However, the evidence for other potential benefits of soy -- such as effects on heart and bone health -- is not clear, a panel of experts has concluded.
''It gets a good score for [menopausal] symptoms," says researcher Wulf Utian, MD, PhD, ScD, a consultant in women's health and executive director emeritus of the North American Menopause Society. ''But the data is really not strong to give a high score for any of the rest."
With a working group of experts in the field, Utian combed through evidence during a two-day symposium in late 2010 to evaluate the health benefits of soy for women at midlife.
The report is published in the journal Menopause.
Benefits of Soy: The Scorecard
They reviewed hundreds of studies. They found mixed results.
They looked at research evaluating soy from foods and supplements. Soy's isoflavones are credited with producing the healthy benefits. The isoflavones were first considered to be ''plant estrogens" and estrogen-like in action. But experts now believe they may also work in other ways, such as having antioxidant properties.
Among the findings of the working group:
- Soy relieved certain menopausal symptoms. Utian says the relief from hot flashes is typically moderate. According to research, soy does not work as well as hormone therapy but was better than placebo, Utian tells WebMD. "If you give estrogen a 9 out of 10 score, and placebo 4 of 10, soy would be about 6.5."
- Supplements with a higher proportion of the isoflavone known as genistein or increased S(-)-equol, which is made by intestinal bacteria from the isoflavone daidzein, seem to provide more benefits than other products.
- Soy from foods is linked with lower risks of breast and endometrial cancer in studies.
- The benefit of soy intake on bones is not yet proven. "On bone health, we really didn't find adequate evidence to recommend its use for preventing or reducing the risk of osteoporosis and osteoporotic fracture," Utian says.
- Soy's heart health benefit is still evolving in research.
- Soy appears to help women under age 65 with cognitive function, but not those over 65. Utian refers to this as a ''critical window" after which women don't seem to derive benefit.
Although evidence was lacking for many of soy's proposed benefits, Utian says that "the good news is that we didn't show it carries any significant risk."
The symposium and report were supported by unrestricted grants from Otsuka Pharmaceutical, Pharmavite LLC, and the Allmen Foundation.
Utian reports consultant or advisory board work for Bayer, Bene Therapeutics, Bionovo, Cleveland Clinic Foundation Innovations Center, Hygeia (Orcas Therapeutics), Lupin, Merck, Novogyne, Pfizer, Pharmavite, and Teva Women's Health. Other experts in the working group report consultant work for the United Soybean Board, food companies, and pharmaceutical firms.
Benefits of Soy: Practical Matters
For menopausal symptoms, the recommended starting dose is 50 mg of total isoflavones a day, the researchers write.
A trial of 12 weeks is typically long enough to see if it works.
Because they found more potential benefits from supplements with higher proportions of genistein or increased in S(-)-equol, Utian advises people to look for products with those substances.
Few American women are capable of converting the daidzen to the S(-)-equol, he says.
Getting soy from foods is encouraged, too. Soybeans, including edamame, are high in isoflavones. Soy flour, soy protein isolate, and miso soup also have high isoflavone content.
Benefits of Soy: Perspective
Overall, the review is supportive of including soy foods for health, says Connie Diekman, RD, director of university nutrition at Washington University in St. Louis. She reviewed the study but did not participate in it.
"But it is clear that more studies are needed in terms of the impact of soy at different ages, soy related to different disease states, and on how soy foods differ from supplements in terms of health," Diekman says.
Although the review does support the idea that soy may help in lowering some cancer risk, relieving hot flashes and other menopausal symptoms, and possibly improve mental abilities, more research is needed, she says.