Soy Health: Tofu and Isoflavones

From hippie icon to health-enhancing,food-industry phenomenon.

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Eat some edamame - a 1/2 cup of raw soybeans has 34 grams of soy protein and 176 mg of isoflavones.

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Remember Johnny Carson? Back in the 1970s and ‘80s, when “He-e-e-e-re’s Johnny” reigned as king of late-night television, he often poked fun at health foods, in particular, tofu. Such comments resonated with meat-and-potatoes Americans, who loved to hate the bland, spongy, soy blocks.

Carson’s successor, Jay Leno, doesn’t put down tofu. During the 1990s, the nation’s relationship to soy foods changed dramatically. Tofu hasn’t exactly replaced burgers and pizza as the nation’s favorite foods, but the jokes have largely ceased. Tofu and other soy foods have become as mainstream as McDonald’s—only they’re lower in fat, much more nutritious, and appeal to a growing number of Americans committed to eating a healthier, more plant-based diet.

Today, more than 300 companies produce soy foods, according to the U.S. SoyFoods Directory. In fact, so many soy products now line the shelves and refrigerated cases of health-food stores and supermarkets that it probably won’t be long before someone opens a store that stocks only soy foods.

The bean grown round the world

Soybeans (Glycine max) were first cultivated in China as early as 2800 b.c., serving as a key source of protein for a culture that was largely vegetarian. The Chinese developed tofu around 200 b.c.

The ancient Japanese adopted soybeans and tofu. During the eighteenth century, soybeans were introduced into Europe, but they didn’t arrive in the United States until Chinese immigrants introduced the bean in the 1880s. Soybeans quickly became a major U.S. crop, raised almost entirely as cattle feed and for export to Asia. Today, the United States grows 49 percent of the world’s soybean crop.

Until the late 1960s, tofu was a curiosity in the United States, served only at Asian restaurants and in Asian American homes. Then the counterculture generation adopted it, and tofu found its way into health-food stores. More than thirty years later, soy foods are as American as Kellogg’s, the cereal company whose Worthington Foods subsidiary sells Morningstar Farms soy franks, soy breakfast links, and soy-based Harvest Burgers.

The soy-health connection

The health benefits of soy come, in part, from a group of compounds known as isoflavones. The two most prominent isoflavones are genistein and daidzein. All soy foods contain varying amounts of isoflavones. Here’s how the various types of soy foods stack up for isoflavones:

Raw soybeans (½ cup): 34 g soy protein, 176 mg isoflavones
Roasted soybeans/soy nuts (½ cup): 30 g, 167 mg
Tempeh (4 ounces): 19 g, 61 mg
Soy protein (1 ounce): 26 g, 57 mg
Soy flour (¼ cup): 8 g, 44 mg
Tofu (4 ounces): 18 g, 38 mg
Textured soy protein (¼ cup): 18 g, 28 mg
Soy milk (8 ounces): 10 g, 20 mg

Soy isoflavones are plant estrogens (phytoestrogens). Similar in chemical structure to human estrogen, they bind to estrogen receptors around the body. However, soy phytoestrogens are much weaker than human estrogen, which is why, scientists believe, soy foods may help prevent breast cancer.

Soy isoflavones have a remarkable number of health benefits. The following are some of the best known.

Cholesterol control. Compared with Americans, Asians have a lower rate of heart disease. Asians get more exercise, eat a lower-fat diet, and live in more cohesive communities, all of which help prevent heart disease. They also eat more soy foods. Estrogen plays a role in the risk of heart disease. Compared with U.S. men, U.S. women have a much lower rate of heart disease—until menopause, when their estrogen production declines. These observations led scientists to investigate soy phytoestrogens for prevention of heart disease. They discovered that soy foods reduce cholesterol, a key risk factor for heart disease.

Many studies show that soy helps lower cholesterol. At Wake Forest University in Winston-Salem, North Carolina, researchers placed 156 men and women with high cholesterol on the American Heart Association’s low-fat diet. In addition, they were given one of five different supplemental drinks: a placebo or four beverages containing differing amounts of soy isoflavones. After nine weeks, the placebo group showed no decrease in cholesterol level, but the soy groups all showed reductions, with the greatest decrease occurring in the group with the highest baseline cholesterol levels whose soy beverage contained the most isoflavones. In this group, total cholesterol declined 10 percent, which translates to a 20 percent reduction in heart-attack risk.

Researchers at the Veterans Affairs Medical Center in Lexington, Kentucky, analyzed thirty-eight studies of soy protein’s effect on cholesterol. Their conclusion: Consuming 47 g of soy protein (1.7 ounces) per day reduces total cholesterol by 23 milligrams per deciliter of blood, a substantial reduction.

Because of all the research showing that soy protein helps cut cholesterol, in 1999 the U.S. Food and Drug Administration (FDA) ruled that foods containing at least 6.25 g of soy protein per serving may make a label claim that they reduce cholesterol and risk of heart disease. Commission E, the expert panel that judges the safety and effectiveness of herbal medicines for the German counterpart of the FDA, approves soy foods for treatment of elevated cholesterol.

Osteoporosis. Hormone replacement therapy (HRT) helps prevent osteoporosis in postmenopausal women. Many studies have shown that the phytoestrogens in soy produce similar benefits. Japanese researchers used a diet survey to estimate 478 menopausal women’s soy consumption, then measured their bone mineral density. Compared with women who ate the fewest soy foods, those who ate the most had significantly greater bone density and less osteoporosis.

In addition to their isoflavone content, some soy foods also contain appreciable amounts of bone-building calcium. The soy food with the most calcium is tofu—130 mg per half-cup serving. The National Academy of Sciences currently recommends that adults consume 1,000 to 1,200 mg of calcium a day. Tofu won’t supply all of the calcium you need, but it can help.

Menopausal complaints. About 85 percent of menopausal American women complain of hot flashes. But studies show that only about 25 percent of Asian women share this problem. Estrogen-rich HRT relieves most hot flashes. But HRT also increases the risk of breast cancer, so only a minority of American women take it. Why are Asian women largely spared hot flashes? Scientific attention has focused on whether the consumption of phytoestrogens in soy foods can relieve hot flashes similar to the effect produced by HRT.

In one study, New Jersey researchers gave 177 menopausal women either a placebo or a soy supplement containing 50 mg per day of genistein and daidzein. After twelve weeks, the soy group reported significantly less severe hot flashes and 60 percent fewer episodes of night sweats. In another study, Italian researchers gave 104 postmenopausal women either a placebo or 60 g per day of soy protein containing 76 mg per day of isoflavones. After twelve weeks, the soy group reported 15 percent fewer hot flashes—1.6 fewer per day. Although more research is needed, anecdotally, many women report a cooling effect on hot flashes from soy consumption.

Breast cancer. Many studies show that compared with Americans, Asian women have a substantially lower risk of breast cancer. Epidemiologists used to believe the reason was that Asian women eat a lower-fat diet. Now increasingly, they believe that the reason is that Asians’ lower-fat diet is also much higher in soy. Human estrogen stimulates the growth of breast tumors. Weaker soy phytoestrogens attach to estrogen receptors on breast cells, and in effect, lock out women’s own estrogen, reducing the stimulating effect it has on breast cancer. A number of studies support soy foods for breast cancer prevention.

Australian researchers conducted diet surveys involving 144 women newly diagnosed with breast cancer and 144 demographically similar women who were cancer-free. After eliminating other risk factors for breast cancer (family history, exercise, etc.), the researchers found that the cancer-free women consumed significantly more soy isoflavones than the breast cancer sufferers did.

At the University of Southern California, researchers tracked the tofu consumption of 597 Asian American women, some Asian-born, the rest American-born. The Asian-born women ate twice as much tofu—an average of sixty-two times a year versus thirty times a year for the American-born. The Asian-born women were also significantly less likely to develop breast cancer. In both groups, as tofu consumption increased, breast cancer risk decreased.

Breast tissue is most sensitive to estrogen during adolescence, as the breasts develop. Researchers at Vanderbilt University in Nashville asked 1,459 Chinese women with breast cancer and 1,556 matched controls what they ate as teens, and then, for confirmation, asked many of their mothers. After adjustment for other breast cancer risk factors, adolescent soy food intake emerged as significantly protective. The women who ate the most soy foods as teens had the lowest breast cancer risk years later as adults.

In recent years, a few studies have challenged soy foods’ ability to prevent breast cancer. Some scientists have theorized that women who eat a great deal of soy might consume enough phytoestrogens to stimulate breast-tumor growth. Studies by researchers at the University of Illinois, Champaign- Urbana, have shown that in mice, genistein (a key isoflavone in soy) spurs the growth of mammary tumors. Meanwhile, other studies show that genistein suppresses breast-tumor growth. Currently, the question is unresolved—and confusing. The epidemiological research generally agrees that soy reduces breast cancer risk. But the studies showing that genistein stimulates breast tumor growth are disturbing. Until this controversy is laid to rest, it would seem prudent for women to eat soy foods in their diets but not to take isolated genistein supplements.

Prostate cancer. Compared with American men, Asian men are at a much lower risk of prostate cancer. The male sex hormone, testosterone, accelerates the growth of prostate tumors. Estrogen interferes with the action of testosterone, so it’s plausible that the phytoestrogens in soy might explain the difference in Asian and American rates of prostate cancer. Several animal studies have shown that soy isoflavones reduce the growth of prostate tumors. Only two studies have investigated this issue in humans, but both give thumbs up to soy.

University of Hawaii researchers conducted diet surveys among 1,619 men with prostate cancer and a similar number of matched controls who were cancer-free. Intake of beans—all beans, not just soybeans—was significantly protective. Norwegian researchers analyzed data from an ongoing study of the diet, lifestyles, and health of more than 12,000 California Seventh Day Adventists, whose religion requires vegetarianism. Compared with those who drank no soy milk, Adventist men who drank at least one glass per day were 70 percent less likely to develop prostate cancer.

Summing up soy

All of the previously mentioned health benefits are intriguing possibilities. Finally, preliminary studies suggest that soy foods may help prevent uterine endometrial cancer and reduce blood levels of the toxic metal lead. Despite the unresolved questions about soy and breast cancer, overall, the research to date strongly supports soy foods as remarkably beneficial to health: fewer menopausal complaints, lower cholesterol, less risk of breast and prostate cancer, and osteoporosis. Not bad for a bean whose signature product was, not too long ago, the butt of jokes on late-night TV.