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 anti-estrogens, 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 was 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.