Johns Hopkins and Georgetown University
researchers conducted a meta-analysis
of 18 epidemiologic studies revealing
that women who eat soy products may
have a slightly lower risk of developing
breast cancer. But the researchers quickly
add that inconsistencies and limitations in
the studies raise doubt about the potential
benefit of soy, and they warn women that
high-dose supplements could do more harm
than good.
"At this point, women should not be taking
high-dose soy supplements, especially
those who are breast cancer survivors and
women at increased risk for the disease," said
Bruce Trock, associate professor of urology,
epidemiology, oncology and environmental
health sciences at
Johns Hopkins' Brady
Urological Institute and
Kimmel Cancer Center. "We don't have long-term data on
the effects of these supplements, and there is
some evidence that they could be harmful."
Tests of refined soy products in animals
reveal increased tumor growth, and shortterm
studies of women taking the supplements
show changes in breast cell growth
that might actually increase risk for breast
cancer.
Results of the review by Trock and his
colleagues at Georgetown, published in the
April 5 issue of the Journal of the National
Cancer Institute, found inconsistencies
among the studies. Many differed in whether
or not—or how—they accounted for the
many factors that may impact development
of breast cancer, such as body mass index.
Studies that took BMI into account showed
that soy had less of a protective effect than
those that ignored BMI. "This is consistent
with the idea that people who eat a lot of soy
most likely eat fewer calorie-heavy foods and
are less likely to have high BMI," Trock said.
"This means that breast cancer risk may not
be due to eating soy products but to other
dietary or lifestyle factors."
After averaging results from the studies,
which span a quarter century, the overall
relative reduction in breast cancer risk for
soy eaters was a modest 14 percent in Caucasian
women. The effect was not statistically
significant in Asian women. Differences in
pre- and postmenopausal women were slight
and potentially biased due to the small number
of studies that accounted for menopausal
status.
Soy became a popular cancer prevention
topic in the 1980s after early laboratory data
showed that high doses of compounds within
soy, called isoflavones, may block estrogens,
promote cell death and have anti-inflammatory
qualities. Soy products like tofu and bean
curd are eaten widely by Asian women, who
have low breast cancer rates, at least until
they move to the United States, where their
daughters' rates begin to approach those of
Caucasian women. Some believe this may be
due to changes in diet, and the observation
triggered research to find which Asian foods,
like soy, might provide cancer protection.
"We need to take into account that
Asian women are more physically active,
drink less alcohol, have children earlier,
and their entire diet is different from Western
women's, all of which decrease their
breast cancer risk," Trock said.
Many of the studies reviewed also failed
to gauge the amount of soy women actually
consumed because it is also found in
products where it is not expected, such as
doughnuts, white bread, instant coffee and
canned tuna. "At the time most of these
studies were done, few Western women were
eating tofu, but many were getting soy without
noticing it," said Trock, who believes
that this confounds self-reporting of soy
intake and may affect the ability to measure
potential health benefits. Future studies, he
said, should include urine samples taken at
different times to improve measurements of
total soy exposure.
Trock and his colleagues also found
inconsistencies between the soy dosage and
biological response. If soy were beneficial,
researchers would expect a "some is good,
more is better" result. It would also mean
that at some point the dose-response "curve"
would flatten, the effect would plateau,
and no additional amount of soy above the
threshold would improve protection.
But instead, the researchers said their
review found that Western women eating
low doses of soy appeared to have protection
from breast cancer equal to or greater than
Asian women with very high intake of soy.
This would suggest that the benefits of soy
reach their peak at a very low dose, but this
effect is not supported by prior animal or tissue
culture data.
Trock also points to evidence that soy
exposure early in life may be most important.
One of the studies he reviewed showed
that Asian-American soy eaters born in Asia
had more protection from breast cancer than
those born in the United States, possibly
reflecting soy exposure in utero and before
puberty. "But there are no studies that have
followed women long enough to solidify
this," he added.
Trock says that definitive studies, which
track participants over time and before they
get cancer, may take decades before the real
impact of soy is known.
For now, Trock says that there is no problem
with adding soy to the diet since it has health
benefits regardless of whether it protects
against breast cancer. "The important aspect
is eating actual soy-based foods like tofu, not
highly purified isoflavone supplements," he
said. "Highly refined components of soy can
have very different biological effects than eating
tofu or drinking soy milk."
Instead of pill supplements for people who
want to add soy to their diet, Trock suggests
replacing some meats with soy foods such as
tofu, soy milk or soy nuts.
Additional study authors are Leena
Hilakivi-Clarke and Robert Clarke, both of
Georgetown University.
Funding was provided by the National
Cancer Institute and the American Institute
of Cancer Research.