I Eat Soy Every Day. Here's What A Dietitian Said About It.

I eat a lot of organic tofu and tempeh. They're easy for me to digest because they're low-FODMAP. More importantly, they don't make my endo symptoms worse. That's why they've been a big part of my vegan life for the past 25 years. But I've always heard that soy and endo don't mix. So, I wondered: am I hurting my body? Not according to Bethany Doerfler, a clinical dietitian at Northwestern Memorial Hospital in Chicago.

"It became this urban myth that phytoestrogen was bad for you," says Doerfler. "But the data is stripped back. We don't need to be so afraid of soy, even if you have a hormone-positive cancer."*

Both the American Cancer Society and American Institute of Cancer Research agree that soy-based foods are safe for women with breast cancer.¹

Phytoestrogens and endometriosis

The effect of phytoestrogens on endometriosis isn't well understood.² But that's mostly because there isn't a lot of research on it. But I trust Doerfler, who provides medical nutrition advice for people with all kinds of digestive or health problems. That includes irritable bowel syndrome (IBS), which has symptoms that overlap with endo. She recommends tofu and tempeh to her patients who wants to follow a plant-based diet. That's because soybeans are a complete protein source. "They provide the whole spectrum of amino acids, similar to what you would find in animal products," says Doerfler.

For me, tofu and tempeh are a GI-friendly way to get my protein without setting off an endo flare. And they're considered traditional soy foods. That means they're not in the ultra-processed soy category. But, after talking to Doerfler, I still wanted to know more about endo and soy. Here's what the research says.

What's the deal with soy and estrogen?

Soy contains isoflavones. These are estrogenic compounds found in plants. But they aren't the same thing as the estrogen your body makes. For one thing, they don't increase C-reactive protein, an inflammatory marker. But human estrogen can.¹ And phytoestrogens can be both estrogenic and anti-estrogenic.^1,2

To be honest, I'm not sure what that means for people with endo. But plant estrogens, which are weaker than your own estrogen, can bind to the same receptors. You may have less overall estrogen activity if phytoestrogens use the same hub.^1,2  But, like everything else that involves endometriosis, more research needs to be done.

What does science say about soy and endometriosis?

There's not a lot of research on soy and the development and progression of endo. (There's even less on soy and pelvic pain.) But one recent study shows that the amount of phytoestrogens in your body doesn't increase the chances you'll get an endometriosis diagnosis. That may be because dietary phytoestrogens — unlike large supplements tested in animal studies — aren't strong enough to disrupt your endometrium.² This adds to previous research that shows dietary soy isoflavones may decrease the risk of endometriosis in Japanese women.³

In general, experts don't agree on the role of diet in endometriosis. But plant-based foods high in fiber may help you get rid of extra estrogen.¹ And soy-based foods may decrease your chances of endometrial cancer.^2,4 Though, more broad research is needed to back up this claim.⁵

On the anti-soy side, there's some evidence that babies fed with soy-based formula are twice as likely to get endometriosis.⁶ They may have worse period pain when they grow up.⁷ And research shows prepubescent rats fed a high-soy diet — more than 10% of their food intake — were more likely to get endometriosis when they became adults.⁸

Why I avoid some soy

I wouldn't take high doses of soy in a supplement. And some processed soy products do aggravate my symptoms. But it's mostly because they're high-FODMAP and have a lot of other ingredients. No-go foods include soy protein isolate, soy milk, soy-based "meats", and soy-based ice creams. And anything made with soybean oil also gives me cramps. That may be because soybean oil is high in omega-6 fats. Soybean oil isn't necessarily bad for you. But having an imbalance of omega-6 to omega-3 fats may promote inflammation.⁹

So, should you eat soy?

I can't answer for anyone else. But I feel fine when I eat traditional soy products like tofu and tempeh. And since I need a good source of protein, I'm definitely going to keep them around. But everyone is different. So if soy makes you feel bad, then skip it. For now, I feel safe with my soy choices. But I'm going to keep a close eye on the research. And if I find any new discoveries, my endo community will be the first to know.

*October 2019