How to Block Breast Cancer’s Estrogen-Producing Enzymes

The vast majority of breast cancers start out “hormone-dependent,” meaning the primary human estrogen, called “estradiol plays a crucial role in [breast cancer] development and progression.” That’s one of the reasons why soy food consumption appears so protective against breast cancer—because soy phytoestrogens, like genistein, act as estrogen-blockers. They block the binding of estrogens, like estradiol, to breast cancer cells.

But, wait a second. “The majority of breast cancers occur [after menopause], when the ovaries have [stopped producing estrogen].” What’s the point of eating estrogen blockers if there’s no estrogen to block? It turns out the breast cancer tumors themselves produce their own estrogen from scratch to fuel their own growth.

Estrogens may be formed in breast tumors by multiple pathways. The breast cancer takes cholesterol, and, using the aromatase enzyme, or two hydroxysteroid dehydrogenase enzymes, produces its own estrogen.

So, there’s two ways to stop breast cancer. One is to use “antiestrogens,” estrogen-blockers, like the soy phytoestrogens, or “the anti-estrogen [drug] tamoxifen…However, another way to block estradiol is by using anti-enzymes” to prevent the breast cancer from making all the estrogen in the first place.

And, indeed, there are a variety of anti-aromatase drugs in current use. In fact, inhibiting the estrogen production has been shown to be “more effective” than just trying to block the effects of the estrogen—”suggesting that the inhibition of estrogen synthesis is clinically very important for the treatment of estrogen-dependent breast cancer.” It turns out soy phytoestrogens can do both.

Using ovary cells taken from women undergoing in vitro fertilization, soy phytoestrogens were found to reduce the expression of the aromatase enzyme. What about in breast cancer cells, though? Breast cancer cells, too—not only suppressing aromatase activity, but the other estrogen-producing enzyme, too.

But, this is in a petri dish. Does soy suppress estrogen production in people too Well, circulating estrogen levels appear significantly lower in Japanese women than American white women. And, Japan does have the highest per capita soy food consumption. But, you don’t know it’s the soy until you put it to the test. Japanese women were randomized to add soymilk to their diet—or not—for a few months. Estrogen levels did seem to drop about a quarter in the soymilk-supplemented group. Interestingly, when they tried the same experiment in men, they got similar results: a significant drop in female hormone levels, with no change in testosterone levels.

These results, though, are in Japanese men and women that were already consuming soy in their baseline diet. So, it’s really just looking at “higher versus lower…soy intake.”

What happens if you give soymilk to women in Texas? Circulating estrogen levels cut in half. Since increased estrogen levels are a “[marker] for high risk for breast cancer,” the effectiveness of soy to reduce estrogen levels may help explain why Chinese and Japanese women have such low rates of breast cancer.

And, what was truly remarkable is that estrogen levels stayed down a month or two, even after they stopped drinking it. This suggests you don’t have to consume soy every day to have the cancer-protective benefit.

Doctor’s Note

Wait, soy protects against breast cancer? Yes, in study after study after study. Even in women at high risk? See BRCA Breast Cancer Genes & Soy.

Even if you already have breast cancer? See Is Soy Healthy for Breast Cancer Survivors?

Even GMO soy? See GMO Soy & Breast Cancer.

Okay, then, Who Shouldn’t Eat Soy? Watch that video too! 🙂

What else can we do to decrease breast cancer risk? See:

If you haven’t yet, you can subscribe to my videos for free by clicking here.

In health,
Michael Greger, M.D.

Michael Greger, M.D., is a physician, New York Times bestselling author, and internationally recognized professional speaker on a number of important public health issues. Dr. Greger has lectured at the Conference on World Affairs, the National Institutes of Health, and the International Bird Flu Summit, testified before Congress, appeared on The Dr. Oz Show and The Colbert Report, and was invited as an expert witness in defense of Oprah Winfrey at the infamous “meat defamation” trial. Currently Dr. Greger proudly serves as the Director of Public Health and Animal Agriculture at the Humane Society of the United

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