The Bottom Line on Soy and Breast Cancer Risk

By Marji McCullough, ScD, RD


EDITOR’S NOTE: Dr. McCullough added the following statement 4/8/14 in response to questions related to sources of isoflavones:

Research on soy and cancer is highly complex, controversial, and evolving.

When concerns about soy are raised, they generally focus on findings from rodent models of cancer which tend to use isolated soy compounds like soy protein isolate or high doses of isoflavones (compounds found in soy).  However, soy is metabolized differently in humans than it is in mice and rats, so findings in rodents may not apply to people. (See: for more on this.)(Setchell, AJCN, 2011).  There is no evidence in the medical literature that soy protein isolate is bad for humans, compared to other forms of soy. Soy protein isolate is often used as a supplement in randomized studies of the effects of soy on health and none of these studies have shown harm.

Most of the studies suggesting benefits of soy consumption in people have measured how much soy foods people are eating, including tofu, soybeans, and soy milk.  These foods are more commonly eaten in Asian countries. In the U.S., purified forms of soy are used in the food supply, including in energy bars and soy hot dogs.  The few US studies that have measured these forms of soy do not suggest harm.    

More research is needed to understand the relationship between specific forms of soy and doses of isoflavones on cancer risk and recurrence. We also need to learn more about childhood exposure to isoflavones and risk of cancer. Until more is known, if you enjoy eating soy foods, the evidence indicates that this is safe, and may be beneficial (but note that miso, a fermented soy product, is high in sodium.)  It is prudent to avoid high doses of isolated soy compounds found specifically in supplements, as less is known about their health effects. As for other “hidden” sources of soy proteins, the evidence to date does not suggest harm or benefit. However, if you are concerned about these products, you can choose to avoid them.  


Before writing a blog about soy and breast cancer, I took an informal poll of a few friends to get a sense of what women believe about soy.  I asked them, “What do you know about eating soy food?  Is it good for you? Not good for you?” (I didn’t even mention breast cancer.)  The responses I got were,  “I think it acts like estrogen in the body”; “Consuming any soy products increases the risk of breast cancer”; “I don’t eat it a lot because I heard something negative but I can’t remember what it was;” and “I’ve heard you should only have it in moderation.”  Apparently, people are hearing that soy may not be good. But what’s the truth? In this blog I’ll walk you through what we know and what we don’t know about soy and breast cancer, and give you some practical tips on eating soy. [more]

Soy can act like estrogen

Soy foods (such as tofu, tempeh, edamame, miso, many veggie burgers, and other products made with soy flour) contain isoflavones, which are chemically similar to estrogens. Two major types, genistein and daidzein, can act like estrogen in the body, although at a very small fraction of the potency of circulating free estrogen in women.  These effects can be good or bad.  Let me explain.    

It is well established that estrogen is linked to hormonally-sensitive cancers in women, such as breast and endometrial cancer.  Breast cells contain estrogen receptors, and when the “key” (estrogen) joins with the “lock” (the estrogen receptor), a series of signals are sent which can spur on estrogen-receptor (ER) positive breast tumor growth. Common risk factors for breast cancer include conditions that involve longer exposure of breast tissue to estrogen:


  • Having children late or not having children – because pregnancy gives the breast a break from higher estrogen levels
  • Being obese after menopause – because estrogen is formed in fat tissue and becomes the major source of estrogen after menopause unless women are taking hormone replacement therapy

In women who have been diagnosed with ER-positive breast tumors, tamoxifen and raloxifene are often used to block estrogen receptors so that estrogen can’t reach them and stimulate breast tumor growth. 


Animal, human studies show different results

Ok. Now back to soy. Most of the concern about soy has come from studies in laboratory animals.  Rats injected with ER-positive tumor cells were given varying doses of genestein or daidzen.  Those given more of the isoflavones had a greater growth of the breast tumors compared to rats given little or no isoflavones.  But not all animal studies have shown harmful effects.

Furthermore, while isoflavones may act like estrogen, they also have anti-estrogen properties. That is, they can block the more potent natural estrogens from binding to the estrogen receptor.  In addition, they stop the formation of estrogens in fat tissue and stimulate production of a protein that binds estrogen in the blood (to make it less able to bind to the receptor).  They also have anti-oxidant and anti-inflammatory properties and work in other ways to reduce cancer growth.

Epidemiological studies, in which large populations of healthy women have reported details about their usual diet and were followed for many years, have either shown no association between soy and breast cancer, or a protective association, meaning that people who ate more soy had less breast cancer

In general, studies in Asian women have found a lower risk of breast cancer with eating more soy, whereas studies in the U.S. have tended to not find any association between how much soy a woman consumes and her risk of breast cancer.  Indeed, a recent study combined data from 14 epidemiologic studies on this topic and found that in Asian countries, women who ate the most (compared to the least) soy isoflavones had a 24% lower risk of developing breast cancer, while there was no association in Western countries such as the U.S. 

Part of the challenge in studying this topic in the U.S. and other Western countries is that women eat much less soy food than in Asia, so evaluating the risk of breast cancer with high levels of soy consumption is difficult.  For example, women who ate and drank the most soy in a study in Shanghai consumed about 40 mg isoflavones/day (roughly equal to 4 servings daily) and those who ate and drank the least ate 5 mg/day (or half a serving). But in the U.S., the highest category of consumption was 1-2 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 U.S. would still fall into the lowest categories in China. 

Keep in mind also that women in Asia likely have other cultural dietary patterns that may not be assumed in the U.S., such as lifelong consumption of soy and other foods that may have influenced breast development during adolescence in ways not yet fully understood.  A study in Shanghai found that women who ate a high amount of soy protein consistently during adolescence and adulthood had a significantly reduced risk of breast cancer before menopause, but not after menopause.


Soy for cancer survivors

What about women who have had breast cancer, especially ER-positive breast cancer? A recent study looked at soy consumption in the diets of more than 9,000 breast cancer survivors who were participating in 3 studies of eating habits and other lifestyle factors after breast cancer. Two of the studies were from the U.S. and 1 was from China. Women from both the U.S. and China who consumed 10 mg/day or more of soy had a 25% lower risk of breast cancer recurrence. These protective associations were slightly stronger in women with ER-negative tumors. In women with ER-positive tumors, the associations also seemed protective (though not strongly so) for women regardless of whether they were taking tamoxifen or not.    

But to find out for sure whether we should recommend soy foods to women, researchers would need to repeat these findings, ideally through a controlled study (considered the gold standard in research). At the very least, the evidence from the studies in women reassures scientists that moderate consumption is likely to be safe. 

The 2012 American Cancer Society Guidelines on Nutrition and Physical Activity for Cancer Survivors, written by a panel of experts, including researchers with expertise in this area, concluded that current research finds no harmful effects to breast cancer survivors from eating soy.  Both the guidelines for cancer prevention and the guidelines for survivors recommend against taking soy supplements because they contain much higher isoflavone concentrations than what you would normally find in the foods you eat, haven’t been as rigorously tested, and may have other potent effects on body tissues.

When you make the decision about consuming soy, it’s also important to remember that breast cancer survivors (and the rest of the population) are also at risk for other cancers and cardiovascular disease. Tofu and other soy foods are linked to lower rates of heart disease because they are excellent sources of protein, may replace other less healthy foods in the diet (e.g. animal fats, red and processed meats), and may help lower cholesterol, and so can be a good meal choice for anyone. 


Bottom line: Even though animal studies have shown mixed effects on breast cancer with soy supplements, studies in humans have not shown harm from eating soy foods. Moderate consumption of soy foods appears safe for both breast cancer survivors and the general population, and may even lower breast cancer risk. Avoid soy supplements until more research is done. So, enjoy your occasional tofu stir-fry or tofu burger – they are unlikely to increase your risk of breast cancer and, on balance, are some of the healthier foods you can eat!

Dr. McCullough is strategic director of nutritional epidemiology for the American Cancer Society.

17 thoughts on “The Bottom Line on Soy and Breast Cancer Risk

  1. As a 2 time breast cancer survivor (er+, 2006 and er-, 2007) and a vegetarian I appreciate this article as I eat tofu and since finishing treatments in 2009 I've tried to live in moderation with less stress. However, it really stresses me out when people tell me-"You shouldn't be eating tofu! It will give you breast cancer!" I've read many different articles on it, and it's really positive to have it explained the way you did in this article! Now I can tell these people…"I can eat my tofu!" Thank you!

  2. Although I am very sympathetic to women diagnosed with breast cancer as my own mother is a breast cancer survivor. I also lost my father to lung cancer and my grandmother to vulvar cancer. I find the ACS a deplorable organization since we are still in month of August and there has been no mention of Pediatric Cancer. While October has been designated as Breast Cancer Awareness Month, September was designated as Pediatric Cancer Awareness Month and I have yet to hear or see one piece of material the would make ANYONE aware of Pediatric Cancer. The ACS designates 1 cent per dollar of their budget to pediatric cancer. While breast cancer and lung cancer receive the bulk of the budget. I guess the ACS or the public believe that a women's breasts are more important than a childs life. ABSOLUTELY UNACCEPTABLE.

  3. I have a totally opposite opinion. Soy is one of the most toxic things we can consume. Not only do women in Asian countries NOT eat more soy, they do not eat it like we do at all. Most soy they eat is fermented and it is in very small amounts. I am so sick of this misconception. I would advise any woman to greatly reduce their soy intake. All of it. Also their intake of dairy and of grains. Do some real research yourself.

  4. Soy takes on a completely different form when fermented like in miso, tempeh and natto and has many health benefits, whereas soy milk and tofu are just over-processed soy products that may affect your thyroid and general health.

    Look at soy in the Asian diet. If they eat tofu, they eat small slivers of it in their soup or stir-fry. they don't consume huge soy burgers or drink tall glasses of soy milk (they don't drink ANY milk in fact)

    It's a complex issue, because you really can't just talk about "soy" unless you define it. Unfortunately it's muddied by the dairy council's bloggers who are seeing their profits slip away to other forms of milk as the public gets smart about dairy in general, and the unhealthy aspects of the modern dairy industry.

  5. I am a vegetarian, but I really am not a fan of soy. I think there are many variances in lifestyle between Asian and Western cultures and more of the health issues most likely stem from other factors besides soy.

  6. The American diet is full of soy. Bad soy. Bad cheap fillers and preservatives derived from soy. The asian diet is full of fermented soy that has not been badly processed. Go into a cabinet read a few food labels and then try to tell people that soy intake in western civilization is low. Worst argument ever…

    Here's a list of things that are derived from soy:
    citric acid
    dough conditioner
    glycerol monostearate
    hydrolyzed plant protein (HPP)
    hydrolyzed vegetable protein (HVP)
    liquid smoke
    mono and diglycerides
    monosodium glutamate (MSG)
    mixed tocopheryls
    natural and artificial flavorings
    protein concentrate
    protein isolate
    thiamine mononitrate
    thickening agents
    vegetable broth
    vegetable fat
    vegetable fiber
    vegetable gum
    vegetable oil
    vegetable paste
    vegetable protein
    vegetable shortening
    vegetable starch
    vitamin E

  7. @Erik Bravin
    Pediatric cancer is a very sad part of our reality. And while I am not in love with ACS, I think you're statement about them, "I guess the ACS or the public believe that a women's breasts are more important than a childs life. ABSOLUTELY UNACCEPTABLE." No, no, no. It's not just a "women's breasts" at stake here. With 1 in 4 women in the U.S. getting breast cancer (and the numbers continue to rise), and breast cancer being the 2nd leading cause of cancer deaths among women, ACS BETTER be directed most of their funds to breast cancer research. Why? BECAUSE SO MANY PEOPLE IN AMERICA ARE AFFECTED FROM IT. It's MUCH more common than pediatric cancers. Not to say that pediatric cancers aren't important to, but we need to direct our resources smartly. Also, you are overlooking the fact that learning more about more common types of cancers, particularly breast cancer, can tell us more about ALL cancers, including pediatric cancers. You also are overlooking the fact that we have CURES for SOME types of leukemia, by using gene therapy. Leukemia is one of the more common childhood cancers. GET YOUR FACTS STRAIGHT.

  8. Organic Soy and Non-GMO Soy with no fillers is the kind people have eaten in the past. Since 1992 we here in the United States get Genetically Modified IOrganism Soy that has the Round UP treated seed so they can spray tons of Round Up weed killer on it and it will survive. So you are consuming Round Up in your food. 90% of Soy is now GMO. Look for Organic and Non-GMO labels when buying any Soy Products! Silk assures it uses Non GMO Soy. But, only the Organic variaties are Organic because the supply of Organic Soy is so difficult now. DEMAND NON-GMO SOY. Also NON-GMO Corn, Canola, and Organic Dairy! Education of the consumer and increased DEMAND for Non-GMO healthy food will increase supply!

    1. Why should we demand NON-GMO soy?

      All three evaluating agencies (EPA, FDA, USDA) under the Coordinated Framework give transgenic crops a thumbs up and often point out that the environmental conditions are preferred over Organic farming, which uses significantly more land to grow things and still uses herbicides, pesticides and fungicides just like all other farming.

      Glyphosate (roundup) interferes with the shikimic pathway in plants, resulting in the accumulation of shikimic acid in plant tissues and ultimately plant death. The enzyme and pathway do not exist in animals, which is why toxicity is so low. Your presumed, “eating of roundup” wouldn’t harm you even if the amount of exposure was high enough to register which it all but never is.

  9. I agree that when in MENOPAUSE, life after breast cancer should entail avoiding soy products. That was specific information from a very smart surgeon. He has been doing these surgeries for years. I think he would have the knowledge. Also, our life style, body type and make-up are not generally similar to Asian body type. The Asian body has more child-like features. Smaller breasts and bone structure, etc. And again, Asians tend to eat very little soy and soy products. Plus they mix it with many other types of healthy foods. And really check labels. Many variety of crackers are made with soy bean oil. If in doubt, throw it out!

  10. The level of ignorance about Asians and the "Asian diet" by both the researchers and the people that have commented on this blog are pretty ridiculous. There are many different countries and regions in Asia, and not all Asians eat soy products, just like not all Europeans eat the same amount or types of Cheese, etc. And yes, there are Asian communities that drink soy milk, a lot of it. Cow's milk, goat's milk and horse's milk are also commonly consumed in various parts of Asia. Not all Asians eat fermented tofu, and not all soy is processed into tofu for consumption in Asia. All of these facts are of course peripheral to the actual point here, which is that the idea that soy-based foods have a direct causal link to breast cancer risk is weak. It's nice that your mom/surgeon/best friend/smartest doctor in the world may have anecdotal stories to share about cancer or an informed opinion about soy, but large-scale studies don't support it. Cancer is also multi-factorial: maybe it's consumption of soy in conjunction with a diet high in fat or processed foods that leads to heightened cancer risk; or it's soy and lack of sunlight; or it's some completely different combo. Vegetarians and vegans tend to eat a lot of soy and gluten products, I'm sure there's an information study on them and the incidences of breast cancer.

  11. Just wanted to clarify some comments above regardign Asian diet. I am Chinese. In our culture, we drink a lot of soy milk specially in the morning. The tradition is everybody gets one big glass of soy milk to go with the drier food in the morning. Later on, the more expensive cow's milk gradually replaced soy milk as cow milk supposedly has better nutrients. However, we are now back to soy milk as cow's milk has its own issues. There are soymilk machines that make soy milk from soy beans in 20 minutes that have become very popular in the past 5-10 years and a lot of families own one. So the bottom line is soy milk is still a huge part of Asian diet so is tofu.

  12. @misunderstood

    Yours is the most intelligent comment yet. I read through the comments and was about to point out the many things wrong that are said here about Asians and their diet (specifically soy).

    I'm Asian myself. It is not rare that Asians eat multiple portions of soy during one sitting. So claiming that Asians eat only very small quantity of soy is totally ridiculous. Also, claiming that Asians almost exclusively eat fermented soy is equally ridiculous. Some people here don't know what they're talking about.

  13. Asians and soy? I guess it depends on your culture and where you grow up. I am Asian but my family never had soy milk for breakfast or any soy in any meal. The only soy we ate when we were kids were those deep fried soybeans, so bad for you in ways more than one. Once in a while we had tofu but not a common dish on my mom's menu. There were deep fried tofu too but they never made it on our plates.

    My husband and son like miso soup and edamames but they don't eat them often. I personally don't care about soy. I used to eat tofu but it made me feel bloated so it has gone off my menu for years. The only must-have soy product we have at home is soy sauce. But again, you can never use more than a couple of tablespoons at a time.

    Many of my westernized Asian friends don't eat soy products at all. I do have some very traditional Chinese friends who enjoy congee (rice soup) and big white noodles for breakfast. They don't drink a lot of milk but they always keep small amount of cow's milk in the fridge, not soy. I notice they like to add small amount of yellow soy sprouts in their stir fries. Most of my Chinese friends like tofu desserts – some sweetened silk tofu that looks like jiggling pudding.

    I don't believe we need to eliminate soy completely but moderation is the key. We all know the problem with having too much of a good thing! :-p

  14. When studies are done about the amount of soy foods consumed by people they are looking at grams of PROTEIN from the soy. the total amount of protein derived from soy in the western diet is far below what asians eat. afterall soybean oil is DEVOID of protein. you can consume cup after cup of soybean oil and you will NEVER get even one gram of protein. however 3oz of silken firm mori-nu tofu (or 1/4 cup plus 2 TBS) has 4 grams of protein. therefore in order to get 9 grams of protein one would have to eat almost 3/4 of a cup of that particular brand of tofu. According to the mori-nu tofu package that would be approximately 3 SERVINGS! 1 TBS of Tamari (soy sauce made from fermentation) has 2g or protein. To get 9 grams of protein from just tamari one would need to consume 4.5 servings. And miso? One ounce (2 TBS) of miso contains 3 grams of protein. Again, that equates to 3 servings of miso. So do asians really only eat a little bit of soy? According to the math – no, not really. The math supports and intake of 3 servings a day.

    So when one thinks about it, logically, processed soy that strips away the protein like vegetable shortenings, oils, starches, natural flavors, etc; sure it means that one consuming those products is eating a lot of soy – but NOT a lot of soy protein. It's the processed soy that definitely needs to be shunned not whole food sources or even lightly processed (tofu).

  15. Most all processed foods contain some form of soy. I have recently given up dairy, gluten and soy due to problems associated with radiation damage incurred from cancer treatment 2 years ago. I was shocked after reading labels to find most every food that comes in a package has soy, dairy and/or gluten in it. Most people are unaware of how much soy they are actually eating everyday. The only way to avoid soy is to start eating more fresh produce and meats. Read labels and don't buy products that contain soy.

  16. I cannot help but observe that those who disagree with this article (like Kelly, above, who writes "I have a totally opposite opinion") offer no references to any studies or scientific evidence whatsoever (and often, it seems, on the basis of false assumptions concerning Chinese and Japanese dietary habits). They seem to be operating purely on opinion or feeling. Yes, cancer and nutrition are emotional topics. But we owe it to ourselves to argue from evidence. If those who hold a different opinion can muster some robust evidence to convince us that what they have to say is more than simply opinion based on emotional reaction, incomplete evidence and unquestioned assumptions.

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