Taking soy supplements won't ease the symptoms of menopause or protect against bone loss in women, researchers have found.
Over two years, there were no differences in changes in bone density or menopausal symptoms between women taking soy and those taking a placebo, although women taking isoflavones did have more hot flashes, Dr. Silvina Levis of the University of Miami and colleagues reported in the August 8/22 issue of Archives of Internal Medicine.
Since results of the Women's Health Initiative tied hormone replacement therapy to several adverse effects, particularly cardiovascular disease, women have been searching for alternative treatments for menopausal symptoms.
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Many have turned to soy, which contains phytoestrogens, specifically the isolflavones genistein and daidzein, the researchers wrote. Studies have shown a lower prevalence of osteoporotic fractures, breast cancer, and cardiovascular disease in Asian women, which has been largely attributed to their soy-rich diets.
Yet prior evidence for the efficacy of soy supplementation has been limited by poor study design, small sample size, and short duration of treatment.
So to clarify previous findings, the researchers conducted the Soy Phytoestrogens As Replacement Estrogen (SPARE) study, a single-center randomized, controlled, double-blind trial, in 248 women, ages 45 to 60, between July 1, 2004 and March 31, 2009.
All of the women were within five years of the start of menopause, and had a bone mineral density score of -2.0 or higher at the lumbar spine or total hip.
They were randomized to placebo, or to 200 mg daily of soy isoflavone tablets.
After two years, the researchers found no significant differences in any measurements of bone mineral density between those on soy versus those on placebo.
Levis and colleagues found no differences when controlling for confounders, except with regard to vitamin D status. Women whose levels were below 20 ng/mL in the placebo group had a significantly greater decrease in spinal bone density than those taking soy.
With regard to secondary outcomes, the researchers found that significantly more women taking soy supplements had hot flashes.
There no differences in vaginal cytologic characteristics or cholesterol, triglycerides, or thyrotropin levels between groups, they reported.
As well, there were no differences between the groups in adverse effects, except those on soy reported more constipation, a finding of marginal significance.
Overall, there were 12 serious adverse events -- nine in the soy group and three in the placebo group. Seven women in the soy group reported fractures compared with just one on placebo. All these fractures were associated with a traumatic event and not considered to be osteoporotic fractures, the researchers noted.
There was also a higher dropout rate in the placebo group (34.1 percent versus 18.8 percent), but Levis and colleagues said the numbers don't necessarily indicate favorable effects for soy because there was no objective documentation of a benefit. In a post hoc survey, similar proportions of patients in both arms thought they were in the active group.
The dropout rate was one limitation of the study, because it was higher than expected, the researchers said. Also, the small amount of bone loss overall may have precluded detection of any treatment effects. They noted, however, that their population was largely white and overweight, two groups known to have some protection against bone loss.