Although there is a concern regarding the safety of soy food consumption
among breast cancer survivors, researchers have found that women in
China who had breast cancer and a higher intake of soy food had an
associated lower risk of death and breast cancer recurrence, according
to a study in the December 9 issue of JAMA.
"Soy foods are rich in isoflavones, a major group of phytoestrogens
that have been hypothesized to reduce the risk of breast cancer.
However, the estrogen-like effect of isoflavones and the potential
interaction between isoflavones and tamoxifen have led to concern about
soy food consumption among breast cancer patients," the authors write.
Xiao Ou Shu, M.D., Ph.D., of Vanderbilt University Medical Center,
Nashville, Tenn., and colleagues examined the association between soy
isoflavone intake with breast cancer recurrence and survival. The
researchers analyzed data from the Shanghai Breast Cancer Survival
Study, a large, population-based study of 5,042 female breast cancer
survivors in China. Women ages 20 to 75 years with diagnoses of breast
cancer between March 2002 and April 2006 were recruited and followed up
through June 2009. Information on cancer diagnosis and treatment,
lifestyle exposures after cancer diagnosis, and disease progression was
collected at approximately 6 months after cancer diagnosis and was
reassessed at three follow-up interviews conducted at 18, 36, and 60
months after diagnosis. A Shanghai Vital Statistics Registry database
was used to obtain survival information for participants who were lost
to follow-up.
After a median (midpoint) follow-up of 3.9 years, 444 total deaths
and 534 recurrences or breast cancer-related deaths were documented
among the group of 5,033 surgically-treated breast cancer patients. Soy
food intake, as measured by either soy protein or soy isoflavone intake,
was inversely associated with mortality and recurrence. Patients in the
group with the highest intake of soy protein had a 29 percent lower
risk of death during the study period, and a 32 percent lower risk of
breast cancer recurrence compared to patients with the lowest intake of
soy protein. The adjusted 4-year mortality rates were 10.3 percent and
7.4 percent and the 4-year recurrence rates were 11.2 percent and 8.0
percent, respectively, for women with the lowest and highest groups of
soy protein intake.
"The inverse association was evident among women with either estrogen
receptor-positive or -negative breast cancer and was present in both
users and nonusers of tamoxifen," the researchers write.
"In summary, in this population-based prospective study, we found
that soy food intake is safe and was associated with lower mortality and
recurrence among breast cancer patients. The association of soy food
intake with mortality and recurrence appears to follow a linear
dose-response pattern until soy food intake reached 11 grams/day of soy
protein; no additional benefits on mortality and recurrence were
observed with higher intakes of soy food. This study suggests that
moderate soy food intake is safe and potentially beneficial for women
with breast cancer."
Editorial: Challenges in Design and Interpretation of Observational Research on Health Behaviors and Cancer Survival
Rachel Ballard-Barbash, M.D., M.P.H., of the National Cancer
Institute, Bethesda, Md., and Marian L. Neuhouser, Ph.D., of the Fred
Hutchinson Cancer Research Center, Seattle, write in an accompanying
editorial that while this study provides important information, there
are several concerns, including differences in the quality, type and
quantity of soy food intake between China and the U.S. (47 mg/d vs. 1 to
6 mg/d, respectively, average isoflavone intake). Also, they point out
the relatively short median follow-up time of the study (4 years); that
there likely are differences in screening rates in China compared with
the U.S.; and a number of factors may make it difficult to compare
stage- and treatment-specific results in China with outcomes in the U.S.
"Even though the findings by Shu et al suggest that consumption of
soy foods among breast cancer patients is probably safe, studies in
larger cohorts are required to understand the effects of these foods
among diverse clinical subgroups of breast cancer patients and
survivors. In the meantime, clinicians can advise their patients with
breast cancer that soy foods are safe to eat and that these foods may
offer some protective benefit for long-term health. Moreover, the
potential benefits are confined to soy foods, and inferences should not
be made about the risks or benefits of soy-containing dietary
supplements. Patients with breast cancer can be assured that enjoying a
soy latte or indulging in pad thai with tofu causes no harm and, when
consumed in plentiful amounts, may reduce risk of disease recurrence."
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