The Question of Soy and its Effect on Breast Cancer

Soybeans are the most widely used, least expensive, and least caloric way to get large amounts of protein with very little fat and no cholesterol. Soy is the main source of protein for billions of people around the world.

Some doctors are concerned about the safety of eating soy for women diagnosed with breast cancer. That’s because soy contains a protein, called isoflavone, which can act like a weak estrogen. Concentrated soy products, such as powders, pills, and capsules, contain more isoflavones compared to soy foods, such as tofu, soy milk, and the beans themselves (also called edamame). Hormone-like substances in plants are called phytoestrogens. The growth of hormone-receptor-positive breast cancers may be turned on by these substances.

Other doctors think soy might protect breast health because the hormone-like strength of isoflavones is MUCH weaker than the estrogen your body naturally makes. So it might be healthier if soy’s weak isoflavones wash out or replace some of your body’s stronger estrogen.

Doctors in the middle say it’s OK to eat soy foods because they are a healthy source of protein. But avoid — or greatly limit — your use of concentrated soy products.

Let’s keep things in perspective: Soy is NOT a major risk factor for breast cancer. At most, there is a concern about soy foods and a worry about soy products. We’re addressing this issue because a lot of you have asked us about things you can do in your everyday life to lower your risk and asked specifically about soy.

It turns out that the research on soy and its effect on breast cancer risk is unclear. You can find studies showing that soy is helpful, harmful, or harmless related to the risk of being diagnosed with breast cancer or a recurrence.

It’s not clear why the results are so different. It might be due to differences in a woman’s age, diet history, weight, amount of physical activity, and other aspects of her health.

Current studies are starting to figure out how the many different compounds in soy affect our bodies. In general though, most doctors believe that it’s safe to eat a moderate amount of soy foods, with or without a history of breast cancer.

Soy 101

When I say soy, I mean all forms of soy foods, not byproducts such as concentrated soy protein powder and soybean oil that are found in many processed foods. Those are topics for another column.

Soy is promoted as a healthy option for vegetarians or for people who want to cut back on foods that come from animals.

Soymilk is a popular plant-based alternative to cow’s milk for vegans and people who are lactose intolerant. Unsweetened soymilk is comparable to cow’s milk in terms of calories, protein, and fat. It also has less sugar than cow’s milk. (Most plant or animal milks naturally contain some sugar.) Soymilk also provides important nutrients, including calcium, vitamin D, and vitamin B12, along with fiber and potassium.

Other substances in soy foods are thought to help keep cell growth and activity normal, regulate cholesterol, and protect cells from harmful chemicals called free radicals.

As discussed above, soy also contains phytoestrogens — weak estrogen-like compounds found in some plants. Isoflavones are a class of phytoestrogens. Some of the isoflavones in soy include genistein and daidzen. Equol is another isoflavone that comes from soy, but it’s only made by certain people when bacteria in their intestines break down daidzen. Not all people have the intestinal bacteria that create equol, so it’s not found in all people.

Because estrogen plays a significant role in the development, spread, and growth of breast cancers, there has been concern that eating a lot of estrogen-like soy compounds might also affect breast cancer risk.

So, is soy good or bad for breast health? 

The question is especially important for women diagnosed with hormone-receptor-positive breast cancers. Big doses of isoflavones could overstimulate hormone-receptor-positive breast cancer. Small amounts of isoflavones in soy foods are less likely to cause a problem. Isoflavones also may get in the way of hormonal therapy medicine’s ability to do its job. Tamoxifen and isoflavones both work in the estrogen receptors in the body.  IF isoflavones deliver a weaker estrogen signal to the receptor compared to tamoxifen (and your body’s estrogen), then the isoflavones might provide extra protection against this type of breast cancer. But if isoflavones give breast cells a stronger estrogen signal than tamoxifen, that’s a problem. Large amounts of isoflavones could interfere with the goal of the aromatase inhibitors, which are Arimidex (chemical name: anastrazole), Femara (chemical name: letrozole), and Aromasin (chemical name: exemestane). Aromatase inhibitors, another type of hormonal therapy, lower the amount of estrogen available in the body to interact with the estrogen receptor.

So there are reasons why some doctors advise women diagnosed with breast cancer to limit or stop eating soy, while other doctors tell them to eat more.

The research: benefits

Most of the reassuring news about breast cancer and soy comes from studies on groups of people and their consumption of soy foods, not soy products.

There’s an association between eating soy and lower rates of estrogen-receptor-positive breast cancer in women living in Asia. The link is weaker for Asian women living in Western countries. People living in Asia eat up to 10 times more soy than people living in the West. Because of this, experts wondered if eating a lot of soy helped protect against breast cancer. Of course, there are other healthy aspects to the Asian lifestyle besides soy that could help explain the lower rate of breast cancer, such as less obesity, more physical activity, and less alcohol use.

There are also studies that found no association between soy and a higher risk of breast cancer or its recurrence. For example, a 2009 study of postmenopausal women found that soy isoflavones didn’t increase breast density. This was an important finding since dense breasts are linked to a higher risk of cancer. Another 2009 study on more than 5,000 Chinese women diagnosed with breast cancer found that a diet rich in soy did not increase the risk of recurrence.

More recent, bigger studies and research reviews suggest eating soy foods (not concentrated soy products) can be protective for some people.

• A 2014 analysis of 35 studies found eating soy foods was linked to a lower risk of breast cancer both before and after menopause for women in Asian countries, but not women in Western countries. Still, when researchers combined results from three large studies involving nearly 10,000 women, they found a lower risk of breast cancer recurrence in both U.S. and Chinese women who ate soy and had been diagnosed with breast cancer. This was especially true for women who had estrogen-receptor-negative disease. Risk dropped by 25% in women who ate about 10 grams of soy daily, an amount similar to what’s in a standard Japanese diet.
• A study of 15,000 Japanese women found eating moderate-to-high amounts of soy foods was linked to lower breast cancer risk after menopause.
• An analysis of more than 130 studies on soy consumption found that eating about 1½ cups of soy foods daily was linked to a lower risk of recurrence and dying from breast cancer for women who had been diagnosed, regardless of their ethnicity.

Recent research also suggests that the age a woman begins eating soy foods plays a role in soy’s effect on breast cancer risk. Asian women begin eating moderate to high amounts of soy foods as children. Scientists think that may be why the most protective results have been seen in these women. Eating soy foods early in life might reduce the risk of developing breast cancer because soy seems to contribute to breast-tissue differentiation in developing girls, which seems to be protective. Tissue differentiation is when cells line up to function as they’re supposed to. There’s also some evidence suggesting a link between better survival and eating soy foods a year or more after being diagnosed with breast cancer.

While all these results seem encouraging, other studies offer different results.

The research: risks

Most of the troubling results about soy come from studies on lab animals and cells. Research has shown that certain isoflavones in soy seem to encourage cancer cell or tumor growth and spread, particularly around menopause. For example, lab studies have found that low concentrations of the isoflavone genistein stimulated the growth of estrogen-receptor-positive breast tumors and interfered with the effects of tamoxifen.

Another concern is whether soy’s estrogen-like effects worsen outcomes for women newly diagnosed with early-stage breast cancer. A small and short 2014 study suggests that for some of these women, adding about 4 cups of soy milk to their diets turns on several genes that encourage cell growth. The study didn’t last long enough to know whether these genetic changes would cause cancer to grow, and the study also didn’t look at whether soy does or doesn’t reduce the risk of breast cancer.

The research: mixed results

Some research has found that soy has different effects under different conditions.

For example, research done in animals has shown that high concentrations of the isoflavone genistein slow the spread of breast-cancer cells but low concentrations of genistein encourage tumor growth. Still, the amounts of genistein studied were higher than anyone would actually eat. Studies are also looking at how soy affects people of different ethnicities, how isoflavones interact with estrogen receptors, and how the interaction is affected by estrogen levels in the body.

Enjoy some soy

At most, soy’s effect on breast cancer risk is neutral, or small. It’s probably a combination of lifestyle factors (for example, exercising daily, limiting alcohol, not smoking) that all work together to lower risk. Also, people who depend on soy for protein are probably eating a healthy diet.

I’m not trying to be confusing by telling you about all these conflicting results. I just want you to know why doctors are still debating the soy question.

I feel comfortable recommending moderate amounts of soy foods as part of a balanced diet for healthy women and breast cancer survivors. This means two to three ½-cup servings of soy per day, similar to a Japanese diet.

That said, it’s always best to follow the Precautionary Principle:

• Until the soy question becomes clearer, eating soy or increasing the amount of soy you eat is not a proven strategy to reduce your risk of breast cancer or lower your risk of recurrence.
• Eliminating all soy from your diet “just to be sure” is drastic, hard to do, and is unlikely to give you any extra protection against breast cancer.

But we recommend that you:

• Avoid highly concentrated soy products or protein supplements (they come in powders and capsules).
• Avoid hidden soy, found in many packaged foods in various forms (protein isolate and soybean oil). Real whole foods provide the best nutrients. Packaged foods may affect your body differently than whole soy. Eating hidden soy may mean that you’re eating more soy than you wanted to.

• Stick to certified organic soy foods. More than 90% of conventional soybeans come from genetically modified seeds. So far, no research shows that genetically modified foods affect cancer risk or cause long-term health problems. But many crops such as soy are engineered to withstand spraying with certain pesticides, and pesticide residues can cause unhealthy cell changes. Certified organic products don’t use genetically modified organisms (GMOs).

Special circumstances call for extra precautions. Of course, anyone with soy allergies should avoid soy. Limit how much soy you eat if you’re being treated for low thyroid hormone levels, and don’t take your medicine with soy foods. And if you have problems absorbing minerals, you may want to avoid soy because compounds in soy called phytates can slow or block absorption of important nutrients including iron, calcium, and zinc.