"I don't eat soy. I heard it feeds cancer."

I've heard this many times. Or a version of it: "When my mom had breast cancer, her doctor told her to stay away from soy, so I do too." And when I ask when that was, the answer is almost always the same: decades ago.

Where the soy warning came from

That advice made sense to doctors at the time. Estrogen can fuel the growth of many breast cancers, so anything that seemed to act like estrogen raised a red flag.

In early animal studies, rodents were given large doses of genistein, a single compound pulled out of soybeans in a lab. They grew more tumors, and the idea that "soy acts like estrogen" stuck. But mice process soy very differently than we do. And none of us are eating genistein, a lab extract. We're eating real food: tofu, edamame, and soy milk.

One compound is not the whole food

We've seen this before. In the 1990s, a large study giving beta carotene pills to smokers was stopped early because lung cancer rates went up. Yet foods rich in beta carotene, like carrots, are linked to lower cancer rates.

One compound on its own doesn't act like the whole food. It's like judging a salad by drinking the vinegar. Nobody eats it that way, and nobody eats soy that way either.

What the research in women shows now

The research in women has caught up, and it points the other way. Soy's isoflavones are often called plant estrogens, but they don't behave like the estrogen your body makes. In studies of women, soy foods haven't increased breast density or breast cell growth. The old warning just never got updated, so it keeps getting handed down.

For women who have already had breast cancer, the news is even better. A 2024 analysis of six studies with nearly 12,000 breast cancer survivors found that women who ate the most soy had a 26% lower risk of their cancer coming back. A 2025 review of the research concluded that women with breast cancer can safely eat soy foods.

How much soy is that?

The women in these studies ate about two or three servings of soy a day. That adds up quickly. One serving is:

• 1 cup of unsweetened soy milk
• About 3 ounces of tofu
• ½ cup of edamame

Good news for men, too

Men who eat more soy foods tend to have a lower risk of prostate cancer. And studies show soy doesn't lower testosterone.

An easy way to start

If soy has never been part of your meals, start with the easiest one. Edamame tastes a lot like fresh peas, and soy milk works anywhere you'd use regular milk.

Try this tonight: warm a cup of frozen shelled edamame and toss it with a squeeze of lemon and a pinch of salt.

Two things to keep in mind

1 - Stick with whole soy foods. The reassuring research is on food, not soy pills or powders.

2 - If you take tamoxifen or an aromatase inhibitor, the research so far shows soy foods are safe to eat while you're on them. It's still worth mentioning to your oncologist.

Breast surgeon Dr. Kristi Funk and Venus Williams made a short, upbeat video about food and breast cancer. Watch it here.

So go ahead and put soy on your plate. That's advice worth handing down.

Common questions about soy

Does soy mess with your hormones? Soy's isoflavones are called plant estrogens, but they don't act like the estrogen your body makes. In studies of women, soy foods haven't increased breast density or breast cell growth. In men, studies show soy doesn't lower testosterone.

Can I eat soy if I've had breast cancer? The research says yes. In a 2024 analysis of nearly 12,000 breast cancer survivors, women who ate the most soy had a 26% lower risk of their cancer coming back. If you take tamoxifen or an aromatase inhibitor, soy foods appear safe too, but mention it to your oncologist.

Is soy milk healthy? Unsweetened soy milk is an easy way to get a serving of soy. One cup counts as one serving, and it works anywhere you'd use regular milk: in oatmeal, coffee or a smoothie. Check the label and skip the sweetened versions.

What about soy protein powder or isoflavone pills? I'd skip them. The reassuring research is on whole soy foods like tofu, edamame and soy milk, not on extracts. Remember the beta carotene pills: one compound on its own doesn't act like the whole food.

Is edamame the same as soybeans? Yes. Edamame are young green soybeans, usually sold frozen, in or out of the pod. Shelled edamame is the easiest way to add soy to a meal tonight.

Sources

1 - Van Die et al., JNCI Cancer Spectrum, 2024: soy isoflavones and breast cancer recurrence (Johns Hopkins news release)

2 - Current Nutrition Reports, 2025: narrative review of soy foods and breast cancer

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