There has been a lot of controversy about the benefits of soy as it relates to cancer. There are some doctors who believe that soy should be avoided because it can cause cancer or a hormone imbalance, while others believe that it is a healthful protein-rich vegetarian food that can prevent cancer.
For the past 20 years, researchers have studied soy as a potential anticancer food. Soy contains weak estrogen-like compounds called phytoestrogens (or plant estrogens). The theory is that these compounds attach to cells and block the potent cancer-promoting effect of the real hormone.
Although research has sometimes been contradictory, the most recent and comprehensive studies demonstrate that a soy-rich diet does seem to reduce the risk of developing breast, prostate, endometrial, colon and lung cancers. Most of the research, however, has focused on breast cancer. The question about soy and breast cancer that has been most unclear: Does soy help or harm women who already have been diagnosed with breast cancer? Researchers weren’t sure about the estrogen-like effect of soy on breast cancer survivors or soy’s potential interaction with the breast cancer drug tamoxifen.
But a new study from Vanderbilt University Medical Center in Nashville examined soy consumption among 5,000 Chinese women who had been successfully treated for breast cancer. Even though the women in the study ate a diet that is different from women’s diets in the US (the researchers did not specify the type of soy consumed), there is important information to be gleaned from this study.
Study findings: The women who ate the most soy protein — and therefore consumed the greatest amount of specific phytoestrogens called isoflavones — were about one-third less likely to have a recurrence of breast cancer or to die during four years of follow-up.
Researchers also noted that the benefits of high soy consumption applied across the board — for women with either estrogen-positive (cancer cells that need estrogen to grow) or estrogen-negative (cancer cells that don’t need estrogen to grow) breast cancers… pre- and postmenopausal women… and users and nonusers of tamoxifen.
In reviewing this study, I noted other important dietary and lifestyle habits that influenced survival. Women who ate a lot of soy foods also consumed relatively large amounts of cruciferous vegetables (including cancer-preventing broccoli and cauliflower), red and white meat, fish and tea. These women also were more likely to take vitamin supplements and to exercise.
A study from the National Cancer Institute found that soy consumption early in life conferred protection against breast cancer later. The women studied were of Chinese, Japanese and Filipino ancestry and were living in California or Hawaii.
Women who had consumed the largest amounts of soy foods as children (between ages five and 11) had a 60% lower risk for breast cancer as adults. (This study did not specify the type of soy consumed.) When women consumed a lot of soy foods later in life — as adolescents and adults — they had about a 20% to 24% lower risk for breast cancer.
This study provides an important context for the benefits of soy consumption. In Asia, soy foods are a staple, and many people of Asian descent begin consuming them as children. It is very possible that this early exposure to the phytoestrogens in soy permanently modifies how women’s bodies handle the hormone estrogen and reduces the effects of xenoestrogens (estrogen-like compounds in the environment, such as pesticides).
PRACTICAL ADVICE
If you have had breast cancer, these studies don’t mean that you should start eating as much soy food as possible — because excessive amounts of any one food to the exclusion of others is unhealthful. But the evidence does suggest that soy is safe for women with breast cancer. Based on the research so far, here’s my thinking on eating soy foods…
Pay attention to the type of soy you eat. Soy foods do seem to protect against breast cancer recurrence and death from the disease. But the specific types of soy foods may be the real key. Asians tend to eat a diet high in fermented soy foods, such as miso (fermented soy paste used in miso soup and other dishes), tempeh, natto, soy sauce, tamari and tofu. (Tofu is available in fermented and unfermented forms. Fermented is more healthful than unfermented.) The beneficial bacteria (also known as probiotics) involved in fermentation seem to provide other health benefits, including aiding nutrient digestion and enhancing immunity. In contrast, Americans tend to eat many unfermented soy foods, which aren’t healthful — for example, soy turkey (a form of processed soy) or soy chips. I recommend consuming fermented soy foods… minimally processed soybeans (such as edamame) in moderate amounts… and small amounts of nongenetically modified (the label should say “organic”) soy products, such as soy milk.
Minimize exposure to synthetic estrogens. We are surrounded by synthetic estrogens — in the estrogen-mimicking chemicals in pesticides and soft plastics containing bisphenol A.
Important: Soy phytoestrogens are very weak when compared with many of these synthetic estrogens. I think it’s imperative to minimize exposure to synthetic estrogens. Use caution with pesticides, and heat food in glass (not plastic) containers.
Avoid isoflavone supplements. The jury is still out when it comes to the benefit of isoflavone supplements for those with cancer. I recommend that patients with cancer avoid them.
Finally, I want to remind you that focusing on a single food is not the way to prevent or beat cancer. For my other anticancer food and supplement recommendations, see “Foods and Supplements that Prevent Cancer”.
