Deploy Soy
The protective benefits of soy consumption may depend on the time period when soy is consumed
People come to the United States for countless reasons, seeking opportunities, perhaps a chance at a better life. Many adopt the local fashion, new customs and foods prepared in different ways. Yet for Asian-American women, research hints that assimilating their new Westernized eating habits-which include fewer traditional soy foods-might mean more risk of breast cancer.
Preventive medicine researchers at USC/Norris Comprehensive Cancer
Center recently found that eating soy foods regularly, especially
during adolescence, may lower the risk of breast cancer. The
study is part of ongoing research projects in the U.S. and Singapore
to better understand why breast cancer rates are increasing among
Asian and Asian-American women.
If physicians can pick out the reasons behind the growing breast
cancer rates, they may be able to suggest practical steps that
all women can take to reduce cancer risk. One reason behind the
increase might be declining soy consumption.
In the journal Carcinogenesis, the USC/Norris researchers announced
that Asian-American women who consumed soy foods on a weekly
basis during their teen years and adulthood had about half the
risk of developing breast cancer compared to similar women who
ate little soy during the same time periods.
Risk also was somewhat lowered for women who ate soy regularly
during their teen years but ate little soy during adulthood.
The initial data suggest little added benefit, though, for women
who did not eat a lot of soy until adulthood.
"There has been a lot of talk and controversy about the
Asian diet and connections between soy food intake and breast
cancer. We wanted to look at soy carefully, to better understand
if soy by itself is protective or if the level of soy consumption
is just a marker for acculturation," says Anna H. Wu, Ph.D.,
professor of preventive medicine at the Keck School of Medicine
of USC.
Wu and colleagues conducted a case-controlled study of breast
cancer among Chinese, Japanese and Filipino women in Los Angeles
County, specifically looking at the importance of soy. From 1995
to 1998, they interviewed 501 Asian-American breast cancer patients
and compared them to 594 healthy Asian-American women.
The researchers asked about eating habits, including how many
times each week during adolescence they ate tofu. They also asked
about the frequency and amounts of whole soy foods such as tofu,
soy milk, miso and fresh soybeans usually eaten during adulthood.
Intake was highest among Chinese, intermediate among Japanese
and lowest among Filipinas. Women born abroad ate a little more
soy than American-born women did. Most of the Chinese and Filipino
women in this study-more than 90 percent-were born in Asia, compared
to less than 30 percent of the Japanese women.
When women were grouped by how often they ate soy during adolescence
and adult life, researchers found that women who were high consumers
during both time periods had a 47 percent reduction in risk.
Those who ate little soy during adult life but were regular soy
consumers during adolescence showed a 23 percent reduction in
risk.
Women who were low consumers during adolescence and high consumers
during adulthood showed little reduction in risk. However, the
number of such women was small, and researchers note that larger
studies must be conducted to confirm this result.
Scientists are not sure how soy works, but they have some ideas.
Isoflavones, certain plant-based chemicals, may play a part.
Studies in animals, for one, indicate that early life exposure
to genistein-the main isoflavone in soybeans-seems to help protect
against chemically induced breast tumors. Getting genistein early
in life may help the mammary glands develop in a favorable way.
Wu and the Keck School's Mimi C. Yu, Ph.D., professor of preventive
medicine, Frank Z. Stanczyk, Ph.D., professor of research in
obstetrics and gynecology, and colleagues at the National University
of Singapore also have found some intriguing suggestions from
their Singapore Chinese Health Study.
Women in Singapore, China and Japan traditionally had low breast
cancer rates, but these rates have soared over the past few decades-just
as they have among Asian-American women in the U.S. To find out
why, the researchers have been studying the diets and lifestyle
histories of more than 35,000 postmenopausal women in Singapore.
In a recently published paper, the team reported that study participants
who ate the most soy foods had 15 percent less circulating estrone
in their blood than other participating women. Estrone is the
predominant estrogen circulating in postmenopausal women.
Evidence suggests that the more the body is exposed to estrogen,
the greater the breast cancer risk. Estrogen stimulates breast
cells to divide and multiply, which seems to add up over time
to increased breast cancer risk.
They also found circulating estrogen levels rose according to
women's body mass index, a measure of body fat. In addition,
circulating estrogen levels were higher among women who had their
first period at age 16 or younger and those who had their first
child after age 30 or had no children at all. However, soy's
links to estrogen levels were independent of these factors, according
to the study in Cancer Epidemiology, Biomarkers & Prevention.
And in another study involving Yu, researchers found that Singapore
women with diets highest in soy had a nearly 60 percent lower
chance of having high-risk, dense breast tissue than women who
ate the least soy.
Regardless of the protective mechanism, Wu notes that if the
mounting research shows promise, many may ask, "How much
soy is enough?" There may be a point at which eating more
soy does not further reduce risk, she says.
Researchers are still seeking to understand the relationship
between soy food dose and risk reduction. In the Los Angeles
study, the greatest risk reduction was seen among women with
the highest level of consumption: 12.7 mg or more of isoflavones
for every 1,000 kilocalories consumed during adulthood, and eating
soy foods four or more times a week during adolescence.
According to the latest recommendations, a moderately active
50-year-old woman who is 5-foot-5-inches tall and weighs between
111 and 150 pounds, for example, should get about 2,100 to 2,300
kilocalories a day from all foods in her diet.
Isoflavone levels in soy foods vary considerably, but a serving
of store-bought tofu-a quarter of a box-contains about 10 mg
of isoflavones, Wu says.
Wu notes that a much larger study is needed to sort out the benefits
of adult soy food intake and what levels of soy intake are most
helpful. Her study looked only at traditional soy foods, not
at genistein or other isoflavones that may be contained in pills
and supplements.
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