A new study shows soy offers no benefits to older women who seek to reduce post-menopausal bone loss, high cholesterol and memory decline.
The findings, published in the July 7 issue of the Journal of the American Medical Association, stem from an analysis of more than 150 post-menopausal women over age 60 who randomly received either a soy protein supplement or a placebo for a year.
The results are likely to disappoint women who had hoped that soy's natural estrogen-like properties could offer a safe yet effective alternative to hormone-replacement therapy for treating complications of menopause. Recent studies show that risks of long-term estrogen use — which include increased risk of stroke, breast cancer and heart disease — outweigh potential benefits and have dissuaded many women from its use.
The new study was the first to rigorously assess the effects of soy in older women, said Dr. Johanna Lampe, an investigator in the Public Health Sciences Division and one of the study's co-authors.
"Our findings suggest that healthy older post-menopausal women aiming to improve cognitive function, maintain bone density or lower cholesterol levels are unlikely to derive benefits from supplementing their diets with soy protein if they start after age 60," she said. She added that more research is needed to determine conclusively the effects of soy supplements in younger women.
Dr. Sanne Kreijkamp-Kaspers and colleagues at the University Medical Center of Utrecht in the Netherlands led the analysis. Lampe contributed to the study's design and analysis. Her laboratory also conducted chemical analyses of the women's serum samples to monitor compliance to the diet and evaluate whether differences in innate metabolic differences affected a woman's response to treatment.
Estrogen levels drop suddenly in women after menopause, accelerating aging processes including decreased bone-mineral density that can lead to fractures, a decline in memory and other cognitive functions, and increased cholesterol levels. Although research suggests that post-menopausal estrogen therapy — either alone or in combination with progestin — can reverse some of these effects, both forms of hormone therapy have been associated with increased risk of a host of potentially dangerous side effects.
Some plants, including soy, beans, fruits and vegetables, contain compounds known as isoflavones, naturally occurring estrogen-like chemicals that can simulate some of the same biological effects as the estrogen produced in a woman's body. That has prompted many researchers to investigate whether plant-derived estrogens could serve as an alternative to traditional hormone-replacement therapy. Because the typical U.S. diet contains little soy or other sources of isoflavones, many women use soy protein supplements to boost their intake of plant-derived estrogens.
Studies of small numbers of individuals have found that soy-protein supplements had a positive effect on cognitive function and bone loss in women under age 60, and other research has found that soy reduces cholesterol levels in men, Lampe said.
"But there hadn't been a large-scale study that addressed all of these symptoms in a comprehensive manner, particularly in older women," she said. "That was the impetus for our study."
More than 150 healthy post-menopausal Dutch women ages 60 to 75 completed the entire study. Women were randomly assigned to receive a high-isoflavone, soy-protein powder drink or a milk-protein powder every day for one year.
"This was a carefully controlled study in that neither the participants nor the staff who interacted with them could tell which treatment each woman received," Lampe said.
At the beginning and end of the study, researchers measured participants' blood cholesterol levels and the bone-mineral density of their hips and lower spine. Women also completed cognitive tests both pre- and post-study, which measured short- and long-term verbal and visual memory, verbal skills, and concentration and visual attention.
No change after one year
At the end of one year, the researchers found no different effect on cognitive function, bone-mineral density or blood cholesterol levels from the soy-protein treatment than the milk-protein treatment.
"One obvious explanation for the fact that we saw no difference between the two groups of women is that there is no effect of soy protein containing isoflavones on the outcomes we studied," Lampe said. "The earlier studies that found some benefit to soy were smaller, shorter-term and their results were not uniform, so this is a realistic option."
But because the women in the current study were healthy, Lampe said, it's possible that no or few benefits would have been observed. Alternatively, "both bone density and cognitive function decline relatively slowly and a one-year intervention may not have been long enough to detect an effect, although typically a one-year time frame is used for soy studies in other populations," she said.
Lampe advised women to talk with their doctors about the most appropriate way to deal with menopausal symptoms, since treatment approaches are likely to differ depending on which symptoms they are trying to prevent.