the early years following menopause," the authors write. "This inverse association was independent of major risk factors for osteoporotic fractures and other dietary factors, including intake of calcium, nonsoy protein, fruits, and vegetables. Soy or soy isoflavones may exert their effects on bone by suppressing bone resorption, while at the same time stimulating bone formation."

The National Institutes of Health in Bethesda, Maryland, supported this study. The authors report no financial disclosures.

Arch Intern Med. 2005;165:1890-1895

Learning Objectives for This Educational Activity

Upon completion of this activity, participants will be able to:

• Describe the association between soy intake and fractures in postmenopausal Chinese women.
• Compare the effect of soy intake on fractures in postmenopausal Chinese women based on years since menopause.

Clinical Context

Women experience accelerated bone loss at the rate of 3% to 5% per year for five to seven years after the onset of menopause, which increases their risk of bone fracture. It is well-known that hormone replacement therapy prevents postmenopausal osteoporosis and fracture. However, recent studies have shown an increased risk of cardiovascular disease and breast cancer with hormone replacement therapy. According to the authors, the U.S. Food and Drug Administration and new clinical guidelines recommend against the use of hormone replacement therapy for the prevention of osteoporosis, emphasizing instead more exercise, calcium and vitamin D intake, and other alternative approaches. Plant-derived phytoestrogens or isoflavones have attracted attention as natural substitutes for estrogen, with potential benefits for hot flashes, cardiovascular disease, and cancers. Soy or soy isoflavones may exert effects on bone by suppressing bone resorption and stimulating bone formation as well as increasing the production of insulin-like growth factor, enhancing osteoblastic activity. However, soy's effectiveness in preventing fractures has not been studied prospectively, according to the authors.

This is an observational, prospective, cohort study of 24,403 Chinese women from Shanghai, China, aged 40 to 70 years and living in seven urban communities, to examine the association between soy consumption and bone fractures in postmenopausal women.

Study Highlights

• Inclusion criteria were postmenopausal (surgical or natural) with no history of cancer or fractures.
• Participants received in-home face-to-face baseline interviews and follow-up interviews biennially. Anthropometric measures were made at baseline, and medical records from the city cancer and hospital registries were used.
• Usual dietary intake was determined by a comprehensive quantitative FFQ that covered all soy food available in Shanghai, including soy milk, tofu, soy sprouts, fresh soybeans, and other products. Participants were asked how often they consumed products daily, weekly, monthly, or yearly. Amount consumed was determined in grams per unit of time.
• Chinese Food Composition tables were used to determine nutrient content, and isoflavone content was derived using published data.
• Correlation of the FFQ with 24-hour recall was calculated at 0.59 to 0.66 for macronutrients, 0.41 to 0.59 for micronutrients, 0.41 to 0.66 for major food groups, and 0.37 for soy foods.
• Primary outcome was incidence of new fractures by self-report. Participants were asked if they had broken any bones since the last home visit that was confirmed by a physician. Traumatic fractures were not excluded from analysis.
• Mean age was 60 years, mean number of menopausal years was 11, and median daily intake of soy and isoflavones were 8.5 g and 38.0 mg, respectively. Mean body mass index (BMI) was 24.5 kg per m², mean number of hours of exercise weekly was 2.2 hours, and mean daily caloric intake was 1,423 to 1,846 kcal. 5% had ever smoked.
• Women with higher soy intake were more educated, more physically active, had higher intake of calcium, and a higher BMI than those with lower soy intake.
• Mean daily soy protein intake from the lowest to the highest quintiles of intake was 3.3, 6.2, 8.5, 11.4, and 18.5 g, respectively.
• For the same quintiles of soy intake from lowest to highest, the corresponding daily intakes of calcium were 316, 392, 439, 504, and 650 mg, respectively.
• Mean duration of follow-up was 4.5 years (110,423 person-years).
• 1,770 incident fractures were identified including 17.6% of the wrist, 15.1% of the arm, 14.9% of the vertebrae, 13.1% of the ankle, 7.0% of the rib, and 3.3% of the hip.
• After adjustment for age and total caloric intake, higher soy intake was significantly associated with lower risk of fracture.
• Other variables adjusted for included cigarette smoking, alcohol consumption, BMI, exercise, diabetes, education, and income.
• The effect of soy intake on fractures was more pronounced for women with recent menopause than for those later in menopause.
• For women within 10 years of menopause, the RRs of fracture for the quintiles of soy intake from lowest to highest were 1.0, 0.68, 0.60, 0.55, and 0.52 (P < .01 for trend).
• For women beyond 10 years of menopause, the corresponding RRs were 1.0, 0.73, 0.75, 0.70, and 0.71, respectively (P = .009 for trend).
• Similar associations were found for intakes of isoflavones.
• Adjustment for major risk factors for osteoporosis, fractures, socioeconomic status, and dietary factors did not alter the associations.

Pearls for Practice

• Higher intake of soy and isoflavones in postmenopausal Chinese women is associated with lower rates of fractures.
• The effect of soy and isoflavones on incident fractures in postmenopausal Chinese women is greater in those within 10 years compared with those beyond 10 years of menopause.

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News Author

Laurie Barclay, MD
is a freelance writer for Medscape.

Disclosure: Laurie Barclay, MD, has disclosed no relevant financial relationships.

Clinical Reviewer

Gary Vogin, MD
Senior Medical Editor, Medscape

Disclosure: Gary Vogin, MD, has disclosed no relevant financial relationships.

CME Author

Desiree Lie, MD, MSEd
Clinical Professor of Family Medicine; Director, Division of Faculty Development, University of California, Irvine School of Medicine, Irvine, California

Disclosure: Desiree Lie, MD, MSEd, has disclosed no relevant financial relationships.

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Medscape Medical News 2005. © 2005 Medscape

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Soy Foods May Reduce Fracture Risk in Postmenopausal Women(豆制品可以降低绝经后妇女骨折风险)