Healthnotes Newswire (September 21, 2000)—Eating soy products may help prevent bone loss in postmenopausal women, according to a study published in the current issue of the American Journal of Clinical Nutrition. This report offers new hope for millions of women who are at risk of developing osteoporosis, a condition that is responsible for more than 1.2 million fractures per year in the United States.

Researchers at Iowa State University randomly assigned 80 women entering menopause (average age 50 years) to consume, in double-blind fashion, 40 grams per day of one of the following supplements for 24 weeks: 1) soy protein rich in isoflavones (the estrogen-like compounds that occur naturally in soy), 2) soy protein from which most of the isoflavones had been removed, or 3) whey (control group).¹ The isoflavone content of the isoflavone-rich (SPI+) and isoflavone-poor (SPI-) supplements were 80.4 and 4.4 mg per day, respectively. Bone mineral density and bone mineral content, two rough measures of the strength of bone, were determined in the lumbar spine before and after the 24-week treatment period. Compared with the control group, the SPI+ group experienced a statistically significant reduction in the amount of bone loss. The extent of bone loss was also less pronounced in the SPI- group than in the control group, but the difference was not statistically significant.

The onset of menopause is typically accompanied by a rapid decline in bone density for several years, which then settles down to a slower rate of bone loss that continues for the rest of a woman’s life. If this accelerated loss of bone can be minimized, then a woman will be more resistant to fractures in her later years. The results of this new study suggest that eating soy products around the time of menopause can help maintain stronger bones, although longer studies are needed to determine whether this effect will translate into reduced fracture risk. Eating soy protein can also reduce the severity of menopausal hot flashes, according to most^{2 3} but not all⁴ double-blind trials. Other reasons to consume soy products include lowering of serum cholesterol⁵ and possible prevention of cancer,⁶ although the research on cancer has produced conflicting results.

The results of the Iowa State study suggest that isoflavones are at least partly responsible for the bone-sparing effect of soy protein. However, one cannot automatically assume that taking soy-isoflavone supplements (which are widely available in natural food stores) will produce the same effect as eating soy. The SPI- supplement did appear to have a small (though not statistically significant) beneficial effect on bone density, even though almost all of the isoflavones had been removed. Thus, it is possible that isoflavones are not the only compounds in soy that have a positive effect on bone.

It is not known what the optimal intake of soy is for postmenopausal women. The amount of isoflavones that were found to reduce bone loss (80.4 mg per day) can be obtained by consuming two cups of soy milk or two servings of tofu per day. However, while there are many reasons to believe that soy is good for one’s health, eating too much soy can promote the development of iron deficiency, especially in vegetarians,⁷ and might interfere with thyroid function.⁸ People who eat soy foods once or more per day should probably be seen periodically by a doctor, to monitor iron status and thyroid function.