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Which Seaweed Is Most Protective Against Breast Cancer?

Sushi lovers, rejoice. Nori seaweed snacks may favorably alter estrogen metabolism by modulating women’s gut flora, resulting in decreased breast cancer risk.

As traditional East Asian diets have westernized, breast cancer rates have risen. Some researchers have linked this to a quadrupling of animal product consumption. If you click on the above video, you can see the breast cancer rate of Japanese women living in Japan. Within 10 years of immigrating to the United States, their risk increases, and if they hang around long enough their risk goes up even more. Note in the graph in the video, though, it’s still somewhat lower than the U.S. national average. This may be because of some of the dietary habits they carry with them—soy and green tea consumption, perhaps eating more mushrooms—but maybe it’s partially due to seaweed.

We’ve known for over a decade that in vitro (in a Petri dish) seaweed broth is effective at clearing cancer cells. In the video you can see three different types of human breast cancer exposed to either a widely used chemotherapy drug or a sea vegetable. The seaweed worked even better. And unlike the chemo, it didn’t hurt normal, non-cancerous breast cells. What about outside of the test tube, in people?

Well, a population study comparing women with breast cancer to women without found that consuming a single sheet of nori a day may cut a woman’s odds of breast cancer in half. We think it’s because seaweed favorably alters estrogen metabolism, likely due to modulating women’s gut bacteria. The more seaweed we eat, the less estrogen we have in our system, which may lower breast cancer risk.

The breast cancer protection may be because of all the fiber in sea vegetables, or because seaweed may block the enzyme that undermines our body’s attempt to flush out excess hormones. Or seaweed may somehow interfere with estrogen binding to estrogen receptors. Whatever the cause, to effectively lower their estrogen levels, Asian women may be able to get away with about one sheet of nori a day, but American women are physically so much larger that it may take closer to two. There are lots of yummy seaweed snacks out there to make it a tasty experience—just try to get some low-fat, low-sodium ones. They’re just like kale chips, munchies made out of dark green leafy vegetables—can’t beat that!

For more on lowering breast cancer through diet:

The seaweed used fresh in seaweed salads, wakame, unfortunately did not appear to reduce breast cancer risk. Wakame consumption has, however, been found to lower blood pressure in hypertensives (people with high blood pressure). Just two teaspoons of seaweed salad a day for a month dropped people’s blood pressure 14 points, and two months of wakame was associated with up to a 2 inch skinnier waistline.

As I’ve mentioned before, I’d recommend avoiding hijike, which tends to have too much arsenic (see Avoiding Iodine Deficiency), and kelp, which tends to have too much iodine (see Too Much Iodine Can Be as Bad as Too Little). In fact, too much seaweed of any type may actually increase one’s risk for thyroid cancer because of the amount of iodine you’d be taking in, but there does not appear to be any increased risk at the levels of consumption I’m talking about, like a sheet of nori every day. And a study of seaweed eaters in California actually found decreased risk, but, again, we’re talking modest levels of intake.

I’ve frequently talked about the benefits of dietary diversity, eating different families of fruits and vegetables, eating different parts of individual plants—such as beets and beet greens. If we just stick to land plants, though, we’re missing out on all the plants from the other 70% of planet earth. Sea vegetables have phytonutrients found nowhere else, special types of fiber, and unique carotenoids and polysaccharides, and various polyphenol defense compounds, each of which may have anti-cancer properties. I encourage everyone to try experimenting until you find a sea vegetable you like, even if that means just sprinkling some powdered dulse on your food. More on the importance of dietary diversity in Garden Variety Anti-Inflammation, Apples and Oranges: Dietary Diversity, and Constructing a Cognitive Portfolio.

For more on some of the other protective dietary components in the diets of Japanese women, check out Why Do Asian Women Have Less Breast Cancer? and Breast Cancer Survival and Soy.

And if you dig anti-cancer comparison videos, make sure you also check out Which Fruit Fights Cancer Better? and #1 Anticancer Vegetable.

In health,
Michael Greger, M.D.

PS: If you haven’t yet, you can subscribe to my free videos here and watch my live year-in-review presentations Uprooting the Leading Causes of Death and More Than an Apple a Day.

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Read more: Health, Cancer, Diet & Nutrition, Eating for Health, General Health, Videos, Women's Health, , , ,

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Dr. Michael Greger

A founding member of the American College of Lifestyle Medicine, Michael Greger, M.D., is a physician, author, and internationally recognized speaker on nutrition, food safety, and public health issues. Currently Dr. Greger serves as the Director of Public Health and Animal Agriculture at The Humane Society of the United States. Hundreds of his nutrition videos are freely available at


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8:48PM PST on Dec 9, 2013


10:11AM PST on Dec 9, 2013

thanks. I think live in peace no stress feeling good in life every moment is the key for having a

good health....

1.. shrimp.....pomegranate.......lemon......beet....strawberry ......ginger.....are the best to fight breast cancer....

7:28PM PST on Dec 6, 2013

Thank you.

7:56PM PST on Dec 5, 2013

It's quite obscene how I can eat a whole package of seaweed snacks like if it was a canister of I don't feel so bad about it ;O)

6:24PM PST on Dec 5, 2013

Sushi is good! Not only on seaweeds but also those deliciously carefully cut salmon. Yummmmy!

4:14PM PST on Dec 5, 2013

My 12 year old grandson introduced me to seaweed and it is one of our snack staples.

2:39PM PST on Dec 5, 2013

Seaweed has benefits -- it has been known to be healthy for you for a very long time -- but there are other causes of breast cancer other than dietary habits or behavior -- genetics plays a role...

2:04PM PST on Dec 5, 2013

What's with the seductive looking lips there ??

Doesn't have anything to do with breast cancer.....

1:06PM PST on Dec 5, 2013

Thanks Dr. Greger for the video and interesting article.

12:20PM PST on Dec 5, 2013

So, for the actual seaweed-averse, are the seaweed derivatives used in foods (carageenan, agar) also beneficial?

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