Osteoporosis is a disease in which the bones become weak and brittle, making them more susceptible to fracture . Postmenopausal women are at increased risk for the condition; women lose three to five percent of their bone mass each year for five to seven years after they reach menopause .

Studies have shown that hormone replacement therapy (either estrogen alone or a combination of estrogen and progestin) can prevent postmenopausal osteoporosis and bone fracture. Unfortunately, estrogen has been shown to increase the risk of cardiovascular disease and breast cancer . As a result, most physicians no longer recommend it for the long-term prevention of postmenopausal osteoporosis.

Some plants produce estrogen-like compounds, known as phytoestrogens . The phytoestrogens made by soybeans are called isoflavones , and they have been studied in recent years as a natural substitute for hormone therapy. Preliminary studies linking soy to the prevention of postmenopausal bone loss have been promising. For example, research has linked dietary intake of soy in postmenopausal women with higher bone mineral density. But no studies to date have examined the crucial question—do isoflavone-rich foods actually decrease the risk of fracture in postmenopausal women?

In a study published in the September 12, 2005 Archives of Internal Medicine , researchers examined the relationship between soy consumption and risk of fracture. They found that postmenopausal women who consumed the most soy protein or isoflavones were significantly less likely to have a fracture than postmenopausal women who consumed the least soy protein or isoflavones. The relationship between soy consumption and decreased risk of fracture was strongest during the first ten years after menopause, when rapid bone loss normally occurs.

About the Study

The researchers studied 24,403 postmenopausal women, aged 40 to 70 years, who were enrolled in the Shanghai Women’s Health Study. All of the study participants had been in menopause (either natural or surgically induced) for at least one year, had never used hormone therapy, and had no history of fracture or cancer.

At the beginning of the study, the women were asked about their diet and lifestyle habits, menstrual and reproductive history, medical history, and sociodemographic characteristics.

The study subjects also filled out food frequency questionnaires (FFQs) at the beginning of the study and then again after two or three years. The FFQs asked how often and how much of several soy foods (soy milk and tofu, for example) the study subjects consumed. The researchers calculated the isoflavone content of each soy food from published data. Based on their responses to the FFQs, the researchers broke the women down into five categories based on the quantity of soy protein or isoflavone in their diets.

The study participants reported any fractures that occurred during an average follow-up period of 4.5 years. The researchers examined the relationship between soy protein or isoflavone consumption and bone fractures in the study subjects.

The Findings

Even after adjusting for age, other major risk factors for osteoporosis (smoking, for example), and other dietary factors, postmenopausal women who consumed the most soy protein (≥ 13.27 grams per day (g/d)) were 37% less likely to have a fracture than postmenopausal women who consumed the least soy protein (< 4.98 g/d). This was a significant difference.

This relationship was strongest for women in early menopause. The researchers found that women who had been in menopause for fewer than ten years and consumed the most soy protein had a 48% reduced risk of fracture compared to women in early menopause who ate the least soy protein. On the other hand, women who had been in menopause for ten years or more had a 29% reduced risk of fracture compared to women who ate the least soy protein.

Isoflavone intake was also associated with reduced risk of fracture. Postmenopausal women who consumed the most soy isoflavones (≥ 60.27 milligrams per day (mg/d)) were 35% less likely to have a fracture than postmenopausal women who consumed the least soy isoflavones (< 21.16 mg/d). Again, this was a significant difference.

While this study demonstrates a strong association between soy consumption and reduced risk of fracture, it is important to bear in mind that an observational study such as this cannot establish a cause and effect relationship. In addition, although the researchers did control for a variety of relevant factors such as diet and lifestyle factors, and major risk factors for osteoporosis, it is possible that other factors that were not measured could have influenced the study results.

How Does This Affect You?

This study found that postmenopausal women who ate more soy foods had a significantly reduced risk of fracture compared to women who ate less soy foods.

Soy foods can be readily found in most supermarkets and can be easily incorporated into virtually anyone’s diet. Soy foods include soy milk, tofu, soy sprouts, and soybeans (also known as edamame).

Although soy foods are safe and healthful, high-dose soy supplements may not be. At least one study has shown that high-dose soy supplements may trigger an abnormal increase in the growth of certain breast cells, making it potentially unsafe for women who have already had breast cancer.

The best way to prevent osteoporosis is by building strong bones throughout your lifetime. That means eating a balanced diet with plenty of calcium and vitamin D, regular exercise (weight-bearing exercise is particularly beneficial for osteoporosis prevention), moderate alcohol consumption, and no smoking. For women who are at increased risk for osteoporosis, medications that slow or stop the breakdown of bone might also be appropriate. These medications include alendronate sodium (Fosamax), risedronate sodium (Actonel), and raloxifene (Evista).