Does Soy Cause Breast Cancer?

No. In case I lose you, the most important thing is that you walk away with this: soy does not cause breast cancer.

I used to avoid soy like the plague. I thought, like many people, that soy consumption is rocket fuel for breast cancer. If soy led to breast cancer, what other health implications could it cause? Seemed best to just keep my distance and never find out.

The day I discovered that soy does not, in fact, cause breast cancer or any other health complications for that matter, is etched clearly in my memory because it wasn’t an ordinary day. I was getting a facial treatment from my health-conscious friend and those days are always memorable regardless of the conversation.

Her and I would always exchange notes and talk health related stuff, and this was long before a food and health related website was even on my radar and let me tell you what – it’s not easy to find someone else to nerd out on this stuff with.

But on this particular day we were talking about different plant-based food items and she asked me if I liked soy milk. I told her that I didn’t drink it. “Why?” she asked, I could hear the curiosity in her tone. I told her I didn’t drink soy milk because of the phytoestrogens, “They’re estrogen mimickers,” I stated, with zero backing behind my bold claim.

My eyes were closed but I could feel her reaction. She paused briefly, probably out of shock that I didn’t know any better. “Phytoestrogen is a lot different than estrogen,” she said carefully, “It doesn’t mimic estrogen. Soy milk is very healthy.” Hmm…soy milk healthy? That’s the first I’d heard about that. I made a mental note to look into this later on, hoping my friend’s health wasn’t in danger.

That afternoon, feeling zen after my facial, I set up my chair in the sunny yard to keep my relaxing day rolling. Sitting in the warm dappled shade, I took a sip of my lemon mint cooler and flipped my book, How Not To Die by Dr. Michael Greger, open to where I left off. And this is where things were so unusual that it became memorable.

Two paragraphs in, I kid you not, the topic was suddenly about soy and phytoestrogens. The timing was eerily impeccable. This is a 412 page book, and the coincidence was blaring. Everything my friend had briefly mentioned was not only accurate, but there was way more to it.

Not only does soy not cause breast cancer, but soy is protective against it. I could tell she was hesitant to say any more about the topic because she was at work and being professional, but in hindsight I made such a blatantly incorrect statement that she should have grabbed me by the shoulders and shaken some sense into me.

What IS soy?

Soy is not some scary substance, oozing with cancer-causing estrogen. It’s a legume! I never associated eating black beans, pinto beans, lentils, garbanzo beans as risky or unhealthy – I’ve always given myself a gold star when eating those legumes. Why would soy beans be any different? Turns out, they’re not.

Why does everyone think soy is linked to breast cancer?

I was under the impression that soy caused breast cancer but I can’t pinpoint when and where I got this information. Can you? I had such a negative conception towards soy, I avoided it at all costs.

“Playing with fire,” I’d think to myself, shaking my head and passing by a soy product at the grocery store. Where did this thinking stem from? No one bats an eye when you reach for a handful of almonds, an apple, or eat a bowl of strawberries in a single sitting – yet those all have phytoestrogens, too.

Soy got its bad reputation in the 70s and 80s, and it’s the reason why we all have the same belief bookmarked in our minds that soy is linked to breast cancer. Here’s what happened: studies were done using mice, the scientists grafted human breast cancer cells on the rodents and then fed them a diet filled with soy. The cancer cells grew. Aha! The isoflavones (phytoestrogen) must fuel the breast cancer growth! Well, yes, it certainly did in mice. The problem is, we aren’t mice. Sometimes human bodies behave in a similar way as mice and sometimes they don’t. In the case of soy, humans metabolize isoflavones differently than mice do. But they didn’t know that at first.

Finally, after decades of assumption that the isoflavones in soy fuels breast cancer growth, studies were done on actual humans in 2009. Not a single study showed cancer growth with soy consumption. Whats more, the studies only showed beneficial effects from soy consumption, including lowered risk of breast cancer occurrence, recurrence, and death. ¹

The original study, despite being debunked, ruined soy’s reputation. It’s difficult to get a population to change their mind about something once they’ve formed an opinion. All these years I thought I was dodging a cancer bullet by avoiding soy products but I was actually missing out on a pretty amazing nutrition source.

Why is soy healthy?

Here comes the science, but stick with me! It’s interesting, I promise.

Soybeans contain natural plant compounds, isoflavones, which are responsible for most of the health benefits. Isoflavones are phytoestrogen. This is part of the problem: people hear the word “estrogen” in “phytoestrogen” and assume it’s exactly the same as the estrogen that we and other mammals produce. But that isn’t the case, phytoestrogens are significantly weaker than the estrogen your body makes.

Phytoestrogen doesn’t function entirely like estrogen, either. It also has anti-estrogenic properties.²³ What does that mean? When you consume phytoestrogens, your body produces less active forms of estrogen, which is an anti-estrogenic effect. This is the reason soy products have the ability to lower breast cancer risk. Phytoestrogens can also help reduce menopausal hot flashes and potentially increase bone density, a pro-estrogenic effect.⁴

Bone density is an especially important issue in postmenopausal women! Due to the decline in estrogen, bone loss and osteoporosis can occur. Studies have found that soy isoflavones may significantly reduce the risk of bone loss and fractures by increasing bone mineral density.⁵⁶

Why doesn’t soy phytoestrogens cause breast cancer?

Not only is soy phytoestrogen significantly weaker than mammalian estrogen, but there’s more to it than that.

There are two types of estrogen receptors: alpha and beta. Alpha receptors signal breast cell proliferation while the beta receptors have the opposite action – they oppose breast cell proliferation. The phytoestrogens in soy bind to the ERβ, the beta receptors. This is why soy can lower breast cancer risk and reduce menopausal symptoms!

Is there anything that soy can’t do?

It can’t cause breast cancer! Haha. But seriously, it can’t.

What if someone has already had breast cancer? Researches have found that women diagnosed with breast cancer who ate the most soy lived significantly longer and had a lower risk of recurrence than those who ate less. Even just a single cup of soy milk a day could lower the risk of breast cancer returning by 25%!⁷

Heart disease is the #1 killer in the United States. Can soy help in that department? It certainly can! Isoflavones also reduce blood vessel inflammation and improve their elasticity, reducing risk of stroke and heart disease.

What about Man Boobs?

Gynecomastia, the medical term for man boobs, can be caused by one of several factors: obesity, diet, hormones, medication, and/or genetics.

But what about soy, can it contribute to man boobs? Nope! Well, I guess that’s another thing soy can’t do: it can’t cause man boobs.

Studies have been done and not a single one has shown any correlation between soy intake and gynecomastia.

“If you see a shirtless man at the beach and he has very prominent man boobs, go up to him and ask if he eats a lot of tofu. I guarantee the answer will be no.”
– Dr. Neal Barnard, on the misconception that soy causes man boobs

Beer and…breast cancer?

Beer is toted as the manly man drink while a glass of soy milk is shunned as a feminine drink that contributes to breast cancer and man boobs.

But did you know that beer has phytoestrogens, too? Actually, the hops in beer contain the strongest phytoestrogen of them all, known as 8-PN or hopein.⁸

Now, these phytoestrogens do behave more like mammalian estrogen, so much so that it’s a common ingredient in breast enhancing supplements. Rather than binding to the beta receptors like soy does, 8-PN prefers to bind to alpha receptors – which signals breast cell proliferation.⁹

This could be why beer may be more carcinogenic to the breast than, say, a glass of red wine.

If you’re worried about man boobs, put down the beer and pour yourself a glass of soy milk. Seriously. If anything is going to cause moobs, it’s going to be beer.

What about milk?

If everyone is worried about the phytoestrogen in soy milk, why aren’t they sounding the alarm with cow’s milk?

Cows don’t just have milk magically appear in their udders at any given time, ready for the taking. Just like human mothers, in order for a cow to produce milk she has to have given birth to a baby. The milk she produces during her pregnancy is a perfect concoction of nutrients designed to make her calf grow very quickly. Her milk does, in fact, contain sex hormones such as estrogen and progesterone.

Sadly, her baby calf doesn’t get to drink his milk, and is instead taken away from her in a wheelbarrow after birth so that her milk can be taken for human consumption. It’s monstrous and highly disturbing, and it’s the reason I quit dairy.

But back to the dairy itself.

Dairy has been linked to all sorts of hormone-related conditions including acne, lowered male reproductive potential, and premature puberty. Along with all the mammalian estrogens is also IGF-1 (insulin-like growth factor-1), which increase hormonal cancers such as breast, prostate, and endometrial.¹⁰ Women who drink milk appear to have five times the rate of twin births compared with women who don’t drink milk, and it’s thought to be the hormone content that can be thanked for that.¹¹

"Women who consumed 1/4 to 1/3 cup of cow’s milk per day had a 30% increased chance for breast cancer. One cup per day increased the risk by 50%, and 2-3 cups were associated with an 80% increased chance of breast cancer. Vegans (no dairy, no cheese, no eggs) experience less breast cancer than non-vegetarians."¹²¹³

“It is thought that the great concern about dairy’s hormone content is steroid hormones, especially estrogens, due to current epidemiological evidence, which indicating most probable effects of them in initiating and provoking of breast and prostate cancers.”¹⁴

Incredibly, corporations aren’t required to label everything that’s in dairy products. Would you buy that package of butter if the progesterone, estrogen, IGF-1, and even the pus concentrations were listed on the label? I would have given it up decades ago if I knew.

Mountain views and chocolate milk

If you read my post about why cheese is addictive, this will be a repeat story. But it was relevant then and it’s relevant now and too fitting to leave out.

I was sitting at a hike summit with a friend, we stopped for lunch and found a nice spot facing the rugged Icicle Peaks. I pull out my tempeh sandwich and he gave it a quizzical look, “What is that?!” My plant-based diet has always puzzled him. He said he’d never heard of tempeh so I explained what it was. “Oh, it’s SOY,” he scowled. “Aren’t you worried about getting breast cancer eating that?” he asked me as he took a swig of his thermos of hot chocolate. The timing of his question was impeccable. I explained the soy misconception and he grunted in vague acknowledgement. I know that grunt; it was in one ear and out the other. I pointed at his hot chocolate and asked him if he was worried about breast cancer drinking that. He chuckled with disbelief, why on earth would milk give anyone breast cancer?

He’s 70 years old and one of the most stubborn people I know, so I’m not going to change his mind on anything. The majority of the general public is still, years after the soy misconception has been debunked, under the impression that soy is dangerous while milk is essential for strong bones (it’s not). I was under that same impression until a mere few years ago! In this age of information with all sorts of facts at our fingertips, it’s incredible that these misconceptions aren’t immediately debunked and quickly public knowledge overnight.

1 out of 8 women will get breast cancer in her lifetime. Those are terrible and tragic odds. Breast cancer doesn’t occur overnight; by the time a lump is detected it’s been growing for potentially a decade or longer.

Soy does not cause breast cancer, it can actually help prevent it! If you take care of your body, it will take care of you. So remember, eat like your life depends on it!

Thanks for reading! If you enjoyed my article, leave me a comment! 🙂

4 thoughts on “Does Soy Cause Breast Cancer?”

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David says:

March 8, 2022 at 12:25 pm

Wow ! It’s surprising to me that these days , in the year 2022 , with all of the world’s knowledge at your fingertips , that myth-conceptions about soy are still rampant . As this article explains quite clearly , eating soy products is both a component of a healthy diet , as well as a way to maximize your longevity on this planet ! WooHoo 🥳 ! !

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