A recent study has found that high soy consumption may be linked to breast cancer—but the results are far from conclusive. Published in the Journal of the National Cancer Institute, the study recruited 140 women with invasive, early-stage breast cancer and divided them into two groups; one group received 51.6 grams of soy protein daily for 7 to 30 days, while the other received a placebo. Among the women taking soy, researchers observed a statistically significant increase in the expression of certain genes thought to promote cancer cell growth. Here is where the study results get murky:
Although gene expression increased in the soy group, there was no statistically significant increase in a marker of actual cancer cell growth compared to the placebo.
Women consumed soy protein in amounts equivalent to about 1.5 liters of soy milk daily, which may be much higher than in a typical diet, and is much higher than one serving of some soy protein supplements.
In observational studies of Asian populations that regularly consume soy, soy intake is associated with a decreased risk of breast cancer.
These considerations indicate that, at the very least, the present study has some important limitations that future research should address.
Source: Journal of the National Cancer Institute