Part of the pea family, this plant is commonly found in southeastern Asia. It makes up a large part of the Asian diet (in foods like tofu and tempeh). Because the rate of prostate cancer is very low in Asia, its high consumption is believed to play a role in cancer prevention. But does it?
Soy products and some of its components—which are called isoflavones—have been widely studied for their effects on cancer, including prostate cancer.
Researchers believe that some isoflavones (in particular one called genistein) may have estrogen-like effects. What isn’t yet clear is whether isoflavones have the ability to stimulate or stop the effects of the hormone estrogen.
While we know of one radiation oncologist who advocates it for all men with advanced prostate cancer, a recent study by Bosland et al (JAMA, July 10, 2013) showed that men at high risk of PSA failure who drank a beverage powder supplement that contained a soy protein isolate every day for two years after undergoing radical prostatectomy did not have a reduced biochemical recurrence of prostate cancer compared to men who received placebo. This was the first randomized clinical trial with cancer as the endpoint. Investigators stopped the trial early for lack of treatment effects.
Hwang et al (2009) concluded in their comprehensive meta-analysis of five cohort studies and 8 case-control studies that consumption of soybean milk, miso, or natto did not significantly reduce the risk of prostate cancer.
On the flip side, some earlier studies suggested potential benefits in men with prostate cancer:
- Pendleton et al concluded from a Phase II study of 20 men with a rising PSA after prior local therapy that dietary intervention with isoflavone supplements may have biologic activity in men with biochemical recurrent prostate cancer. In this study, men drank soy milk containing 47 mg of isoflavonoid per 8 oz serving 3 times per day for a year. PSA had increased 56% per year before the men entered the study, and only increased 20% per year for the 12-month study period. While this was a small study, the decline in the slope of PSA results was statistically significant
- Yan and Spitznagel did an extensive review of all the available data (called a meta analysis) and concluded that dietary soy is associated with a reduced risk of prostate cancer. They also concluded that this benefit may be due to the type of products a man consumes and how much he consumes
- Ahmad et al studied the use of these isoflavones in conjunction with radiation therapy in men with prostate cancer. In this small study, men who had 200 mg when they started radiation experienced less dripping/leakage of urine, less rectal cramping/diarrhea, and less pain with bowel movements after 6 months than men who did not take it. There was also a higher overall ability to have erections.
Researchers, nutritionists, and prostate cancer specialists have not provided guidelines for what types of foods and/or supplements are ideal for men with prostate cancer and how much men should consume each day. Studies should also be conducted to determine if long-term consumption is safe for men with cancer.
Ahmad IU, Forman JD, Sarkar FH, et al. Nutr Cancer. 2010 Oct;62(7):996-1000.
Bosland MC, Kato I, Zeleniuch-Jacquotte A, et al. JAMA. 2013 Jul 10;310(2):170-8.
Hwang YW, Kim SY, Jee SH, Kim YN, Nam CM. Nutr Cancer. 2009;5(61:598-606.
Pendleton JM, Tan WW, Anai S, Chang M, Hou W, Shiverick KT, Rosser CJ. BMC Cancer.2008;5(8):132.
Yan L, Spitznagel EL. Am J Clin Nutr. 2009;89(4):1155-63.