beneficial and harmful effects of soy consumption in breast cancer
patients have been a longstanding debate.
- The timing
of soy's introduction into a patient's diet can have a positive or
negative impact during breast cancer treatment, finds a new study.
consumption of soy helps build up the immunity against breast cancer and
reduce the recurrence.
The positive and negative impact of soy consumption on a common breast
cancer treatment has been revealed using animal models by a research team at
the Georgetown Lombardi Comprehensive Cancer Center.
The scientists have found the biological pathways in a rat model by
which regular consumption of soy improves the effectiveness of tamoxifen
and reduces breast cancer
recurrence. The team also showed the negative impact of eating soy-based foods
for the first time while being treated with tamoxifen, which reduce the
effectiveness of the drug and promote recurrence.
‘Breast cancer patients who start eating soy after the diagnosis may be at higher risk of cancer recurrence following tamoxifen treatment.’
How Soy Consumption Affect Breast Cancer
The research team uncovered how the active compounds in soy -
isoflavone and genistein - can affect tamoxifen
both positively and negatively.
"The study also mirrors what has been observed in breast cancer
patients," said the study's senior investigator Leena Hilakivi-Clarke,
Ph.D., professor of oncology at Georgetown Lombardi.
"There has long been a paradox concerning genistein, which has the
similar structure as estrogen and activates both human estrogen receptors to a
degree. Estrogen drives most breast cancer growth, yet high soy intake among
women in Asian countries has been linked to a breast cancer
rate that is five times lower than
Western women, who eat much less soy," said Hilakivi-Clarke.
"So why is soy, which mimics estrogen, protective in Asian
About 1.67 million women were diagnosed with breast cancer worldwide.
More than 70 percent of the breast cancer cases were estrogen-receptor
positive. Tamoxifen and other endocrine therapies are the common drugs used for
estrogen-receptor positive breast cancers to reduce the ability of estrogen to
promote cancer growth. Endocrine therapies are effective in treating
. However, about half of the breast cancer patients on
endocrine therapies exhibit resistance or have cancer recurrence.
Timing of Soy's Introduction is the Key
The research team used a more advanced rat model of breast cancer and
tamoxifen and found that the timing of genistein intake is the critical issue.
of soy-based diet prior to breast cancer development can improve
immunity against cancer and reduce the risk of breast cancer recurrence.
"Longtime sustained use of genistein before the development of breast cancer
improves overall immunity against
cancer, thus protecting against cancer development and recurrence," said the
study's lead researcher, Xiyuan Zhang, Ph.D.
"It also inhibits a mechanism called autophagy that would allow
cancer cells to survive, which explains why it helps tamoxifen work,"
Previously conducted studies have shown no evidence of adverse effects
of soy intake on breast cancer outcome. The research team said that Asian
and Caucasian women who consumed one-third cup of soymilk daily (10mg of
isoflavones) had the lowest risk of breast cancer recurrence.
In the animal model, the research team found that
consumption of soy after the diagnosis of breast cancer did not trigger the
"We do not know yet why this made the animals resistant to the
beneficial effects of tamoxifen and increased risk of cancer recurrence,"
Animals consuming soy had a 7 percent chance of breast cancer
recurrence after tamoxifen treatment, compared with a 33 percent recurrence
when exposed to soy only after breast cancer developed.
"We have solved the puzzle of soy and breast cancer in our rat
model, which perfectly explains the paradox seen in earlier animal studies and
patients," said Hilakivi-Clarke.
"While many oncologists advise their patients not to take
isoflavone supplements or consume soy foods, our findings suggest a more
nuanced message -- if these results hold true for women. Our results suggest
that breast cancer patients should continue consuming soy foods after
diagnosis, but not to start them if they have not consumed genistein
- Xiyuan Zhang, Katherine L. Cook, Anni Warri, Idalia M. Cruz, Mariana Rosim, Jeffrey Riskin, William Helferich, Daniel Doerge, Robert Clarke, Leena Hilakivi-Clarke. Lifetime Genistein Intake Increases the Response of Mammary Tumors to Tamoxifen in Rats. Clinical Cancer Research, 2017; 23 (3): 814 DOI: 10.1158/1078-0432.CCR-16-1735