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Prostate Cancer, Nutrition, and Dietary Supplements (PDQ®)

  • Last Modified: 10/16/2014

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Questions and Answers About Soy

Current Clinical Trials

  1. What is soy?

    The soybean plant has been grown in Asia for food since ancient times. Soy first arrived in Europe and North America in the 18th century. The soybean can be processed into a wide variety of products including soy milk, miso, tofu, soy flour, and oil.

    Soy foods contain many phytochemicals that may have health benefits. Isoflavones are the most widely researched compounds in soy. Major isoflavones in the soybean include genistein (which may be the most bioactive isoflavone), daidzein, and glycitein. Isoflavones protect the soybean plant from stress and have antioxidant, antimicrobial, and antifungal actions.

    Isoflavones are phytoestrogens (estrogen -like substances found in plants) that attach to estrogen receptors in cells. Genistein has been shown to affect many pathways in prostate cancer cells involved in the growth and spread of cancer.

  2. How is soy administered or consumed?

    Soy may be consumed in the diet or taken in dietary supplements.

  3. Have any preclinical (laboratory or animal) studies been conducted using soy?

    Laboratory research and animal studies have been done to find out if soy may be useful in preventing or treating prostate cancer.

    Studies of soy in the laboratory have shown the following:

    • Several laboratory studies have found that treating human prostate cancer cells with isoflavones (such as genistein or daidzein) interferes with pathways in prostate cancer cells related to inflammation and cancer growth and spread.

    • Some laboratory studies have found that treating prostate cancer cells with whole soy extract (containing all the major isoflavones) or combining other plant-based compounds with isoflavones may have more anticancer effects than using single isoflavones. One study compared treating human prostate cancer cells with soy isoflavones, curcumin (a yellow pigment of the spice turmeric that is being studied in cancer prevention), or a combination of the two compounds. Results showed that combining curcumin and isoflavones was more effective in lowering prostate-specific antigen (PSA) levels than using either compound alone.

    Studies of animal models of prostate cancer treated with soy have shown the following mixed results:

    • Strains of mice created to develop prostate cancer that acts like human cancer were fed a diet with genistein or a control diet. Compared with mice on the control diet, the mice fed the genistein diet had less prostate cancer cell growth and lower levels of growth promoting proteins.

    • A study of mice that were genetically modified to produce prostate cancer found that mice fed a low-dose genistein diet had more spread of cancer than mice fed either a high-dose genistein diet or a diet with no genistein. This suggests that the effects of genistein on prostate cancer may vary depending on dose and on how early it is given.

    • A study in mice implanted with advanced human prostate cancer found that those given daily genistein in peanut oil developed more tumors in other parts of the body than mice given peanut oil without genistein.

    • In a study of combining radiation therapy with soy isoflavones, mice implanted with prostate cancer cells were treated with genistein, mixed isoflavones (genistein, daidzein, and glycitein), and/ or radiation. Mixed isoflavones were found to be more effective than genistein in slowing prostate tumor growth. Combining mixed isoflavones with radiation was found to be most effective in slowing tumor growth.

  4. Have any population studies or clinical trials (research studies with people) of soy been conducted?

    Many population studies and clinical trials have been done to find out if soy may be useful in preventing or treating prostate cancer. Soy products studied include dietary supplements, drinks, and bread.

    Population studies

    Population studies look for risk factors and ways to control disease in large groups of people. Population studies of soy intake and prostate cancer risk have shown the following mixed results:

    • A 2009 review of many studies combined showed that men eating large amounts of nonfermented soy foods (for example, tofu and soybean milk) had a lower risk of prostate cancer. Eating large amounts of fermented foods (for example, miso) was not found to affect the risk of prostate cancer.

    • A 2013 review showed that PSA levels and sex hormone levels were not markedly different in men treated with soy, compared with men who were not treated with soy.

    Clinical trials of preventing prostate cancer

    • In a study of Japanese men who underwent a prostate biopsy but who did not have cancer, some received a daily supplement of soy isoflavones (40 mg) and curcumin (100 mg), while others received a placebo. After 6 months, there were no differences in PSA levels between the supplement group and the placebo group overall. However, among patients with higher PSA levels at the start, those who received the supplement had meaningful decreases in PSA levels compared to patients in the placebo group.

    • A study was done to find out if a soy diet standard in Asia would be practical for European men to follow. Healthy men ate either a high-soy (2 daily soy servings) or low-soy (usual) diet for 3 months, then crossed over to the other diet. Lower PSA levels were seen with the high-soy diet. Results showed that study volunteers were able to follow the high-soy diet.

    • Men at risk for prostate cancer or with low-grade prostate cancer were given either soy protein, alcohol -washed soy protein (which is lower in isoflavones), or milk protein (which has no isoflavones) for 6 months. PSA levels did not differ among the groups at 3 months or 6 months. Fewer cases of prostate cancer were found after 6 months in men who consumed either type of soy protein than in men who consumed milk protein.

    Clinical trials of treating prostate cancer

    • In a trial of soy isoflavones, prostate cancer patients with rising PSA levels who had been treated with radiation therapy consumed a soy drink daily for 6 months. The soy drink contained 65-90 mg of isoflavones. Results showed that the soy drink had very few side effects and slowed PSA doubling time (how long it takes PSA levels in the blood to increase by 100 percent). These findings indicate that consuming the soy drink may have helped slow the progression of prostate cancer.

    • In a trial of genistein (a major isoflavone), prostate cancer patients scheduled for radical prostatectomy received either a placebo or genistein (30 mg/ day) for 3-6 weeks before surgery. PSA levels in patients who received genistein decreased slightly while PSA levels in those who received the placebo increased slightly.

    • In a trial of soy isoflavone, prostate cancer patients scheduled for prostatectomy received either capsules (containing 80 mg/day of isoflavones) or a placebo for up to 6 weeks before surgery. There was no difference in PSA, testosterone, or cholesterol level changes between the two groups.

    • A trial of a soy protein supplement (containing 60 mg/ day of isoflavones) studied patients with early-stage prostate cancer. Those who received the supplement for 12 weeks had slightly greater decreases in PSA and testosterone levels than those who received placebo.

    • Trials of whole soy were done in men scheduled for surgery to remove the prostate. In one study, patients given soy supplements for 2 weeks before surgery showed much higher levels of isoflavones in prostate tissue than in blood. In another study, patients who ate high-phytoestrogen bread (containing soy or soy and linseed) had greater decreases in PSA levels than those who ate wheat bread.

    • A trial of a soy isoflavone supplement was done in prostate cancer patients receiving antiandrogen therapy. Side effects of antiandrogen therapy may include sexual dysfunction, lower quality of life, and changes in mental functioning. Men who received the isoflavone supplement (160 mg/day) for 12 weeks showed no improvement in side effects of antiandrogen therapy compared to men who received a placebo.

  5. Have any side effects or risks been reported from soy?

    Soy products and isoflavones have been consumed by prostate cancer patients with very few side effects in many clinical trials. The most commonly reported side effects were minor gastrointestinal symptoms.

  6. Is soy approved by the U.S. Food and Drug Administration (FDA) for use to prevent or treat cancer in the United States?

    The U.S. Food and Drug Administration has not approved the use of soy as a treatment for cancer or any other medical condition.

    Soy is available in the United States in food products and dietary supplements. Because dietary supplements are regulated as foods, not as drugs, FDA approval is not required unless specific claims about disease prevention or treatment are made.

Current Clinical Trials

Check NCI’s list of cancer clinical trials for CAM clinical trials on soy isoflavones for prostate cancer and soy protein isolate for prostate cancer that are actively enrolling patients.

General information about clinical trials is also available from the NCI Web site.