EVIDENCE-BASED ANSWER:
No, it doesn’t affect the risk of primary breast cancer, but it does (favorably) affect the risk of cancer recurrence.
Compared with diets low in soy, high dietary intake of soy protein or soy isoflavones isn’t associated with any alteration in the risk of developing primary breast cancer (strength of recommendation [SOR]: B, systematic review of prospective cohort studies). In patients with breast cancer, however, consuming a diet high in soy is associated with a 25% decrease in cancer recurrence and a 15% decrease in mortality (SOR: B, prospective cohort studies).
EVIDENCE SUMMARY
A large systematic review evaluated the relationship between dietary soy intake and risk of a primary breast cancer diagnosis. It included 7 prospective cohort studies, which comprised the best quality evidence available (numerous other reviewed studies were of lower quality). The review found no significant association between dietary soy intake and primary breast cancer (TABLE1-6).
Investigators either surveyed women for intake of soy isoflavones or soy foods or products (tofu, soybeans, lentils, miso) or measured urinary or plasma levels of soy isoflavones. They adjusted for age, alcohol use, smoking status, body mass index, caloric intake, and hormone replacement therapy, then followed subjects for 7 to 23 years, comparing the risk of breast cancer for the lowest and highest levels of soy intake.
Six of the prospective cohort studies found no association between soy intake and breast cancer risk; one study, comprising 4% of the total population, found a lower risk with higher soy intake (effect size=0.44; 95% confidence interval [CI], 0.26-0.73; an effect size of 0.2 is considered small, 0.6 medium, and 1.2 large). The authors didn’t do a meta-analysis of the prospective cohort studies.
Other cohort studies yield similar findings
Four other large systematic reviews evaluating soy intake and breast cancer risk incorporated a total of 6 individual prospective cohort studies that weren’t included in the previously described review (again, these studies comprised the best quality evidence within the reviews). The 6 studies found no association between soy intake and breast cancer risk.
In 2 of the studies, investigators surveyed postmenopausal women and followed them for 4 to 8 years.2 Investigators in another study adjusted for age, family and gynecologic history, hormone and medication use, exercise, and other factors.3 In 2 other studies, investigators evaluated population subsets that consumed the most vs the fewest servings per week or kilograms per year of soy foods.4 The sixth study compared low with high intake of soy foods and miso.5