About a year ago I went looking for links between soy and estrogen when an aquaintance expressed alarm that I was giving my son soy. I found at that time that the blogs were full of this, but they never seemed to have any sources except each other. I searched for sources myself and could find no evidence or warnings at NIH, USDA, Mayo Clinic, or the American Dietetic Association. On the contrary, research looking for ill effects among Japanese eating a lot more soy than we do found none.
A friend of mine recently went vegan and said she was avoiding soy because her friend told her it was linked to cancer. I said she could easily be vegan and never eat soy. If it worries you, don’t eat it. But I went to look for evidence about soy and cancer. Those sites I visited before now acknowledge that there is concern about soy’s health effects, but I’m not seeing any general warnings. I emailed to my friend what I found, and thought perhaps others would be interested. I’m not a doctor, just a Googling Know-it-All. Here’s what I wrote to her:
“Before I bore you on soy, I wanted to suggest that vrg.org is a great source for vegan diet info. They are very reputable, very calm and reasonable. Click on Veg Nutrition and then choose a topic such as calcium, protein, or B12. A vegan should probably know at least those three topics. Getting back to soy, you can look at this page at VRG and the second FAQ is about soy. The answer gives a great overview and includes servings: people in Asian countries eat 2-3 servings per day (and do very well).
Here are a group of studies I found on soy, mostly through National Institutes of Health, the official U.S. government medical entity. I didn’t “cherry-pick” them at all – this is the order I found them in Google, actively looking for anything negative, until I got sick of looking:
1. This scientific article says soy consumption in Japan seemed to reduce risk of colon cancer slightly among women only.
2. This scientific article about breast cancer in Japan (where many were exposed to the bomb) has a confusing abstract but I think the key sentence is: “The risk for breast cancer was not significantly associated with consumption of soya foods.” Not sure if they were looking for increased risk or decreased risk, but they found neither.
3. Another study from Japan shows “frequent miso soup and isoflavone consumption was associated with a reduced risk of breast cancer.”
4. Another study says: “The present study provides modest support for the preventive role of soy against stomach cancer and heart disease death.”
5. Another study, and this one assumes everyone knows soy is good against stomach cancer: “Soy food is known to contribute greatly to a reduction in the risk of gastric cancer (GC). However, both Japanese and Korean populations have high incidence rates of GC despite the consumption of a wide variety of soy foods. One primary reason is that they consume fermented rather than non-fermented soy foods….These findings show that a high level of consumption of non-fermented soy foods, rather than fermented soy foods, is important in reducing GC risk.” (Miso and tempeh are fermented. It isn’t clear to me whether they’re suggesting the high rate of GC is caused by fermented tofu or just isn’t helped by it.)
Studies can be flawed and contradictory, so it is important to look at wider views from professionals who have drawn what conclusions they can from carefully examining many studies:
At a general page at NIH where they sum up research, under the general statement that “Soy is considered safe for most people when used as a food or when taken for short periods as a dietary supplement” they do have this one conditional negative statement: “Soy’s possible role in breast cancer risk is uncertain. Until more is known about soy’s effect on estrogen levels, women who have or who are at increased risk of developing breast cancer or other hormone-sensitive conditions (such as ovarian or uterine cancer) should be particularly careful about using soy and should discuss it with their health care providers.” I’m not sure what research leads them to worry about soy – I can’t find anything specific online – but that’s what they say.
Another article from NIH. This one is pretty good, seems comprehensive, and again (I mean like everything except the previous article) has nothing to say about soy increasing the risk of breast cancer. The only thing they say about cancer (and note dementia, too) is:
“The original interest in soy was fueled by…the observation that populations that consume a lot of soy, particularly those in eastern Asia, have less breast cancer, prostate cancer, and cardiovascular disease, and fewer bone fractures. Additionally, women in these populations report fewer menopausal symptoms, such as hot flashes, and both men and women have a lower incidence of aging-related brain diseases. Since lifestyle can affect chronic disease development, and diet is a major lifestyle factor, traditional Asian diets drew considerable attention. …[T]he cumulative evidence of numerous biomarker studies has confirmed that their diets are significantly higher in both isoflavones and lignans (another phytoestrogen) compared to the typical Western diet. Studies have further shown that when Asians emigrate to Western nations such as the United States and adopt the prevailing diet, their disease rates change.”
This last sentence indicates that the lower incidence of those diseases mentioned is therefore not a genetic effect but something around them that changed when they moved to the U.S., probably diet.
For another point of view, the Mayo clinic, who were mum on the topic last time I looked, now say:
“The high intake of soy foods in Asian countries has long been credited, at least by some researchers, for the lower rate of breast cancer among Asian women, compared with women in countries where little soy is consumed. But some confusion arises when you look at genistein, the main soy isoflavone and a plant estrogen. Does it protect against breast cancer or, on the other hand, promote the growth of existing cancer cells? Some studies have suggested the latter. Researchers at the Mayo Clinic reviewed all the evidence and concluded that soy has not been shown to fuel breast cancer cells. “If breast cancer patients enjoy soy products,” they concluded, “it seems reasonable for them to continue to use them.” Whether soy actually protects against breast cancer is still unknown.”
Lastly (I promise) John’s Hopkins University has this very clear page:
10 Myths About Breast Cancer Survivorship
MYTH 1: Eating soy products after having hormone receptor positive breast cancer increases my chance of a recurrence.
FACT: Research on soy has been conflicting over the years. It has the capacity to mimic as well as block certain estrogens. Overall, natural dietary soy in the form of soy milk, soy bean sprouts, tofu or tempeh appears to be safe and may provide significant health benefits when it replaces animal sources of milk and protein. However, soy in concentrated forms such as pills, powders and supplements has the strongest potential for estrogenic activity and probably should be avoided by anyone who has been diagnosed with hormonal receptive breast cancer.
I’m not pooh-poohing your friend’s research. You’ll just want to compare her sources to these and decide for yourself.”