"This is the kind of thing that could potentially be used as therapy," said Christine Skibola, a Berkeley toxicologist and lead author of the study. "I was anxious to get it out there, because at this point there aren't any true dietary anti-estrogen compounds."

Though the research was in rats, the findings could have benefits for humans as well. In Japan, where brown kelp seaweed makes up about 10 percent of the diet, women have lower rates of breast cancer than in the United States.

Skibola was inspired to do the rat study after she learned that kelp supplements had helped some women with irregular menstrual cycles and endometriosis, a common disease in women.

Skibola gave female rats doses of powdered kelp similar to the amount of kelp in typical Japanese diets. The kelp supplements lowered the rats' levels of estradiol and lengthened their menstrual cycles by about 37 percent from a little more than four days to 5½ or six days.

Estradiol is known to cause mutations that can lead to cancer and also to encourage the growth of already existing cancerous cells.

Previous studies in humans have linked longer menstrual cycles with lower rates of breast, ovarian and endometrial cancers. This is likely because it reduces the number of menstrual cycles a woman has in her lifetime, and consequently her exposure to estradiol, which peaks just before ovulation.

"During the month, you get this surge of estradiol," Skibola said. "So the more exposure you get to these high levels, the greater risk you have."

Need clinical trials

Skibola's team used a north Atlantic kelp species called bladderwrack, which is the most common type of kelp sold in the United States. It is closely related to the types of brown seaweed most often eaten in Japan, wakame and kombu. Skibola plans to do similar studies on the Japanese kelp in the future to see if it has the same effects.

The results show a lot of promise for potential cancer prevention or treatment, but the findings are preliminary, and the scientists warn against taking in too much kelp. The iodine and metals it contains could be toxic at high levels.

"We don't want people to run out and take large quantities of kelp supplements," said Skibola. "That could be dangerous in some people."

Increasing the amount of whole kelp, rather than concentrated supplements, in your diet is a safer way to go, said Skibola.

The team is currently working on isolating the compounds in the kelp that are acting to lower the estradiol, so that it could be taken without the risk of toxicity from metals and iodine. The next step will be clinical trials in women.

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