Dr. Susan Love Research Foundation | New Study Sheds Light on Soy and Breast Tumor Growth

Cross-cultural research has found that women living in Asian countries have a lower rate of breast cancer than do women in the US. One suggested reason for this has been the greater amount of soy products consumed by Japanese women throughout their lifetime. In an attempt to decrease their own breast cancer risk, many American women began to add more soy to their diet, and the food and supplement industry was quick to pick up on the trend, creating soy powders and supplements and adding soy to everything from cereal to energy bars.

Women who have been diagnosed with breast cancer, however, often question whether they should eat soy. They are often concerned that the isoflavones in soy, which act like estrogen, may cause their tumors to grow. An article published online May 6, 2004, in the journal Carcinogenesis, adds new information to what we know about soy and its potential to influence tumor growth.

The study, "Soy Processing Influences Growth of Estrogen-Dependent Breast Cancer Tumors," was conducted by William Helferich, a professor of food science and human nutrition at the University of Illinois at Urbana-Champaign.

Soy is a food source of isoflavones. Isoflavones in soy contain two important compounds, genistin and daidzin. When soy is eaten, bacteria in the stomach converts genistin into genistein and daidzin into daidzein, compounds the human body can absorb.

Although they are often referred to as phytoestrogens (estrogens from plant sources), genistin and daidzin are actually more like selective estrogen receptor modulators (SERMs). SERMs, like tamoxifen, act like estrogen in some organs but appear to block estrogen in others.

In Asian countries, women eat whole soy foods, like soybeans or soy flour. In contrast, in the US, women often take products that contain large amounts of soy protein and isoflavones, such as soy extract capsules and genistein powder, rather than eat soy foods. But supplements do not contain all the biologically active components of whole soy foods—like tofu and edamame (boiled soybeans). And when these other components are not present, soy may not have the same benefits—or may even be harmful.

Helferich's study evaluated the effects different soy foods and supplements had on mice that had a preexisting breast cancer. He fed the mice one of five different diets: soy flour plus mixed isoflavones, soy molasses (a soy extract), Novasoy (a soy extract), mixed isoflavones, and purified genistin. Each diet contained the same concentration of genistin, which allowed Helferich to assess whether genistin's effects differed depending on whether it was being consumed with the other bioactive compounds found in soy foods. (The soy flour, a whole soy food, had all of these compounds; the genistin, which was purified, had none.) Helferich found that the five different diets had differing effects on breast tumor growth. In the mice fed soy flour plus mixed isoflavones, the tumors remained the same size. But in the mice fed soy molasses, Novasoy, mixed isoflavones (alone), or genistin, the tumors grew. Further, the rate of growth depended on how purified the product was. Soy molasses, the least purified product, resulted in the smallest amount of tumor growth, while genistin, the most purified product, produced the largest tumors.

"If the level of dietary exposure to genistein was the principal factor that influenced tumor growth, various dietary treatments would have resulted in similar tumor growth," writes Helferich. "This was not the case...The results here demonstrate that there is a significant difference in how soy processing can alter potential health benefits of soy foods." Notably, Helferich continues, the soy flour diet "is more representative of the soy-containing foods consumed in the Asian diet" whereas "diets in this study containing purified isoflavones are more representative of the use of dietary supplements containing isoflavones, which is how many Americans consume these compounds."

Based on his findings, Helferich concludes, "eating soy foods that are made from soy flour, such as the types of soy foods that are consumed in Asian countries, may be more advisable than consuming soy-derived supplements that do not contain the full complement of bioactive compounds and nutritive components originally present in soy itself."

Susan says:

This study confirms the findings of other studies that have looked at how soy affects tumor growth.

Soy has a lot of good things going for it. As little as 25 grams of soy protein a day can lower cholesterol in people with high cholesterol, which makes soy good for the heart. Further, the isoflavones in soy may help the body hold on to calcium, which is good for the bones and may help ward off osteoporosis.

But as this study found, soy food and soy supplements are not the same.

After soy was found to be beneficial, the supplement industry started marketing many different types of soy capsules, genistein powders, and isoflavone concentrates, knowing that many women would find it easier to take a pill than to actually add soy foods to their diet. But as we are learning, supplements do not always offer the same benefits of whole foods.

Should women who have had breast cancer eat soy? Soy as food is probably safe for women with breast cancer, but the final answers aren't in yet. You will have to decide for yourself whether you want to have soy in your diet. If you do, try eating one serving (40 grams) of soy a day. But if the idea of eating soy worries you, then don't eat it. There are many other good protein alternatives. What if you are taking tamoxifen? Soy will not counteract tamoxifen. In fact, a study in mice found that soy appeared to actually make tamoxifen work better. Still, once again, if the idea of eating soy worries you, don't eat it.

What if you haven't had breast cancer? Soy is an excellent source of protein and you may want to consider adding it to your diet.

What should you eat? Tofu, miso soup, soybeans (edamame), and soy milk are all good options. So is soy protein powder, which you can add to milk or orange juice. What you don't want to do is take soy tablets or capsules that contain isoflavones or genistein. These purified forms of soy are not the same as soy, and they appear to do more harm than good.

Reference:

Allred CD, Allred KF, Ju YH, Goeppinger TS, Doerge DR, Helferich WG. Soy Processing Influences Growth of Estrogen-Dependent Breast Cancer Tumors. Carcinogenesis 2004 Sep;25(9):1649–57. Published online 2004 May 6.