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Soy and Cancer?

Soy and Cancer?

Soy is a 3 letter word that causes so much confusion! For health, we are encouraged to eat plant sources of protein but remain fearful that choosing soy contributes to cancer, particularly hormone sensitive cancers such as breast and endometrial. Let’s take a look at what we know about soy.

Tofu, tempeh, edamame, soymilk and miso are soy foods that are enjoyed around the world. Soy, a plant-based protein, has all the amino acids your body needs to make protein. Soy is also a good source of fiber and minerals such as potassium, magnesium, copper and manganese. Soy foods like soymilk and tofu are excellent sources of calcium.

As a plant, soy is rich in Isoflavones, a phytonutrient. Isoflavones include genistein, daidzein and glycitein. The isoflavones in some ways mimic the action of estrogen; hence they may be referred to as “phytoestrogens”. Since high levels of estrogen are linked to increased breast cancer risk, this causes soy fear. YET, human studies show soy foods do NOT increase risk of primary breast cancer or recurrence. Initial studies in rodents left many questions, but we now understand that rats and humans metabolize estrogen very differently. In fact cell research shows that soy isoflavones act as a potential tumor suppressor that inhibits cell growth. This tumor suppression from soy foods comes mostly from studies where women had been eating soy foods over a lifetime and therefore had early exposure. We do not know if starting to eat soy foods later in life has the same benefits.


  • Ten grams of soy protein per day (the amount in a cup of soy milk) seems to offer the most benefit with regard to breast cancer recurrence and total mortality. Moderate amounts of whole soy foods are safe according to long-term studies in Asian populations and does not contribute to breast cancer risk. A moderate amount of soy is considered to be up to 3 servings a day with a serving equal to 8-10 grams of protein.
  • Choose whole soy foods. There is less research on concentrated soy products, such as soy protein isolate, used in meat analogs as well as some protein powders and protein bars. These are processed foods and likely not the best for health for a number of reasons.
  • Most soy crops grown in the United States are genetically modified unless they are labeled certified organic, in which case you're in the clear. If you are concerned with GMO, this is one food where you'll want to check labels.
  • Ingredients like soybean oil and soy lecithin are often found in small amounts in dietary supplement pills. This amount of soy is trivial unless, of course, one has an extreme allergic reaction to soy.
  • Soybean oil isn’t necessarily unhealthy, but it does have a higher ratio of omega-6 to omega-3 fat compared with, say, and olive oil. So stick with the latter for you salad dressings and for sautés.

Using Soy in Meals
Unsweetened soy milk can be substituted for dairy milk in any recipe. Toss edamame on a salad, start your meal with a cup of miso soup, use marinated tempeh as a meat substitute in a stir fry or sandwich, or blend silken tofu into smoothies or salad dressing.

Charis W. Spielman, MPH, RD, CSO, CNSC
Certified Specialist in Oncology Nutrition
Certified Nutrition Support Clinician

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