Vegetarians have always known about the low-fat, high-protein soybean–the one bean that contains all of the amino acids essential to the healthy functioning of the human body. Recent research, though, has catapulted the humble bean and products made from it into a health spotlight that’s glaring enough to have generated its own backlash.
Last fall, soy got a boost when the U.S. Food and Drug Administration (FDA) announced that products rich in soy protein could carry labels stating that the products may help lower cholesterol and reduce the risk of heart disease. According to its website, the FDA made its decision after reviewing dozens of controlled clinical studies showing that a low-fat, low-cholesterol, soy-rich diet (containing 25 g or more of soy protein a day) could benefit the heart. Additional research indicates that soy might help prevent some cancers, protect against osteoporosis, and reduce menopausal symptoms such as hot flashes.
Recently, though, conflicting studies have muddied the waters in terms of understanding exactly how soy affects the body. Most health professionals say that the benefits of soy are indisputable but that more research is needed to pinpoint how soy works and to determine who will benefit and who should refrain from eating soy. What is also required, they say, is a common-sense approach.
“There are no miracle foods,” says Marilynn Schnepf, Ph.D., R.D., and chair of the Department of Nutritional Science and Dietetics at the University of Nebraska–Lincoln. Soy, however, is “certainly a good addition to anyone’s diet. It is a high-quality protein and contains those isoflavones that we’re just starting to understand.”
Some practitioners fear that overconsumption of soy supplements–not soy foods–could backfire in some people, particularly postmenopausal women or women with a family history of breast cancer.
Researchers believe that isoflavones act as weak, estrogen-like compounds in the human body. Naturally occurring estrogen–whether it’s made in your body or harvested from animals–can promote breast cancer. But scientists speculate that soy isoflavones, particularly two of them, genistein and daidzein, may offer estrogen’s benefits of reduced menopausal symptoms and bone protection without the cancer risks that estrogen carries.
Researchers also suspect that isoflavones are the key to soy’s potential ability to prevent other hormone-related cancers, including prostate cancer in men and possibly ovarian cancer in women.
Soy’s potential problems
However, some practitioners fear that overconsumption of soy supplements–not soy foods–could backfire in some people, particularly postmenopausal women or women with a family history of breast cancer. (The studies responsible for raising this concern involved products with concentrated soy isoflavones, including soy powder, a soy beverage, and isolated genistein–not soy foods such as tofu or tempeh.) Experts fear that such oversupplementation could actually stimulate the growth of cancer cells.
“The potential for benefits is there,” Schnepf says. “We just need a lot more research, and that’s where people are getting confused.”
That research is on the way. The National Cancer Institute is currently exploring more than forty foods and supplements, including soy isoflavones, that might help prevent breast, prostate, colon, and lung cancer. “Because of their safety and the fact that they are not perceived as ‘medicine,’ food-derived products are highly interesting for development as chemopreventive agents . . .” researchers reported in the April Journal of Nutrition. (Other food com-pounds being explored in- clude green and black tea polyphenols, curcumin, lyco-pene, perillyl alcohol, vita- min D, vitamin E, selenium, and calcium.)
So what is known for sure about what soy can do for you? Below is a summary of the most recent research.
Heart health and cholesterol
Coronary artery disease is the leading cause of death for American women, and its incidence increases dramatically after menopause. But researchers at Stanford
University Medical Center report that women who eat plant-produced estrogen-like compounds, known collectively as phytosterols, part- icularly the isoflavones in soy products, have less cardiovascular disease (as well as less breast and uterine cancer and fewer menopausal symptoms) than those who eat a traditional meat-based Western diet.
Researchers at St. Michael’s Medical Center in Toronto recently attempted to clarify just how phytoestrogens may prevent heart disease and other illnesses. The researchers were aware of concerns that phytoestrogens may work by disrupting the endocrine system. So they studied thirty-one people with high levels of LDL, the so-called “bad” cholesterol, during two one-month periods. The test group ate soy protein foods (33 g of soy protein per day, providing 86 mg of isoflavones). The researchers decided that soy may reduce cardiovascular risk without increasing the risk for hormone-dependent cancers.
Researchers at the University of Illinois at Urbana-Champaign Division of Nutritional Sciences also recently found that eating as little as 20 g of soy per day can reduce levels of LDL. In their randomized, controlled study, eighty-one men with moderately high cholesterol levels ate 50 g of protein per day, including isolated soy protein (ISP). The men were divided into five groups, including one control group and four groups that received different amounts of ISP. The researchers found that eating 20 g of soy protein per day instead of animal protein for six weeks can reduce concentrations of LDL cholesterol.
Finally, researchers at the University of Minnesota reported in June that their three-month study of thirteen healthy, premenopausal women showed that including isoflavones in the women’s daily diets significantly improved cholesterol levels. The women lowered their LDL cholesterol readings by 7.6 percent to 10 percent; improved their ratio of total cholesterol to HDL, or “good” cholesterol, by 10.2 percent; and improved their ratios of LDL to HDL cholesterol by 13.8 percent. These effects, the researchers wrote, “could contribute to a lower risk of developing coronary heart disease in healthy people who consume soy over many years.”
Postmenopausal women in the United States have a high incidence of osteoporosis; as many as half will suffer a bone fracture due to the disorder. Although the loss of estrogen that happens during meno-pause causes other symptoms, too, doctors are always in search of alternatives to estrogen replacement therapy (ERT), because it often carries risks and side effects itself.
But a survey of animal studies, conducted by researchers at Loma Linda University, showed that the isoflavones in soy may retard bone loss almost as effectively as supplemental estrogen, at least in animals. The researchers proposed that substituting soy protein for animal protein may also indirectly enhance bone strength in humans because soy decreases calcium excretion whereas animal protein increases it.
Medical researchers are also excited about ipriflavone, an isoflavone that is synthesized from daidzein in the lab. Ipriflavone holds great promise in the prevention and treatment of osteoporosis and some other bone diseases. More than 150 studies of ipriflavone’s safety and effectiveness have been conducted in Italy, Hungary, and Japan, according to the June issue of Alternative Medicine Review.
The University of Cincinnati College of Medicine reviewed studies of ipriflavone and isoflavones conducted up to 1999. It concluded that although isoflavone studies are limited, the compounds do reduce bone loss caused by estrogen deficiency. The college’s report also concluded that the results of the many studies of ipriflavone showed that the compound was a safe and effective alternative to estrogen therapy.
The evidence of soy’s bene- fits for menopausal symptoms is mixed. Some studies show that soy has no effect while others show that the weak estrogen-like activity of iso-flavones offers potential as an alternative to hormone replacement therapy. Taking hormone replacements holds risks, including an increased risk of breast cancer. Experts suspect that isoflavones could cut those risks.
But there have been recent concerns about women, especially postmenopausal women and those with a family history of breast cancer, overconsuming soy. Animal studies at the University of Illinois at Urbana-Champaign showed that genistein suppressed breast cancer if it was taken before exposure to a cancer-causing agent; but in some of the studies, genistein stimulated tumor growth if it was given after cancer had begun to develop. Doctors fear that large amounts of phytoestrogens such as soy will behave just like naturally occurring estrogens and encourage cancer growth. Most doctors are waiting for additional research, however, before advocating or nixing soy as a menopausal antidote.
“You have to have a balanced diet,” says Jane Beran, owner of Jane’s Benson Health Market and Deli in Omaha, Nebraska. Beran, who has been in the health-food business for twenty-five years, takes soy to offset her menopause symptoms. The products “reduce my hot flashes and I’m sleeping better, and other women come in and tell me the same,” she says.
But soy should be used like any other food, she adds. “Soy is good, but you have to eat fruits, grains, and vegetables, too. Nothing works alone. Put together, they work well.”
Recent studies show that soy isoflavones, particularly genistein, inhibit prostate cancer cell growth. Most of this research focuses on one type of prostate cancer cell, the androgen-dependent kind. But researchers at Wayne State University recently explored genistein’s ability to inhibit androgen-independent cancer cells. They reported earlier this year that genistein effectively inhibits the growth of this second type of prostate cancer cell as well.
Breast cancer strikes American women more than any other type of cancer; one in every eight to nine women will have breast cancer in her lifetime. Researchers believe that 10 to 15 percent of breast cancer cases have a genetic cause, but that environment–including diet–contributes significantly.
Asian women have lower rates of breast cancer than American women, and researchers believe that the traditional soy-rich Asian diet may be the reason. This belief suggests that including soy as part of a healthy diet, beginning at an early age, may contribute to preventing breast cancer, according to researchers at the University of Alabama.
In a report published in the June issue of the American Journal of Clinical Nutrition, the Alabama researchers wrote that genistein fed to young animals promoted cell differentiation that, by adulthood, suppressed the development of mammary cancer. Other studies, however, suggest that lifestyle differences may account for differences in breast cancer risk as much or more than how much soy a woman eats. So at this point, it’s still speculative to say there’s a link.
The University of Nebraska’s Schnepf says that consumers should consider studies as pieces of a larger whole. Scientists are waiting for many more studies to be completed before they make judgments about how soy works, she says.
“So far, the research is definitely good in terms of soy’s benefits,” says Schnepf, who shared that she enjoys smoothies sprinkled with soy protein.
Overconsumption of any food is never wise, she added. “Eat in moderation, eat a variety of foods, and whole foods are always best. That’s the bottom-line message.”
What’s really in your soy?
Soy milk and other soy “dairy” products
Soy protein isolate powder
Dry whole soybeans
Textured soy protein (TSP)
Isoflavone and ipriflavone capsules
Jan Knight is the former editor of Herbs for Health and a doctoral student focusing on environmental journalism at Ohio University.
The reference list for this article is extensive. If you would like a copy, please send a self-addressed, stamped envelope to “Soy meets world,” Herbs for Health, 428 N. Cleveland Ave., Loveland, Colorado 80537-5655, or e-mail us at HerbsforHealth@HCPress.com.