Posted on

Is Soy Safe?

Dr Susan Blum, Functional MedicineComing on the heels of breast cancer awareness month in October, I decided to write about soy, a food that causes much confusion about whether or not it is safe to eat. Women are cautioned not to eat soy because of its effects on hormones and the thyroid gland. I would like to weigh in here, because just like everything else in nutrition and medicine, this is a personalized decision, and I will give you information to help you make the choice that is right for

First, let’s talk about soy and hormones…

Soy contains phytonutrients called isoflavones that are structurally similar to estrogen and bind to estrogen receptors, thus they are referred to as phytoestrogens. While this might suggest that soy is the same as estrogen in the body, it is not a hormone and, therefore, does not act in the body like the hormone estrogen. Instead, it is called a SERM: selective estrogen receptor modulator. The isoflavones in soy can bind as a blocker or a stimulator, depending on the tissue of the body, and therefore has both estrogenic and anti-estrogenic activity. This turns out to be good since it has stimulating effects where we want it, such as the bone, and blocking effects where we want it, such as the breast.


Soy isoflavones simulate the estrogen receptors in the bone and blood vessels, thus making it beneficial in preventing and treating osteoporosis and cardiovascular disease. It has also been found to have an anti-depressant effect; reduces hot flashes in menopausal women; and reduces LDL cholesterol.

What about cancer? The evidence is pretty clear that soy is protective. Here are some facts…

The incidence of breast and ovarian cancers in vegetarians is low, and studies have confirmed that a phytoestrogen-rich diet may offer protective benefits. While initial studies done on mice showed concern about stimulating growth of breast cancer, recent studies in humans show different results. Mice metabolize the soy differently and now we think the studies weren’t valid for humans. Epidemiological studies all show that soy reduces risk of breast cancer recurrence, and it is clear that soy does not stimulate breast tissue growth like conventional hormone replacement with estrogen.


Now that the concern about cancer is cleared up, the other issue to discuss is the thyroid and the type of soy food to choose.

First, let’s discuss thyroid. Soy is considered a goitrogen, which means that it can cause an enlarged thyroid and hypothyroidism by binding to iodine and preventing the thyroid from using it. This is much like kale and cruciferous vegetables as I talked about in a previous newsletter. This does not mean that you can’t eat soy as part of a regular, balanced diet. It just means that you need to make sure that you are eating food with iodine, using iodized salt, and making sure that your multi has iodine in it. This will ensure adequate iodine intake and prevent the soy from stealing all of it. I believe the goitrogenic effect of soy happens when there is also an iodine deficiency. I generally eat soy 2-3 times each week, and feel this is beneficial.

What kind of soy is good for you?

Mary will talk more about this in the Nutrition section that follows, but the focus is on organic, non-GMO soy in its whole form, like tempeh, tofu, edamame, and miso is great too. Avoid processed soy, like soy protein shakes, soy hot dogs, soy burgers, and all foods that have soy protein isolate added to it. The problem is that when soy is processed this way, it loses the integrity of the protein and no longer has the benefits or acts normally in the body, like whole soy does. And most of this soy is GMO (genetically modified organism), which is a topic for another newsletter!

And finally, some people do have food sensitivity to soy, and therefore should avoid it. The best way to know is to remove it 100% from your diet for 3 weeks and then eat it again, paying attention for any kind of reaction, whether gut, joints, headache, fatigue. If you react, then it’s not for you!

And to answer a question I am asked all the time, soy lecithin is made from the fat in the soy, and is not typically a problem if you have soy sensitivity. But, we recommend avoiding it if you have a true soy allergy requiring the use of an epi-pen. Soy lecithin is commonly used in vitamins and supplements.