Keith I. Block, M.D. follow us read our blog

Soy Products and Breast Cancer

There has been much written about soy consumption and a possible link to breast cancer. Very recently, a woman wrote in to our blog to ask if her consumption of soy drinks had perhaps caused her breast cancer, a hypothesis set forth by her oncologist. She explained that she’d eaten a very healthy diet her entire life and she was the only woman in her family – including 5 sisters – who’d been diagnosed with breast cancer. In fact, the question of whether soy consumption causes breast cancer is one we are asked so frequently, we thought it time to address.

The following is our response to the woman's comment from our blog. You can read the original comment and the blog post at

The question of what causes an individual person to get breast cancer (and many other cancers as well) is one that still confounds scientists as much as it does patients. It sounds like you have been doing very close to the right thing all along. One thing that scientists have found is that 80% of breast cancer is what is called “sporadic,” that is, unrelated to any family history or genetic background. This suggests to us that there are potentially many reasons that a person can get breast cancer – perhaps many causes – all acting at the same time. Hormone replacement therapy is certainly one thing that has been linked to breast cancer, because of its stimulation of breast cancer cells that grow in the presence of female hormones, and I am glad to see that you apparently have not been taking it.

I suspect from your use of soy drinks that you may be post-menopausal, and perhaps using the drinks to help with hot flashes. We do find that soy has some estrogen-like activity in laboratory assays due to its content of phytoestrogens (sometimes called isoflavones). However, soy also has many other cancer-inhibiting properties owing to other phytochemicals. The most recent, reliable epidemiological data indicate that postmenopausal women whose diets include a lot of phytoestrogens actually have a somewhat lower risk of cancer ( Further, when women were given a soy isoflavone extract for 3 years, there were no effects on the lining of the uterus – the ultimate test for estrogen stimulation in the body ( A study of supplement use among 35000 American women found that there was no higher risk of breast cancer in postmenopausal women using soy supplements or other menopausal supplements ( Furthermore, even among women who already had breast cancer, who were being treated with anti-estrogen therapy, those who had the highest levels of phytoestrogens in their diets were less likely to get a recurrence ( So I think we can rule out your soy supplements as a cause for your breast cancer. Articles like these are not usually communicated to oncologists, so it’s easy for them to get confused by findings of some older laboratory studies and the general anti-supplement bias of medical journals.

But what might have, in fact, caused your breast cancer? The answer is that there are many factors that we don’t usually think about that can cause breast cancer. First, there are well-known factors that contribute to breast cancer risk. Drinking alcohol, even at relatively low levels, can raise breast cancer risk. In fact, recent research suggests that even 1 alcoholic beverage per day can raise a woman’s risk of breast cancer! Having your children late, or not having children, also contributes to breast cancer risk. Starting menstruation early is another. These are all well-validated risks for breast cancer, and it can sometimes be hard to overcome the risks that you may have been exposed to from these factors.

One breast cancer risk factor that is very seldom acknowledged is chronic, low-grade inflammation. Inflammation is known to stimulate the growth of many cancer cells. Research has shown that women who take drugs like aspirin long-term have a lower risk of breast cancer. However, this isn’t really a good idea, since aspirin can cause bleeding stomach ulcers, so no one is recommending the long-term, regular use of aspirin as a way to prevent breast cancer. Low-grade inflammation can come from several sources, one of which is diet. Even the “healthy” American diet can be a source of inflammation-promoting omega-6 fats. Chicken and poultry, and vegetable oils other than canola and olive oil, can cause inflammation; many margarines also contain omega-6 fats. Over-exercising can cause inflammation. You may have had a background of inflammation from the diet you grew up with as a child that predisposed you to breast cancer. The same study that found that soy supplements did not raise breast cancer in the 35000 American women also found that fish oil, a supplement that lowers inflammation, was significantly associated with lower breast cancer risk. Sometimes a healthy diet is not quite enough to combat these background levels of inflammation, and that’s when you need a supplement.

Another risk factor potentially related to diet is blood glucose and insulin. It’s becoming clear that insulin resistance, in which muscle cells don’t take up insulin as well as they should, can contribute to breast cancer risk. This can be modified with diet and some kinds of exercise, and in some cases with supplements or medications.

Other risk factors are even more subtle. We’ve recently learned that your internal “biological clock” can be strongly related to breast cancer. Jobs or lifestyle habits that upset your biological clock by keeping you up at night may contribute to breast cancer risk. Exposure to radiation on the job can contribute to breast cancer risk. There are concerns about the many artificial chemicals in our environment that have estrogenic effects. What’s more, there are likely other factors that contribute to breast cancer risk that scientists don’t really know about, or have not confirmed yet. So, it’s a mistake to blame your efforts at maintaining a healthy diet, much less your soy drinks, for your breast cancer. The fact that your sisters have not (yet) been diagnosed with breast cancer, in spite of their less healthy lifestyles, does not affect your situation: you may have had other risk factor exposures, or have other genetic predispositions that make you more vulnerable to breast cancer than they are. Your lifestyle, in fact, may have protected you from getting breast cancer much earlier in your life!

If you were my patient, either before or after your diagnosis of breast cancer, what I’d do is encourage you to keep up your efforts to maintain a healthy lifestyle, and, in fact increase them. I test all my patients for the main cancer risk factors, which I call “terrain factors,” since they are the main elements of the terrain or “soil” in which a cancer grows in the body. These include inflammation, oxidation, blood sugar and insulin problems, stress and biological clock problems, immunity and blood coagulation issues and they can all be monitored with standard laboratory tests. If you are found to have even mild abnormalities in any of these tests, you can target changes in your diet, exercise, stress care and supplement regimens to help bring them back in line. There are likely some places in your diet where we could point out ways for you to make it healthier and less likely to promote cancer growth. Most importantly, I’d advise you to go back to your healthy diet and exercise program. Both randomized studies and studies in which women are observed medically for long periods have found that a healthy diet is definitely associated with lower death rates in breast cancer patients. Furthermore, breast cancer patients who walk as little as 3-5 hours a week (a little more than half an hour daily) have a 50% lower risk of death ( So this is not the time to give up on your healthy lifestyle – it’s time to go back to it, and perhaps encourage your sisters to take it up also!