Survivorship

Soy: Is it safe for cancer survivors?

by Ami Karnosh, a Seattle-area certified nutritionist

Consuming organic soy is an easy, inexpensive and healthy way to add more protein, vegetables and fiber to your diet. But many cancer survivors are mistakenly told to avoid soy because such foods contain phytoestrogens, chemicals that can mimic the hormone estrogen. Because estrogen fuels many breast cancers, soy has long been a source of concern. People worry that it might raise their risk of hormone-related cancers (like breast, uterine and ovarian) or increase the odds of recurrence.

Soy is actually one of hundreds of foods, including garlic, oats, carrots and more, containing phytoestrogens. But plant-based estrogens are not the same as the hormone estrogen. True estrogens—which attach to certain cells to turn on growth—are only found in animals. Soy is not a hormonal food. While phytoestrogens can attach to the same cell sites as estrogens, they have about 1,000 times less potency than actual estrogens and they cannot turn on the same cells that estrogen would to the same degree. Because of that, they can be very protective. Phytoestrogens help block cell receptor sites, decreasing the areas where estrogen in the body can attach. Given enough fiber in the diet and avoidance of estrogen foods (like high fat dairy and meats), this will decrease overall estrogen.

Some of the concern comes from a series of lab studies in which the protein in soy was isolated and placed with cancer cells in petri dishes. The cancer cells grew, but the researchers acknowledged that any protein, even from other sources like wheat, corn, grass or tomatoes, would make the cells would grow.

In human studies, scientists have not found that diets high in soy increase breast cancer risk. In fact, most have found the opposite. All long-term human studies of soy have shown a protective effect against cancer, fewer cancer recurrences, or had neutral findings (neither negative nor positive effects). There have never been any direct correlations between soy and cancer. Soy has also been shown to protect heart and breast health in women who ate it during puberty.

Like many plant-based foods, soy can be a healthy part of a nutritious diet, even for cancer survivors. If you choose to eat soy, bear in mind two caveats. Soy is one of the most genetically modified foods in our country; 92 percent of the soybeans grown in the U.S. are genetically modified and that is likely not a healthy form. Choose organic forms of soy instead. Also, avoid "isolated soy protein" (found in a lot of bars, shakes, etc.) as an ingredient. Manufacturers take all of the other nutrients out of the soybean and retain only the protein as a cheap, easy way to increase the protein in these foods. Instead, seek out the whole forms of soy.

Frequently asked questions about soy

Has soy been shown to cause breast cancer?

No, that’s misinformation. Soy has actually never been shown to cause cancer in human studies. All such studies have shown a protective effect against cancer, fewer cancer recurrences, or had neutral findings (neither negative nor positive effects). There have never been any direct correlations between soy and cancer.

Can I eat soy if I’m taking Tamoxifen, Herceptin, or other medications to lower my estrogen levels?

You should always discuss these issues with your health care provider in consideration of your individual situation. Some people are told to avoid soy as they’re trying to get rid of estrogen. But soy has been shown to improve the effectiveness of these estrogen-blocking drugs because soy blocks estrogen receptors so the estrogen floats in the blood, making it easier for those drugs to bind to it and break it down.

How can I make soy easier to digest?

Like all high-protein foods, some people can find soy a little hard to digest. Choosing fermented forms of soy like tempeh, miso or natto are the easiest to digest. Tofu is fine, too, but just like meat, marinating it helps break down its proteins, making more nutrients available and aiding digestion of it.

What about soy convenience foods like bars?

Such foods are often designed as grab-and-go items or as alternatives for many meat, chicken or dairy products. While high in protein, they usually contain isolated soy protein and are stripped of many nutrients. Avoid highly processed forms of soy, including supplements containing soy and any food product containing isolated soy protein, soy protein concentrate, TSP or TVP. Instead, choose whole forms of soy, like organic edamame, soy nuts, miso, tempeh and tofu.