Soy and Cancer: Decoded


Soy remains an important and somewhat contentious health topic.

Is there a connection between eating soy and being diagnosed with breast cancer?

Soy remains an important and somewhat contentious health topic especially when it comes to breast cancer. Soy contains high amounts of isoflavones, which belong to a group of substances called phytoestrogens. As the name implies, they have a structure similar to estrogen. For this reason, many people adopt the idea that it behaves like estrogen to promote hormone-sensitive tumor growth. However, if there is one solid conclusion that can be drawn from all the data on soy and breast cancer, it’s that eating moderate amounts of soy foods is safe and does not increase the risk of breast cancer. If anything, rather the opposite. The majority of high-quality studies have consistently found that soy foods do not increase risk of breast cancer, even when eaten at levels much higher than those typically seen in the US.

Because soy contains estrogen-like compounds, there was fear that it may raise the risk of hormone-driven cancers. Early animal studies seemed to support this. We have a lot more research now and evidence shows that this is not true. Human studies consistently show that soy does not increase breast cancer risk. Humans do not process isoflavones in the same way that animals do. In fact, research shows that soy is not only safe but may also be protective, lowering risk for cancer survivors when eaten in moderate amounts. A moderate amount of soy is one to two servings per day. This represents around 30-50mg of isoflavone per day as is common in Japan and Shanghai, China. In contrast, the US daily isoflavone intake is less than 3 mg.

According to the American Cancer Society, for breast cancer survivors, current evidence suggests no adverse effects on recurrence or survival from consuming soy and soy foods and there is a potential for these foods to exert a positive synergistic effect with tamoxifen, a common anti-estrogen drug for estrogen sensitive breast cancer.

In the largest study to date, a pooled analysis of studies that included almost 10,000 breast cancer patients consuming at least 10mg isoflavones daily was linked to a 25 percent reduction in breast cancer recurrence. This effect was seen among both women from the United States and Asia and was most evident in survivors of ER-negative cancer and those with ER-positive cancer taking tamoxifen. A theory is that soy isoflavones protect estrogen receptors from excessive stimulation by the hormone estrogen.

This continues to be a very confusing area for the public. There is plenty of research from the US and globally that for breast cancer patients and survivors, eating moderate amounts of whole soy foods does not increase a woman’s risk for breast cancer.

Each serving of whole food sources of soy provide only 30-60mg of soy isoflavones. Whole food sources of soy include edamame, soybeans, soy milk, soy nuts, soy yogurt, miso soup, tempeh and tofu. Given the available data, I do not restrict these whole food sources. These whole food sources provide only low-moderate levels of soy isoflavones in a healthy combination with protein, fiber, and fat, potassium, magnesium, copper, manganese.

Processed soy foods like sausages, cheese and hot dogs actually lose isoflavone potency in processing. These things, like any processed food, lose much of their healthy components like micronutrient concentration and often have additives, salt, stabilizers etc.

Isoflavone concentrated in soy protein powders and pills are actually much higher. Women with a history of estrogen sensitive breast cancer should avoid soy protein meal supplement bars and shakes often listed as soy flour or soy protein isolate because these contain very high levels of isolated soy isoflavones, which may raise the risk of breast cancer in post-menopausal women and interfere with the action of selective estrogen receptor drugs like tamoxifen.

I would not recommend soy supplements or powders as there is one 2014 study that indicates altered expression of genes promoting breast cancer. The level of isoflavones consumed was 100mg per day, which is above those recognized as safe. The clinical significance of this is unclear but more is not always better.

Moderate soy intake can be part of an overall healthy diet. It is important to not get lost while focusing too closely on one food or nutrient but to consider the overall diet. Aim to eat 5-9 servings of fresh, whole vegetables and fruit per day. Aim for a variety of vegetables and fruit. Include healthy whole grains and legumes, lean proteins. Stay hydrated and drink water. Stay active. Manage stress and anxiety. And if you like tofu, edamame, miso or tempeh, enjoy it.

Is there a notable amount of estrogen in soy or other foods?

In the human diet, phytochemicals provide a plant with color, aroma, flavor and protection from infection and predators. Phytochemicals work together to fend off cancer and other disease. They have potential to stimulate the immune system, slow the growth rate of cancer cells and prevent DNA damage that can lead to cancer. Confusion about soy arises from the very term phytoestrogen. Phytoestrogens are actually a class of molecules under the polyphenol umbrella, like flavonols in almonds or anthocyanins in berries. If we instead thought of soy isoflavones or phytoestrogens as the polyphenols they are, I think generalized public fear would wane. This may sound like a lesson in semantics but it is important because once a fear has been planted that a certain food is unsafe or unhealthy, a woman is forever panicking about being “exposed” to that food.

Like most other vitamins, minerals or phytochemicals, a particular substance can be found in many sources. Similar to how we achieve adequate vitamin C from not just oranges but also strawberries, spinach and potatoes. Isoflavones are also found in legumes, alfalfa and licorice root among others.

Another type of phytoestrogen is lignans. Lignans are compounds found in fiber-rich foods including flaxseed, whole grains, legumes and vegetables. In post-menopausal women, lignans can cause the body to produce less active forms of estrogens which may potentially reduce breast cancer risk among other proposed mechanisms. Flaxseed may also reduce breast tumor growth and spread in non-estrogen sensitive cancers suggesting that flax may have anti-cancer benefits unrelated to estrogen or estrogen metabolism. Multiple studies with mice indicate that dietary flaxseed strengthen the anti-tumor effects of tamoxifen. Studies in women have shown that higher intake of lignans is associated with reduced risk of breast cancer. Further, dietary lignans have been associated with less aggressive tumor characteristics in women who have been diagnosed with breast cancer, meaning that those already eating flax at the time of diagnosis seem to have tumors that are less aggressive. I recommend that dietary intake of ground flaxseed, sometimes called flax meal, up to two tablespoons per day is safe.

Are there other foods connected with breast cancer?

Other foods that have been associated with decreased risk of breast cancer are many but all require further study. Some of these include cruciferous vegetables like broccoli, Brussels sprouts, cabbage, collards, cauliflower, kale, mustard greens, bok choy, etc. Others include flavonoid rich foods like onions, cherries, berries, herbs like parsley, thyme, whole grains, citrus fruits and tea. It is also important to note that convincing evidence exists that regular alcohol consumption increases the risk of breast cancer in both premenopausal and postmenopausal women in a dose-responsive manner, regardless of the type of alcohol consumed. Meaning, the more you drink, the higher your risk. Adequate folate status may partially mitigate the increased risk of breast cancer associated with moderate alcohol consumption so folate adequacy should be ensured in women who drink. Beans, lentils and dark green vegetables like broccoli and spinach are all good sources of dietary folate. If consumed at all, alcohol should be limited to one drink per day for women, two for men.

Amanda Bontempo, MS RD CSO CDN

Twitter @amandabontempo

Instagram @amandabonbon

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