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Seaweed May Lower Breast Cancer Risk

But Don't Rush Out to Buy Kelp Just Yet, Say Researchers
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WebMD Health News

Feb. 2, 2005 -- The oceans may hold a new clue about preventing breast cancer.

In studies on rats, dietary kelp (seaweed) lowered estrogen production, a hormone needed for some breast cancers to grow. The studies also show that kelp can block the action of certain hormones, which may help lower the risk even further.

It's the latest lead in a hunt for foods that protect against breast cancer. The results suggest that kelp might echo soy's presumed protective, anti-estrogen effect.

But don't make room in your refrigerator for seaweed just yet. The researchers want to learn a lot more about kelp first. Dietary sources of kelp include sushi wraps, and miso soup may come sprinkled with brown kelp.

"People should be careful about excessive kelp intake," says researcher Christine Skibola in a news release. "The high levels of iodine and low levels of heavy metals contained in kelp means that it's not recommended for people who are pregnant, nursing, or who have hyperthyroid conditions."

Seaweed: A Staple of Japanese Diets

The researchers' interest stems from Japanese cuisine. "Brown kelp seaweed makes up more than 10% of the Japanese diet," says Skibola, an assistant research toxicologist at the University of California, Berkeley.

Japan has one of the world's lowest rates of breast, endometrial, and ovarian cancers. The low cancer rates have been attributed to the high dietary intake of soy. But there's more to the Japanese diet than soy.

Kelp is also widely eaten in Japan. Having seen kelp help two women with endometriosis and menstrual irregularities, Skibola wanted to study it further.

Around 2 million women living in the U.S. have been treated for breast cancer, and about 211,000 will be diagnosed this year, says the American Cancer Society. Approximately two-thirds of those breast cancers are estrogen-dependent.

Testing Kelp's Powers

The researchers studied 24 female rats. Some rats got standard food with no kelp. Others received a diet of high or low doses of kelp. Both kelp doses were in line with Japanese kelp consumption.

Rats don't normally eat kelp, so the researchers sprinkled apple wedges with dried bladderwrack seaweed from Maine. The rats gobbled up the kelp-laced apples, following the diets for four weeks.

After four weeks on the kelp-supplemented diet, the rats showed a 25%-38% reduction in estrogen levels compared with the start of the study.

Longer Cycles, Less Estrogen Exposure

Kelp also lengthened the rats' menstrual cycle by up to 37%.

High estrogen levels stimulate the division of breast cells with gene mutations and increase the chances of new mutations. That can raise breast cancer risk, says a news release.

Women with longer menstrual cycles have fewer cycles over their lifetime. They spend less time over the years in their cycle's high-estrogen phase. Japanese women have longer menstrual cycles and lower estrogen levels than their Western counterparts, write the researchers.

The researchers also tested kelp on human ovarian cells. The cells cut estrogen production by 18%-35%, depending on the kelp dose. Kelp also was able to block estrogen's ability to bind with the ovarian cells.

More Work Needed

Skibola's team is searching for kelp's active ingredients, which may yield new breast cancer treatments. The researchers also want to study wakame and kombu seaweeds, which are favored in Japan. They also call for bigger studies to see how kelp affects another female reproductive hormone, progesterone.

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