Initial Posting: 4/3/2018    Last Update: 4/3/2018
Answer: The relationship between soy or soy isoflavone intake and the risk of breast cancer and prostate cancer is not clear-cut. Soy contains compounds called phytoestrogens, such as isoflavones, which have weak estrogen-like activity. Soy and soy isoflavones may have varying effects depending on age, estrogen status and type of cancer.
In women, high soy consumption early in life may help to lower the risk of breast cancer but there is concern that high doses of isoflavones may not be safe in women with estrogen receptor-positive breast cancer and may also slightly increase the risk of uterine cancer. For more information, see the What It Does and Concerns and Cautions sections of the Menopause Supplements Review.
In men, higher soy intakes are generally associated with a lower risk of prostate cancer, but higher intakes of soy isoflavones are associated with an increased risk of advanced prostate cancer. For more details about this, see the Soy section of the Protein Powders and Shakes Review.