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Is soy helpful or harmful for cancer survivors?

For years, cancer survivors were told to be cautious about eating soy foods, but current data shows foods such as tofu, tempeh, soymilk, and edamame are safe to eat. These foods may even reduce the risk for ovarian and endometrial cancers, and protect against prostate cancer and breast cancer recurrence.

The studies show an association between American and Asian women who ate soy products and lower rates of hormone related cancer and recurrence, whether they ate them early or later in life. The risk of breast cancer was reduced even among women who have the BRCA 1 or 2 mutation, a risk factor for breast and ovarian cancers.

Cover picture of edamame on Food & Nutrition

Among a population of women who were followed in the studies, there was no additional risk of hormone related cancers such as breast, uterine (endometrial) cancer, or ovarian cancers.

More recent studies show additional benefits: eating soy helps with treatment-related symptoms. Women experienced less menopausal and other symptoms while undergoing cancer treatments, such as:

  • hot flashes and night sweats

  • joint pain

  • fatigue

  • hair thinning/loss

  • memory problems

Most promising, according to the Academy of Nutrition and Dietetics, soy foods may enhance the effectiveness of tamoxifen, and "improve the breast cancer blocking actions" of the medication for ER+ breast cancer.

What led to the decision that soy foods may contribute to cancer risk? Some of the chemical structures of soy nutrients were initially thought to behave like estrogen in a women's body. Estrogen can fuel cancer growth.

Treatments for hormone related cancers work by blocking estrogen from binding with cells to stop cancer growth. But soy does not behave like estrogen in the body, although the compound phytoestrogen, which occurs naturally in plant foods like soy and flax, may sound the same.

Edamame salad

Eating diverse types of foods is beneficial for the gut and our mood, and soy foods are an important source of nutrients. They are high in fiber, protein, B vitamins, and minerals such as iron. Some soy products contain beneficial fats, and are fortified with calcium and vitamin D, important for bone health. They are a filling and satisfying substitute for meat, and can be eaten raw or cooked, or in plant milks.

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Soy is also found in many types of foods, and a snack bar is fine at times, but the studies focused on whole foods and not supplements. The most recent study showed that under 2 ounces of soy food from whole foods was enough to reduce cancer risk. Most recommendations suggest 1-2 servings per day of soy foods. A serving is 1/2 cup tofu, 1/4 cup edamame beans or soy nuts, and 1 cup soy milk.

Medical food supplements with soy protein necessary for extra nutrition are safe. However, foods processed with protein isolates, isoflavone powders, and soy pills are best avoided.

People with soy allergies and some thyroid conditions should also not eat soy products.

It’s useful to know that soy foods were not the only foods associated with reducing treatment-related symptoms. As little as 2.5 ounces a day of cruciferous vegetables such as broccoli, cabbage, kale, and bok choy, among others, showed the same benefit. And these foods happen to be nutrient powerhouses as well.

Fresh broccoli

Soy foods are often eaten by vegetarians, vegans, and the oldest-living humans on the planet: among them, Seventh-Day Adventists in Loma Linda, California. They are worth keeping in our diets to help enhance the nutrition and broaden our food choices.

For a Creamy Breakfast Smoothie made with soymilk, see my recipe on the blog Creamy Breakfast Smoothie.

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