Key Takeaways

  • It is safe to include soy foods as a part of a healthy dietary pattern, even after a breast cancer diagnosis.

  • Early fears about soy foods causing breast cancer have been debunked by a large body of research.

  • It’s beneficial to eat whole soy foods, such as tofu and edamame. Soy protein powders are also safe to eat but don’t contain all the nutrients and plant compounds found in whole soy foods.

Soy foods top the list of food myths that I get asked about most often, and for good reason. There are lots of myths about soy foods and cancer. Here are some of the most common rumors about soy, along with an update on what the current research shows.

Bottom line: Soy foods are safe to eat and do not cause cancer.

Myth: Soy isoflavones act like estrogen in the body, raising the risk of cancer

How did the myth start?

Isoflavones are natural plant compounds found in soy foods. They have a similar chemical structure to human estrogen, but they function differently in the body. Not understanding the differences between estrogen and isoflavones led to many false assumptions and concerns.

In some early studies in mice, soy isoflavones promoted the growth of estrogen receptor-positive (ER+) breast cancer cells. However, this involved much higher isoflavone blood levels than would occur in humans eating soy. Scientists now know that rodents break down isoflavones differently than humans. The same effect of stimulating cancer cells has not been seen in humans.

Facts:

Studies do not link eating soy to an increased risk of any cancer, including breast cancer. In fact, studies show that soy isoflavones may reduce the risk of developing breast cancer.

Researchers now know that isoflavones and estrogen differ in how they interact with estrogen receptors in the body. One way that cells can respond to their environment is by signaling through receptors. It’s like a lock and key. When a substance binds to a receptor in the body, the receptor can then send a signal to switch certain genes on or off. Different estrogen receptors are present in different parts of the body.

  • Activation of some receptors promotes cell growth.

  • Isoflavones bind to estrogen receptors, potentially acting as a tumor suppressor.

  • Isoflavones may “turn on” genes that slow growth of, or destroy, cancer cells.

  • Isoflavones may support the body’s antioxidant defenses and DNA repair, which can help protect against cancer.

Studies show that soy protein powder or soy isoflavone supplements show no effect on markers of breast cancer risk such as:

  • Hormone levels

  • Markers of breast cell growth

  • Breast density

Some studies from East Asia show that eating one to two servings of soy foods per day helps lower breast cancer risk. In East Asia, including China and Japan, soy foods are commonly consumed throughout life. Evidence is not strong enough to advocate including soy foods specifically to reduce risk of breast cancer, but it does allay fears of risk.

Myth: Avoid soy foods after a breast cancer diagnosis

How did the myth start?

This myth to avoid soy foods after breast (or prostate) cancer diagnosis came from the same misunderstanding as above: Thinking that isoflavones act the same as human estrogen. However, research in cancer survivors confirms that there are no harmful effects from soy foods. Soy can be part of a healthy diet.

Facts:

Population studies show no increased risk for breast cancer survivors who eat soy foods. Quite the opposite, in fact. Soy may be protective against breast cancer recurrence.

The AICR/WCRF CUP Global Report on breast cancer survivors rated the evidence as limited suggestive that eating soy is associated with greater survival and decreased recurrence (including in women who had ER+ cancer).

Studies in women who have been diagnosed with breast cancer show better outcomes with moderate consumption of soy foods. The amount is usually one or two standard servings of soy per day. Eating more soy foods per day is not linked with additional improvements.

Soy and Prostate Cancer

Among men with prostate cancer, supplements of soy protein or isoflavones have shown either no effect or a decrease in progression of prostate cancer. This is true for men with prostate cancer who were:

  • Preparing for surgery

  • Recovering from surgery

  • Following a “watchful waiting” approach after diagnosis

Although the benefit was unclear, none of these studies showed an increased risk associated with soy or isoflavone consumption.

Soy foods are also good for heart health, which is important for many cancer survivors. Soy foods provide unsaturated fat, which reduces LDL-cholesterol when replacing saturated fat. And its plant protein modestly lowers LDL-cholesterol in clinical trials in which participants eat several servings of foods with plant-based protein every day.

Bonus: Emerging research links eating soy foods with a greater variety of health-promoting bacteria in the gut microbiome. Researchers are exploring how this may benefit blood vessel health. More studies are needed to understand the potential for soy protein, fiber and polyphenol compounds in supporting heart health.

Myth: Whole soybeans are okay, but any processing makes soy foods an unhealthy choice

How did the myth start?

There were concerns that food processing may concentrate soy isoflavones to potentially harmful levels.

Facts:

Soy protein powder and textured soy protein don’t contain more isoflavones than the same portion size of plain roasted soybeans. In fact, some soy protein concentrate even loses as much as 80 to 90 percent of isoflavones during processing. The isoflavone content of veggie burgers, protein bars and soy protein cereal are usually less than what’s found in a standard serving of soy food, such as tofu.

Clinical trials with soy have often used isolated soy protein or isoflavones to allow more precise control of isoflavone amounts. These studies provide evidence about the benefits and safety of isoflavones, but they can’t completely represent the effects of eating whole soy foods.

Foods with isolated soy flour or soy protein are categorized as ultra-processed foods because of how they are made. Several recent studies link a high intake of ultra-processed foods with increased calorie consumption, weight gain and possibly increased cancer risk. But remember: These studies group together a wide range of foods, and soy protein would be in the same category of ultra-processed foods as cake, fries and candy. Conclusions from these studies are unlikely to apply equally to all ultra-processed foods in this group.

Soy contains a variety of nutrients that may contribute to health, including protein and fiber. Isolated soy protein doesn’t provide the dietary fiber or complete range of nutrients found in whole soy foods.  Although isolated soy protein does not pose risk, it also doesn’t turn a low-nutrient, sugar-or sodium-laden bar, drink or convenience food into a health food.

Practical Advice

If you choose to include soy foods in your diet, choose whole soy most often. It has a lot of beneficial nutrients, including protein, isoflavones and fiber. When studies suggest “one or two servings” servings of soy foods, each serving has about 7 g protein and 25 mg isoflavones. Here are examples of one standard serving of soy:

  • 1/3 cup tofu
  • 1/2 cup tempeh
  • 1/2 cup shelled edamame
  • 1 cup soy milk
  • 1/4 cup soy nuts
  • 3/4 cup (6 oz.) soy yogurt

Foods such as veggie burgers and protein bars made with soy ingredients (like isolated soy protein or soy fiber) can be part of a healthy diet. However, they don’t count as a “serving” of soy food (as the term is used in clinical studies) because they usually have lower levels of isoflavones.

This announcement was originally released on October 8, 2024, by the American Institute for Cancer Research. It is republished with permission.