A type of vegetation that can often be found washed
ashore on beaches may soon emerge as a new player in
the field of cancer-fighting foods. A new study led
by researchers at the University of California, Berkeley,
has found that a diet containing kelp seaweed lowered
levels of the potent sex hormone estradiol in rats,
and raised hopes that it might decrease the risk of
estrogen-dependent diseases such as breast cancer in
"This study opens up a new avenue for research leading
to cancer preventive agents," said Martyn Smith, UC
Berkeley professor of environmental health sciences
and co-author of the study. "Kelp is a little studied
nutrient, but there's good reason to look at it more
These new results, to be published Feb. 2 in the Journal
of Nutrition, shine a new light onto the Japanese diet.
Prior studies have shown that Japanese women have longer
menstrual cycles and lower serum estradiol levels than
their Western counterparts, which researchers say may
contribute to their lower rates of breast, endometrial
and ovarian cancers. Scientists have been searching
Asian diets for clues to the lower rates of cancer,
with the lion's share of attention being given to soy.
"Brown kelp seaweed makes up more than 10 percent of
the Japanese diet," said Christine Skibola, assistant
research toxicologist at UC Berkeley's School of Public
Health and lead author of the study. "Soy has gotten
most of the attention, but our study suggests that kelp
may also contribute to these reduced cancer rates among
The researchers say that the type of kelp used in this
study, bladderwrack seaweed (Fucus vesiculosus), is
closely related to wakame and kombu, the brown seaweeds
that are most commonly consumed in Japan. Bladderwrack
seaweed is the primary form of kelp sold in the United
States. They say these study results support the need
for more research on wakame and kombu.
Skibola said she began the animal study after obtaining
encouraging results from earlier case studies of women
with highly irregular menstrual cycles.
"The most profound thing I found was that two women
with endometriosis and a lot of menstrual irregularities
experienced significant improvement in their symptoms
after three months of taking 700 milligrams of seaweed
capsules per day," said Skibola. "It reduced much of
the pain associated with endometriosis and significantly
lengthened the total number of days of their menstrual
cycles. In one of these women with high estrogen levels,
I also saw a drop in blood estradiol levels from 600
picograms per milliliter down to 90 picograms per milliliter
after she included kelp in her diet. That led me to
believe it was worth doing further controlled studies
For the new study, the researchers randomly divided
24 female rats into three groups. One group was fed
a high daily dose of 70 milligrams of dried, powdered
kelp for four weeks, while a second group was fed a
low daily dose of 35 milligrams. Both groups were compared
with a third control group of rats that did not receive
kelp. To ensure that all the kelp was eaten, Skibola
and study co-author John Curry, a UC Berkeley post-doctoral
fellow in molecular and cell biology, sprinkled the
powdered kelp onto apple wedges, one of the rats' favorite
The researchers said the experimental doses of kelp
consumed by the rats were roughly equivalent to the
amount of brown seaweed eaten by people in Japan.
Skibola and Curry took on the task of taking daily
vaginal swabs to monitor the rats' menstrual cycles.
The researchers found that the rats' estrous cycles
increased from an average of 4.3 to 5.4 days for the
low dose kelp group, and to 5.9 days for the high dose
kelp group. Overall, dietary kelp resulted in a 37 percent
increase in the length of the rat estrous cycle.
Studies in humans have linked longer menstrual cycle
lengths to lower risk of breast, ovarian and endometrial
cancers. "If you have longer cycles, you actually have
fewer periods over a lifetime, which means less time
is spent overall in the phases where hormone levels
and breast and endometrial cell proliferation are at
their highest," said Skibola.
During the early part of a woman's menstrual cycle,
estradiol levels remain relatively constant. Almost
halfway through the cycle, estradiol levels surge, peaking
just before ovulation. These cyclic periods of high
estrogen, which continues over a span of about 40 years
from puberty to menopause, stimulates the division of
breast cells that already have DNA mutations, as well
as increases the chances of developing new mutations,
factors that may increase one's risk of breast cancer.
To test the impact of dietary kelp on estradiol levels,
researchers took baseline blood samples from 19 rats
immediately before their low dose diet of kelp began.
After just two weeks of eating 35 milligrams a day,
estradiol levels were reduced from an average of 48.9
nanograms per liter to 40.2 nanograms per liter. After
four weeks, estradiol levels dropped further to 36.7
nanograms per liter.
In a separate test of human ovarian cell cultures,
conducted in collaboration with colleagues at UC Davis,
dosing with kelp extract led to a 23 to 35 percent decrease
in estradiol levels.
"One possibility is that the kelp may be acting as
an estrogen antagonist by preventing estradiol from
binding with its estrogen receptors," said Skibola.
"Our next step is to try to isolate the active compound
in kelp that is having this hormone-modulating effect."
She noted that seaweed contains several complex compounds,
including polyphenols that are considered antioxidants.
Kelp supplements are available in health food stores
since they are taken as a source of iodine by people
with thyroid conditions. However, the researchers caution
against a run on kelp because of these early results,
particularly because kelp can accumulate heavy metals.
"People should be careful about excessive kelp intake,"
said Skibola. "The high levels of iodine and the 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."
The researchers say they are working to isolate the
active compounds in kelp that affect estradiol levels
to avoid the possible toxicity of the iodine and metals.
They say there is hope that kelp could eventually be
used as an anti-estrogen in the treatment of hormone-dependent
cancers if further tests demonstrate its effectiveness
"It's a study that points to the need for more studies,"
said Smith, the study co-author. "But this certainly
suggests that there are other elements of the Asian
diet beyond soy that should be explored."