Which Seaweed is Protective Against Cancer?

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Which Seaweed to Help Prevent Breast Cancer?

Sushi lovers, rejoice. Nori seaweed 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. In my video Which Seaweed Is Most Protective Against Breast Cancer, 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, although it is 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 and 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 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 two 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 our risk for thyroid cancer because of the amount of iodine we’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 a modest level 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 enjoy anti-cancer comparison videos, make sure to also check out Which Fruit Fights Cancer Better? and #1 Anticancer Vegetable.

-Michael Greger, M.D.

PS: If you haven’t yet, you can subscribe to my videos for free by clicking here and watch my full 2012 – 2015 presentations Uprooting the Leading Causes of Death, More than an Apple a Day, From Table to Able, and Food as Medicine.


Michael Greger M.D., FACLM

Michael Greger, M.D. FACLM, 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.

26 responses to “Which Seaweed to Help Prevent Breast Cancer?

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  1. I am wondering about fat-free organic plain yogurt. Since it has all the fat removed, this form of dairy has very little toxins – PCBs – hormones and other pollutants since, I have read, most of this junk is in the fats, not the proteins. Are there any compelling reasons why just the yogurt protein would be harmful for humans? Even if one does not have any allergy?

      1. I had considered the same thing, but I thought that these xenoestrogens were only present in the fat, not in the protein, thus making fat-free yogurt void of xenoestrogens, no?

        1. I cannot speak directly to knowing if xenoestrogen is only present when fat is. I doubt this, but will have to do a more in depth search to be certain.

    1. Dairy protein, casein, is probably the worst for increasing IGF-1 in the body, a significant cancer risk increase. Full fat or fat free makes no difference.

    2. Nevo: To expand a tiny bit on the other good replies you have already gotten: Diary protein is animal protein. Animal protein has been linked to harmful levels of IGF-1 increases, which is linked to getting cancer.

      As Geoffrey pointed out, there are some compelling studies showing that one of the dairy proteins is particularly good at feeding cancer. You can read The China Studies and/or watch Forks Over Knives to learn more about those studies.

      I also recommend finding the series of videos on this NutritionFacts website about IGF-1. After watching the series, I think you will understand why/how animal proteins, even with the fat removed and even if there were guaranteed to be no contaminants, are (all else being equal) a problem.

  2. I used to eat a lot of seaweed for the reasons stated above, but now I’m a bit skeptical because of the radioactivity in a lot of the seaweed from the nuclear leak in Japan after the tsunami. Any updates or news on this?

  3. I have some nori at home ever since Dr. G’s video on the subject, but I have a hard time stomaching its “fishy” flavor. Still trying little bits of it from time to time.

    1. MacSmiley: It may not work, but like you, I too try to make myself eat various foods from time to time in order to build up a taste for them. It does work.

      But if you are having a tough time, I have some ideas that might help:

      1) don’t eat it separately. Crumple it up into little bits and put it in your soup or casseroles, etc. I find when I do this that the nori seems to add to the dish without really affecting flavor

      2) deliberately add nori to foods that you want to taste “fishy”. For example, lots of people make a ‘tuna salad’ out of tofu and/or beans. And while the nori would give the dish an un-authentic color, it would add an authentic taste.

      3) go against Dr. Greger’s recommendation and put a *tiny* pinch of dried kelp in your oatmeal. This is something that I have actually started doing because my favorite nori sheets come in those packages with oil and salt. I don’t dislike the plain nori sheets terribly, but they are hard for me to get excited about and remember to eat daily. So, I take a chance that a granule or two or three of kelp once or twice a week will work out in a way that provides a healthy amount of iodine for me. I have to caution that Dr. Greger has a video about how kelp has too much iodine and another video about how too much iodine is bad for you. So, use this method with a big, “buyer beware” note in your head.

      Hope that helps! (and thanks again for all your great participation on this site. I really enjoy reading your comments.)

    2. Satisfying Quick Wraps:
      Hummus, Kombu Strands, Cilantro, Baby Arugula, Romaine Lettuce. (YUM!)

      High Fat Complicated Wraps:
      Rice, Hummus, Avocado, Julienned Anything: (Apples-Carrots-Cucumber-Zucchini-YellowSquash), Sesame seeds, Romaine Lettuce, Jicama, Bean Sprouts, Tofu, Wasabi, Tamari.

      Remember to only moisten the last inch of nori on the inside and keep the seam on the bottom after rolling. Let it sit for a minute on its seam to let seal itself.

    1. There are two types of Nori as food sold in grocery store. One is greasy and salty (probably has other chemicals); the other is plain. The plain one is for making sushi while the flavored one is usually cut into palm size or smaller, as snacks. I always buy the plain one. You can use it to wrap lettuce if you can’t eat it by itself.

  4. What about radiation and nori. I read an article about the unbelievably high level of radiation found in nori in W. Los Angeles. Many people have stopped eating seaweed altogether because of this issue.

  5. Dr Greger, I recently had a rotary scar removed from my breast. The surgeon/oncologist wants me to take Evista (along with MRIs and mammograms at 6 month rotations). I read up on the Evista. It looks a bit scary in terms of stroke, etc. Is the drug referenced in your article an anti- estrogen drug like Evista? If so, I am relieved to see that the seaweed won out. However, do you know if the seaweed is relatively free of toxins–given the polluted state of our oceans?

  6. Great article as always! Question – is seaweed vegan? I know that it is a sea vegetable, however, there’s this notion going around that seaweed is produced and packed along with seafood and that a lot of residue from the sea food gets stuck to the seaweed that we buy on stores nowadays… Is there any truth in this concept? Or is it bogus? Thank you in advance! And thank you for the work you do!

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