Soyfoods & Cancer
August 27, 2015
After years of conflicting information, many people remain confused about the alleged association between soyfoods and cancer risk. Here’s an update on the latest research that supports their safety and protective effects.
The link between soy and cancer risk has been a controversial topic for some time and has confused many people. Not only is the general public often baffled by whether soy is safe to eat, but many dietitians report being uncertain of what to tell their clients. Finding an answer isn’t always easy with the tremendous amount of misinformation floating around—not to mention the fact the research has appeared contradictory over time.
To get to the bottom of the soy and cancer debate, Today’s Dietitian turned to experts from cancer societies as well as dietitians who have been keeping up with the latest research to help clarify some key issues and give you the best information and message to convey to clients and patients.
A Look at the Research
The confusion about soy stems from the fact that studies on its relationship to cancer have been conflicting, says Sharon Palmer, RD, author of The Plant-Powered Diet. “Some studies found protective benefits against cancer, but others showed concern for increased risk.” However, a closer look at the previous research as well as the most current findings reveals plenty, she adds.
While much of the attention on soy these days is focused on whether it has a role in causing cancer, soy was first studied for its role in fighting the disease. “What started much of the interest in soy was the observation of lower cancer incidence—overall cancer and some specific types such as breast cancer—in Asia compared to the United States,” says Karen Collins, MS, RD, CDN, a nutrition advisor with the American Institute for Cancer Research. “One obvious difference between Asian and US diets is the use of soy. And as we discovered that isoflavones in soy are phytoestrogens, the thought was that they might bind to estrogen to decrease the development of cancers, such as many breast cancers, which are promoted by estrogen.”
However, Collins explains that some of the earlier soy studies suggested that genistein, a primary isoflavone in soy, increased growth of estrogen receptor-positive (ER+) breast cancer cells and promoted breast cancer growth. But as research advanced, scientists found that rats and mice metabolize phytoestrogens such as genistein differently from humans.1
“Rodent metabolism leads to much higher levels of the active form of isoflavones compared to humans,” says Mark Messina, PhD, a nationally recognized expert on soy’s health effects. “Therefore, many researchers now say that the effects of breast cancer growth seen in rodent studies may be linked to their far higher blood levels of isoflavones in active form.”
Messina’s 2011 study in The American Journal of Clinical Nutrition looked at the fact that mice metabolize isoflavones differently than humans. He says that even in the rodent model, not all studies show that genistein stimulates tumor growth.
Human studies have advanced too, Collins says. “Comparing high and low quartiles of soy consumption to see effects means comparing very different levels of soy consumption when looking at Asian populations vs. US populations. Population studies link soy consumption with lower breast cancer risk in Asia, where women consume moderate amounts of soy throughout their life. A moderate amount is approximately one to two servings a day. In the United States, those who eat more soy—especially in studies conducted recently—aren’t necessarily eating much and usually reflect soy consumption later in life.”
In general, studies in Asian women have found a lower risk of breast cancer with greater soyfood consumption, but Marji McCullough, ScD, RD, strategic director of nutritional epidemiology at the American Cancer Society, agrees that part of the challenge in studying this topic in the United States is that women eat much less soy than women in Asia.
“For example, women who ate and drank the most soy in studies in China consumed approximately two or more servings per day, and those who drank the least consumed less than a serving,” she says. “But in the United States, the highest category of consumption was 1 to 2 mg/day, less than half a serving per day, and the lowest category was ‘none.’ In other words, women in the highest categories in the United States would still fall into the lowest categories in China.”
Soy and Cancer Survivors
Collins says the biggest concern expressed by both the public and health professionals has involved breast cancer survivors, especially those who had the ER+ form. “But we now have five population studies and one pooled analysis of several studies involving breast cancer survivors that consistently show moderate amounts of soyfood consumption doesn’t increase a woman’s risk of death or recurrence, including women with ER+ breast cancer,” she says. “Some of the studies, in fact, show improved outcomes with regular soyfood consumption.”
Collins refers to the largest study to date—a pooled analysis of studies that included almost 10,000 breast cancer patients—which showed that consuming at least 10 mg of isoflavones daily was linked to a 25% decrease in breast cancer recurrence.2 “This effect was seen among both women from the United States and Asia,” she says. “Some people had feared that soy’s isoflavones might interfere with a hormone-related treatment, such as tamoxifen, but no evidence of that was seen. In fact, overall, these population studies don’t show any harmful interactions between soyfoods and antiestrogen medications. A small number of studies even suggest soyfoods may be protective for women who take tamoxifen, but more research is needed.”
McCullough adds that the American Cancer Society’s 2012 Nutrition and Physical Activity Guidelines for Cancer Survivors concluded that current research finds no harmful effects from breast cancer survivors eating soy.
Whole vs. Processed
Some of the confusion surrounding soy comes from animal research suggesting it’s safer to eat it in whole foods instead of processed forms. Messina says research indicates the greater the amount of processing, the greater the tumor growth in rodents. “This observation is primarily responsible for healthcare organizations concluding that soyfoods are OK, but supplements may not be,” he says.
Still, while Messina has many reasons for recommending whole soyfoods over supplements—in the same way he’d recommend apples over apple juice—he comes back to the fact that the effects of processing on tumor growth in rodents isn’t applicable to humans. Of course, it doesn’t hurt to remind clients that processed soy is still processed food, and whole food is a healthier choice.
“It’s really important that we choose soy in its most unprocessed form and that we urge our clients to do the same,” adds Janel Funk, MS, RD, LDN, a Boston-based nutrition consultant. “These days that’s not always easy to do. You go to the grocery store and there’s a soy aisle with soy nuggets and burgers. I educate my clients that a soy nugget is no different than a processed chicken nugget—it just doesn’t contain chicken. Minimally processed choices such as tofu, edamame, and tempeh are what we should recommend.”
Palmer agrees that dietitians should recommend minimally processed soyfoods to clients. She sees the RD as having a critical role in educating the public on this topic. “RDs can provide factual information to cut through the hype,” Palmer says. “After all, soy is a very healthful food that has major health benefits. It’s been linked with benefits such as heart health and may even reduce hot flashes in women. It’s a nearly perfect food—packed with vitamins, minerals, high-quality protein, fiber, and phytochemicals. You can feed a lot more people with less, and that’s important too. We need to start moving to a more plant-based diet, and soy can help with this goal.”
Clearing Up Lingering Confusion
While recent research establishes soy’s safety, many previous studies still contribute to the remaining misconceptions. Messina says it’s understandable that even RDs might be confused. After all, the soy controversy has gone on for more than a decade. “But only within the past couple of years have the data become strong enough to allay concerns,” he says. “RDs also have a lot on their plate and have more to stay abreast of than just the voluminous soy literature.”
Both Collins and McCullough add that they get numerous questions about soy’s safety and witness much lingering doubt. Collins says the fact that research has evolved over time means that many health professionals may be giving different answers to questions about soy’s safety. “When people ask their doctor, nurses, or even RDs, depending on what research those health professionals last read, people may get very different answers,” she explains.
Although some people simply have no interest in trying soy, others are afraid to do so, and that’s an area where dietitians can help. Clients who want to reduce their consumption of red and processed meat to reduce colon cancer risk may be good candidates for using soyfoods as a replacement, Collins suggests. Also, people who are lactose intolerant or have milk allergies may want to try soymilk. “However, because of the things they’ve heard about soy potentially increasing [cancer] risk, they’re afraid to do so,” Collins adds. “Many foods include soy ingredients, and some people become very worried when they see this as well. Breast cancer survivors especially tend to be very worried about this.”
The good news is that education on the most recent research can allay some of those fears. Therefore, dietitians have an important responsibility to keep up with the latest research, Palmer says.
McCullough says she’d like to see dietitians become familiar with the American Cancer Society guidelines on nutrition and physical activity for cancer prevention and the guidelines for cancer survivors as well as global guidelines from the World Cancer Research Fund/American Institute for Cancer Research.
The bottom line is that dietitians play an integral role in clearing up some of the misconceptions surrounding soy and need to provide their clients with simple take-home messages that make their soy consumption decision easier. McCullough has three key messages she’d like to see dietitians share with clients:
• Soy is a healthful alternative to meat.
• Fermented soy products are much higher in sodium (eg, miso).
• Soyfood consumption is safe for women and breast cancer survivors, and some studies have shown a lower risk of breast cancer and recurrence.
Messina’s overall message to RDs about soy is to stay focused on the human research, both clinical and epidemiologic. The most recent human research should dissolve previously held fears, and when focusing on these findings, “it’s hard not to conclude that soyfoods warrant a larger role in the US diet than they currently have.”
— Lindsey Getz is a freelance writer based in Royersford, Pennsylvania.
Your clients may be interested in adding more soy to their diet but don’t know how. Karen Collins, MS, RD, CDN, a nutrition advisor for the American Institute for Cancer Research, offers the following suggestions for incorporating soy into the diet:
• Make tofu: Tofu can be stir-fried, grilled, added to stews and soups, and used in mixed dishes such as lasagna.
• Prepare tempeh: Tempeh is a great addition to chili and pasta sauce.
• Drink soymilk: Soymilk can be used in smoothies and on cereal.
• Eat edamame: Edamame can be used in soup, stir-fries, and salads or eaten as a snack.
• Munch on soybeans: Roasted soybeans (aka soynuts) can be eaten as a snack or added to salads.
1. Onoda A, Ueno T, Uchiyama S, et al. Effects of S-equol and natural S-equol supplement (SE5-OH) on the growth of MCF-7 in vitro and as tumors implanted into ovariectomized athymic mice. Food Chem Toxicol. 2011;49:2279-2284.
2. Nechuta SJ, Caan BJ, Chen WY, et. al. Soy food intake after diagnosis of breast cancer and survival: an in-depth analysis of combined evidence from cohort studies of US and Chinese women. Am J Clin Nutr. 2012; 96(1): 123-132.
Diet and Nutrients , Women's Health, Diet and Disease