A Fresh Look at the Science of Soy
By Sharon Palmer, RD
Today’s Dietitian
Vol. 8 No. 4 P. 24

Research continues to yield mixed results on the health benefits of soy. Stay tuned to the debate with this comprehensive update.

The science on the humble soybean seems to be as ubiquitous as the legume itself. One of the most widely researched foods on the planet, soy has been the inspiration for thousands of studies. Soy has even been grown in space during a DuPont research mission to help answer the dilemma of sustaining a longer-term human presence in space. Part of soy’s allure for researchers is that it provides quality protein compatible to that of animal foods; thus, it has been touted as the ideal food to feed the world’s hungry. A plot of land can produce 10 times the amount of protein relying on soy rather than cattle.

One cup of soybeans provides 57.2% of the Recommended Daily Value for protein for less than 300 calories and only 2.2 grams of saturated fat. In addition, soy contains essential fatty acids, numerous vitamins and minerals, and fiber. Phytochemicals such as isoflavones, phytate, saponins, phytosterols, and protease inhibitors found in soy have been of particular interest to researchers. Isoflavones possess antifungal, antimicrobial, and antioxidant properties that help the survival of the soybean.

“Soy is one of the few plant foods that is high in protein and has an excellent amino acid balance. In addition, it has a long history of use in Asia and is related to lower risk of disease,” says John Erdman, Jr, PhD, professor of food science and human nutrition at the University of Illinois.

The Confusing Science of Soy
When it comes to soy’s popularity in research, Thomas Badger, PhD, director and professor, Arkansas Children’s Nutrition Center and the University of Arkansas for Medical Sciences, says, “We are trying to understand why certain populations have better health and especially why there is lower risk for chronic diseases that kill Americans. Heart disease and cancer are the really big ones.”

“Everybody’s looking at soy for potential health benefits. The soy industry has had enormous impact by supporting research. The soy growers have also supported research, and they’ve been smart in realizing health benefits and helping support this research. As the results came, the federal government started being interested and also put money into research,” says Mindy Kurzer, PhD, professor of nutrition, department of food science and nutrition, University of Minnesota.

Even for those with their eyes fixed on nutrition research, it is challenging to keep up with the results of the most recent research on soy. The fact that so many soy studies have yielded inconsistent results further adds to the confusion. So you can imagine how difficult it is for the public to decipher the complex message of soy and its health benefits. Like other diet hypotheses research that didn’t yield the hoped-for results (think beta-carotene), soy is not alone when it comes to its share of controversial research results.

“I think there is confusion regarding many proposed diet-health relationships, so in most ways soy is not unique. With the exception of those professionals who focus entirely on soy, it is really not possible to fully understand the scientific literature relevant to health benefits. Generally, I think both consumers and health professionals understand that soy foods provide high-quality protein but are low in saturated fat and cholesterol. But not unexpectedly, most professionals are not able to articulate the evidence in support of these proposed benefits,” says Mark Messina, PhD, a consultant and president of Nutrition Matters, Inc.

Messina believes there has been so much research conducted on soy that it shouldn’t be surprising to find inconsistent results. “I really think that there is too much coverage of nutrition. Consumers don’t need to know about in vitro and animal studies or even small epidemiologic studies. Unfortunately, the press seemingly reports on every study, regardless of its significance, so consumers get confused when they hear about one study saying one thing and the next day another saying something different,” says Messina, who adds that there are inconsistencies in all research areas involving drugs or foods. Since many human studies have very small sample sizes, it increases the likelihood of being inconsistent in the literature.

On October 30, 2005, researchers met in Chicago to present their findings at the Sixth International Symposium on the Role of Soy in Preventing and Treating Chronic Disease. The symposium, which tackled the latest developments in soy research, was cochaired by Messina and Badger. Although soy has been investigated regarding its relationship with a number of health concerns, some highlights of the symposium included soy’s impact on cancer, cardiovascular disease, menopausal symptoms, cognitive function, weight control, and osteoporosis.1

Heart-Healthy Soy
If consumers understand one message about soy’s health benefits, it is most likely its relationship to heart disease. “Because of the health claim, there is no doubt that many are aware of the cholesterol-lowering properties of soy protein, although the reality is that the hypocholesterolemic effects of soy protein, while relevant, are quite modest,” says Messina.

The FDA approved a health claim for the cholesterol-lowering properties of soy protein in 1999. The ADA released a tip on its Web site in May 2004 recommending 25 grams of soy protein per day to help reduce the risk of heart disease. But in recent years, studies have presented results of less impressive reductions in serum lipoproteins related to soy intake. According to Erdman, soy protein lowers low-density lipoprotein (LDL) cholesterol by approximately 3% to 4% in clinical trials, which lowers the risk of coronary disease by roughly 6% to 8%.

The American Heart Association released a scientific statement on January 17 saying that after analyzing 22 studies, an association committee found that large amounts of soy protein in the diet reduced LDL cholesterol only 3% and had no effect on high-density lipoprotein cholesterol, causing them to reevaluate the statement released in 2000 that recommended the addition of soy protein to a diet low in saturated fat and cholesterol. The statement’s authors added that soy protein products replacing high-fat protein in the diet could be beneficial.2

The results of an updated meta-analysis looking at the effects of soy on serum lipoproteins was presented at the Soy Symposium by James Anderson, MD, of the University of Kentucky. The meta-analysis reviewed 55 randomized controlled studies conducted since 1995 in which soy foods resulted in changes in serum lipoproteins. Observations from the study were that the reduction of cholesterol was fairly constant across a wide range of cholesterol levels, the potency of soy protein is greater when divided into multiple doses than when consumed all at once, fewer than 25 grams per day of soy protein is efficacious, and isolated soy protein (a heated product) results in modest reduction in serum cholesterol compared with soy protein consumed in the form of a liquid or yogurt, which has a higher reduction in serum cholesterol.1

Kurzer, who presented at the Soy Symposium last fall, reports that although the data are strong for soy’s role in lowering cholesterol, the benefits are small and not in the same league as pharmaceuticals.

The Soy-Cancer Connection
Probably the most baffling territory for soy research is in the field of cancer. According to Messina, consumers have good reason to be confused: Reports have indicated that breast cancer risk may be reduced with soy along with reports that soy may be contraindicated for breast cancer patients. A large amount of research has been dedicated to soy’s role in breast cancer in the past 15 years, yet no clear-cut answers to this issue have been revealed.

Badger says, “If you look at people who consume soy over long periods of time—generational exposure—there’s a lower cancer burden. There are good published results that soy reduced the incidence and severity of certain cancers. The biggest controversy comes with isoflavones that are capable of binding to estrogen receptors and can be estrogenic.”

In a 2003 review of 18 studies of phytoestrogens and breast cancer risk, researchers from the University Medical Center Utrecht in The Netherlands concluded that the results do not show protective effects, with the exception of women who consume phytoestrogens at adolescence or at very high doses.3 The American Institute for Cancer Research states that soy is associated with the inhibition of breast cancer cells in some laboratory experiments and that the consumption of soy early in life may help protect against breast cancer later in life.4 But the American Cancer Society suggests that those at risk for breast cancer should not take isoflavonoids, even though soy may have health benefits.5

“At this point, a consensus is building that when soy is consumed early in life, during childhood when the breast is developing, it probably exerts a benefit. Soy activates a change in the structure of the breast that makes it less vulnerable to cancer,” says Kurzer, who adds that later in life, the results on soy and breast cancer are more controversial. “Soy is probably breast cancer preventative when consumed during childhood but likely has very little effect later in life.”

A breast cancer workshop was conducted in conjunction with the Soy Symposium last fall. “The workshop focused on whether soy foods or isoflavone supplements are contraindicated for women with estrogen-receptor positive breast cancer or women at high risk of breast cancer,” says Messina. “Concerns are based primarily on the estrogenlike effects of isoflavones and the results from one rodent model demonstrating that in ovariectomized mice, isoflavones enhance the growth of established estrogen-sensitive mammary tumors. However, in this model, less processed soy foods such as soy flour do not stimulate tumor growth, whereas supplements or isolated soy protein do.”

William Helferich, PhD, and his colleagues from the University of Illinois presented data showing that in immunocompromised mice with their ovaries surgically removed to abolish estrogen production, certain soy products and isoflavones stimulate the growth of existing estrogen-sensitive mammary tumors.

“There is little human evidence to suggest any type of soy products are potentially harmful. Evidence includes the lack of effect of isoflavones or soy foods on breast tissue density, the lack of effect of isoflavones on breast cell proliferation in the contralateral breast of postmenopausal breast cancer patients, and the lack of effect of soy foods on the survival of Chinese breast cancer patients noted in a five-year epidemiologic study. However, clearly none of the human data are definitive, so the controversy remains,” says Messina.

When it comes to cancer, soy research crosses over into other areas as well. Kurzer reports that the data are suggestive that soy probably exerts beneficial effects for prostate cancer and that soy may limit metastasis in cancer. Erdman notes that epidemiological findings support soy’s role in the reduction of prostate and colon cancer, as well as breast cancer, but he points out that there are no human trials.

Soy and Bone Health
The link with soy and bone health is still murky. Kurzer reports, “There are currently two- to three-year ongoing studies with bone density as an end point. In the next three to four years, we will have an answer on bone health and soy, but it looks like there will probably be a benefit.”

Researchers from the University of Messina in Italy presented at the Soy Symposium their first-year interim evaluation of a two-year double-blind, placebo-controlled study. In the study, 389 postmenopausal women were randomly assigned the isoflavone genistein. In comparison with the placebo group, there were significant increases in bone mineral density in the femur and lumbar spine.1

Badger says, “By and large, the studies show benefits for soy in osteoporosis. Soy is not likely to reverse menopausal bone loss, but it may help prevent it by increasing bone density in premenopausal women.”

Is Soy Brain Food?
Another important area that has been investigated is soy’s role in cognitive function, but little research on potential benefits has been conducted. Scientists from the University of Wisconsin presented findings from a randomized, placebo-controlled, double-blind clinical pilot study on the effects of soy isoflavones on cognition in older men and women who are postmenopausal. Although preliminary, the data suggested that isoflavone treatment was associated with improvements in language and executive function when compared with the placebo.

Badger, who has been studying soy formula feeding in children, adds, “As for brain function and development, so far studies indicate children fed soy have normal brain function. Soy-fed kids are closer to breast-fed kids than are milk-fed kids.”

Soy and the Battle of the Scale
With obesity weighing heavily on health professionals’ minds, the research on soy and weight management has been of interest. In a randomized, controlled trial comparing the effects of soy meal replacements with casein meal replacements on body weight for women who are obese participating in an intensive lifestyle intervention, researchers from the University of Kentucky found that subjects consuming soy lost more visceral adipose tissue and had greater reductions in LDL cholesterol than with casein, although the differences were not statistically significant.1

Erdman, who presented his findings on weight loss in a two-day meeting on soy and obesity in May 2005, says, “While there is no evidence that soy protein will enhance weight loss or management of weight loss, it is well-rounded, is cholesterol free, and soy foods have an excellent fatty acid profile. Soy protein would provide an advantage for cholesterol reduction and perhaps renal function.”

The Safety of Soy Formula
Of particular public concern is the safety of soy infant formulas. According to Messina, recent advisories from the United Kingdom, France, and Israel have cautioned people to limit or avoid soy intake during the first three years of life, even though more than 20 million infants have used soy formula since the 1960s. Concerns arise from hormone-related changes observed in some animal studies related to isoflavones and the high isoflavone intake in infants consuming soy formula based on body weight.

“This is an area that is being hyped with lots of things written about it without scientific basis. We have the world’s largest study going on looking at soy formula feeding,” says Badger of the study currently underway at the Arkansas Children’s Nutrition Center at the University of Arkansas for Medical Sciences. “We are starting to release information as we get it. Healthy children on soy formula grow like healthy children should. One interesting thing is that children on soy formula for six months grow normally, but with lower body fat.”

Does Soy Spell Menopausal Relief?
A question posed to many health professionals these days relates to soy and the relief of menopausal symptoms such as hot flashes. Although soy isoflavones were thought to contribute to the low rate of hot flashes in Japanese women in 1992, recent trials have shown inconsistent results.

In a double-blind, randomized, placebo-controlled crossover trial in British women who are postmenopausal using soy isoflavone supplements presented at the Soy Symposium, specific reductions were observed in the feeling of being stirred up and the severity of early-morning awakenings and sleeplessness. Daily counts of hot flashes were also significantly reduced with soy isoflavone intervention. There was no clear relationship with soy isoflavones and other symptoms.1

“The results have been very inconsistent. One cannot conclude that soy isoflavones reduce hot flashes,” says Erdman. Messina reports that roughly 60% of the trials show modest reductions in hot flashes while 40% do not. He adds, “Soy may work primarily in women with frequent hot flashes and it may be that a certain level of genistein, the main isoflavone in soybeans, is required for efficacy.”

The Soy Answer
When rummaging through the enormous vault of soy research, how does a health professional find the most appropriate advice? Messina claims, “Other than soy protein modestly lowering cholesterol, most of the other proposed benefits, especially related to chronic disease, are still speculative.”

Messina notes that U.S. government-funded clinical trials examining the skeletal benefits of soy are using an amount of isoflavones found in approximately three servings of soy. The hot flash studies have used the equivalent of roughly two to three servings of soy per day. And for cholesterol reduction, the FDA has set 25 grams of soy protein per day. “I recommend two to three servings per day. In terms of protein and isoflavones, that is about 15 to 20 grams and 50 to 75 milligrams, respectively,” says Messina.

Replacing animal-based foods to lower cholesterol with the aid of soy seems to be prudent advice, according to Erdman. “I would never say that soy is bad for women,” says Kurzer, who adds that she wouldn’t necessarily recommend soy for a woman at risk for breast cancer, either. Kurzer believes many health recommendations are pointing in the same direction—toward a plant-based diet with less saturated fats and more fruits and vegetables. “Part of that could be more soy. I wouldn’t make blanket recommendations that people should consume soy. It is a personal choice.” Kurzer notes that one or two servings per day of soy approximates the amount of soy Asians typically consume.

Badger adds, “Soy should be eaten in context of an overall healthy diet. It should be treated like any other really good food: Eat it in balance with a wide variety of food sources.”

— Sharon Palmer, RD, is a contributing editor at Today’s Dietitian and a freelance food and nutrition writer in southern California.

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Soy Resource Guide
American Dietetic Association
www.eatright.org

American Soybean Association
www.soygrowers.com

Illinois Center for Soy Foods
www.soyfoodsillinois.uiuc.edu

The Soy Connection
www.talksoy.com/SoyConnection

The Soy Daily
www.thesoydailyclub.com

Soyfoods Association of North America
www.soyfoods.org

The Soyfoods Council
www.thesoyfoodscouncil.com

Talksoy.com
www.talksoy.com

The United Soybean Board
www.unitedsoybean.org

U.S. Soyfoods Directory
www.soyfoods.com

References
1. Sixth International Symposium on the Role of Soy in Preventing and Treating Chronic Disease. October 30, 2005. Available at: http://www.aocs.org/archives/soy

2. Soy Protein Shows Little Effect on “Bad” Cholesterol. American Heart Association scientific statement. January 17, 2006. Available at: http://www.americanheart.org/presenter.jhtml?identifier=3037031

3. Peeters PH, Keinan-Boker L, van der Schouw YT, et al. Phytoestrogens and breast cancer risk. Review of the epidemiological evidence. Breast Cancer Res Treat. 2003;77(2):171-183.

4. Foods that Fight Cancer - Soy. American Institute for Cancer Research.

5. Soy and Breast Cancer. American Cancer Society. July 15, 1999.