The new study, published in the Journal of the American Medical Association, found that plant-based supplements modestly reduced hot flashes and vaginal dryness as a result of phytoestrogens.
"Phytoestrogens are clinical components found in plants that are very similar to the female hormone, estrogen," explained Dr. Taulant Muka, lead author of the new study and a researcher at Erasmus University Medical Center in the Netherlands. Soybeans and soy supplements are the richest sources of one class of phytoestrogens known as isoflavones.
Menopausal symptoms are not only inconvenient and embarrassing, they've been linked to an increased risk of heart disease, says Muka. His previously published analysis that compared women with and without menopausal symptoms suggests that hot flashes, night sweats and other symptoms corresponded to an increased risk of coronary heart disease, stroke and cardiovascular disease.
Hormone therapy, in the form of pills, patches, sprays, gels and vaginal rings, is the prescription used most often to treat hot flashes and other menopausal symptoms. Research has linked
hormone replacement therapy with an increased risk of breast and uterine cancer. Other studies have shown
neutral effects on cardiovascular health when used soon after menopause, but the effects become progressively unfavorable in a woman's later years.
These findings help explain why many women -- up to 50% in Western countries, according to Muka -- choose alternative therapies instead of prescription hormones to soothe their menopausal symptoms.
To understand whether they actually work, Muka and colleagues screened hundreds of scientific articles and analyzed 62 that assessed the effectiveness of plant-based therapies in treating hot flashes, night sweats and vaginal dryness. A total of 6,653 women participated in these studies of alternative, traditional or Chinese medicine therapies, including soy, ginseng, black cohosh and St. John's wort.
The researchers discovered that phytoestrogens were associated with a modest reduction in hot flashes and vaginal dryness, though night sweats continued. Specifically, plant estrogens decreased the total number of a woman's hot flashes by 1.31 per day, on average.
While for some women this might be improvement enough, Dr. JoAnn V. Pinkerton, executive director of the North American Menopause Society
, points out that the Food and Drug Administration requires a reduction by at least two hot flashes per day when evaluating any treatment for approval. However, the FDA does not review or regulate herbal supplements of any kind.
Pinkerton also notes that this new study fails to distinguish between women who have many hot flashes and those who have very few. Based on the frequency of her symptoms, a particular woman might find phytoestrogens more or less effective.
"Overall, we did not find an association between phytoestrogens and night sweats," Muka said, though a single experiment with red clover found a reduction in the frequency of night sweats. This is insufficient, though, to draw any firm conclusions.
"A main concern of our study was the length of follow-up," Muka said. Since most of the research included just 12 to 16 weeks of follow-up, he and his colleagues cannot define the long-term effects of phytoestrogen and whether they might have adverse effects over time.
"In general, if women are going to use any supplements, they should recognize that what they are purchasing may vary in dose and amount and that there may be risks if used long-term," Pinkerton said, noting that it is unclear whether isoflavones might increase a woman's risk of breast cancer.
In fact, the North American Menopause Society reviewed all non-hormonal therapies for menopausal symptoms in 2015.
"NAMS recommends caution until we have a stable supplement that has been well-tested in a randomized control trial so women know exactly what they're taking and that it is working," Pinkerton said.
However, she still has hopes for "equol," a particular soy supplement that is undergoing testing. This phytoestrogen, which is available in Japan, appears to be effective in reducing hot flashes, yet its chemical action does not stimulate breast cancer.