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 Previous nutrition research has linked soy to a longer life, but
prior studies have also suggested soy may help tumors spread by
making hormone-based cancer treatments, or endocrine therapy, less
effective.
For the current study, researchers examined data on 6,235 U.S. and
Canadian women with breast cancer. More than half of the women were
followed for at least nine years.
During the course of the study, women who ate the most foods
containing soy were 21 percent less likely to die than individuals
who consumed the smallest amounts of soy.
“All women or all breast cancer survivors can add soy as a component
of a healthy diet,” said lead study author Dr. Fang Fang Zhang, a
nutrition and science policy researcher at Tufts University in
Boston.
The study focused on isoflavone, a compound in soybeans that can be
found in foods like tofu, miso, edamame and soy milk. Isoflavone is
in a family of plant compounds known as phytoestrogens that are
chemically and structurally similar to the female sex hormone
estrogen.
Researchers found the strongest link between soy and survival for
women with certain aggressive forms of breast cancer that can’t be
treated with hormones. They didn’t see a link between soy
consumption and longevity for women with tumors that depend on
estrogen to grow or women receiving endocrine therapy.
“Our findings, taken together, indicate that soy food consumption
does not have a harmful effect for women treated with endocrine
therapies, but the benefit may be limited to women with negative
tumor hormone receptors or those who were not treated with endocrine
therapies,” Zhang added by email.
For the study, researchers examined data from dietary questionnaires
for women diagnosed with breast cancer from 1996 to 2011. They
excluded women who died within one year of completing the first
questionnaire.
At the start, women were 52 years old on average and most had at
least some education beyond high school.
About 47 percent took hormone therapy for tumors.
On average, women consumed 1.8 milligrams of isoflavone daily,
roughly the amount in one ounce of soy cheese and far less than a
3-ounce serving of tofu or a half-cup portion of edamame.
During the study, 1,224 women died.
Overall, women with the highest quartile of dietary isoflavone
intake, an average of at least 1.5 milligrams a day, were 21 percent
less likely to die than women in the lowest quartile who got less
than 0.3 milligrams a day, researchers report in Cancer.
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For women with tumors not fueled by hormones, the highest amount of
isoflavone intake was tied to 51 percent lower odds of death from
all causes.
Among women who didn’t receive hormone therapy, the highest level of
dietary isoflavone was associated with 32 percent lower odds of
death from all causes.
One limitation of the study is that women who ate more foods with
soy tended to be more affluent and educated, with healthier
lifestyles, making it possible that other factors beyond dietary
isoflavone influences longevity, the authors note.
Researchers also lacked data on the type and length of hormone
therapy for women who received this treatment, which could also
impact survival odds.
Still, the findings build on previous research in Asia that linked
high dietary soy intake to a lower risk of developing breast cancer,
said Dr. Omer Kucuk, a researcher at the Winship Cancer Institute of
Emory University in Atlanta.
“It is not surprising that this study showed women in North America
also benefit from soy intake with reduced mortality from breast
cancer while on treatment,” Kucuk, author of an accompanying
editorial, said by email.
“Up until now physicians generally discouraged their breast cancer
patients from eating soy foods because of potential harm,” Kucuk
said. “We can now encourage women with breast cancer to eat soy
foods because it is safe and it will likely improve their survival.”
SOURCE: http://bit.ly/2mtkC6R Cancer, online March 6, 2017.
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