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For men, prostate cancer is the most prevalent adenocarcinoma in the developed world. Although rates in Asia have historically been far lower than those in Western countries, they are rapidly rising. Between 1978-1997, prostate cancer rates increased by as much as 100% in Japan and Singapore, and by 50% in some areas of the Philippines and China. Some of this rise in prostate cancer may have resulted from enhanced detection and aging of the population, much of the increase is tought to be due to a westernization of lifestyle, including reduction in consumption of soyfoods. This hypothesis is supported by studies showing that prostate cancer incidence increases significantly after native Asians migrate to the Unites States. On the other hand, autopsy series demonstrate that the incidence of latent prostate cancer is approximately equal in men from Asia and the United States (1). Nevertheless African American men have the highest incidence of prostate cancer which is roughly 30 times greater than Japanese men and about 120 times greater than that observed in men from Shangai (China) (2).

One or more factors present in Japanese culture appear to slow the growth and/or delay the onset of prostate tumours, since migration data show that differences in prostate cancer mortality rates are environmentally, not genetically, determined. Men who move from low-risk to high-risk countries early in life can acquire the same cancer risk as that of their adopted country, even within the same generation (3). Delaying the clinical appearance of prostate tumours by even a few years would have a significant public health impact, since prostate cancer is generally a disease of older men. Consequently, men would die with, not of, their cancer.

Most epidemiological studies suggest that soy consumption is associated with reduced risk of prostate cancer. Although case-control studies have reported mixed associations (4-11), the best prospective studies have shown that soy intake is associated with lowered prostate cancer risk (12-15). In fact, a meta-analysis of eight studies published in 2005 reported that consumption of soy foods is associated with a 30% decrease in prostate cancer risk (16). A prospective study showed in 2007 that soy intake was associated with an even greater 50% lowering of risk of localized, but not advanced prostate cancer in Japanese men older than 60 years of age (15).


Mechanism

A number of mechanisms have been proposed to explain how soy consumption may lower risk of prostate cancer. Given that prostate cancer is a hormone-dependent cancer, it has been proposed that soy consumption reduces prostate cancer risk as a result of effects on circulating androgen and estrogen concentrations. Prostate cancer cells are stimulated by testosterone, thus a decrease in testosterone concentrations would be expected to reduce prostate carcinogenesis. Similarly, an increase in estrogens, known to antagonize testosterone, would be expected to reduce prostate carcinogenesis. But this hypothesis has not been confirmed in clinical trials: it has been proved that consumption of soyfoods or soy isoflavones does not alter total or free androgen concentrations. Similarly, circulating estrogens appear to be unaffected by soy or isoflavone consumption. Another suggested mechanism by which soy may reduce prostate cancer risk is via effects on concentrations of prostate specific antigen (PSA). However the effects of soy and isoflavone supplementation on circulating PSA concentrations have been mixed. In men with prostate cancer, soyfood interventions have significantly decreased serum PSA (17-18), but several controlled studies have not shown a statistically significant effect of soy or isoflavone consumption. Importantly, some studies have found a significant decrease in rate of rise in PSA that occurs over time in prostate cancer patients (19-20). Just recently it was shown in prostate cancer patients that the consumption of 700 ml of soymilk per day for 1 year resulted in slower rise of PSA-levels (21).

In conclusion

The American Cancer Society includes eating soya as one step men can take to reduce their risk of prostate cancer. The International Prostate Health Council concluded that soya prevents the progression of the latent form of prostate cancer to the more advanced form of this disease (22).

References

  1. Shimizu H, Ross RK, Bernstein L. Possible underestimation of the incidence rate of prostate cancer in Japan. Jpn J Cancer Res 1991;82:483-5.

  2. Moyad MA, Sakr WA, Hirano D, Miller GJ. Complementary medicine for prostate cancer: effects of soy and fat consumption. Rev Urol 2001;3 Suppl 2:S20-30.:S20-S30.

  3. Shimizu H. Cancers of the prostate and breast among Japanse and white immigrants in Los Angeles County. Br J Cancer 1991;63:963-966.
  4. Kolonel LN, Hankin JH, Whittemore AS, et al. Vegetables, fruits, legumes and prostate cancer: a multiethnic case-control study. Cancer Epidemiol Biomarkers Prev. Aug 2000;9(8):795-804.

  5. Sonoda T, Nagata Y, Mori M, et al. A case-control study of diet and prostate cancer in Japan: possible protective effect of traditional Japanese diet. Cancer Sci. Mar 2004;95(3):238-242.

  6. Ozasa K, Nakao M, Watanabe Y, et al. Serum phytoestrogens and prostate cancer risk in a nested case-control study among Japanese men. Cancer Sci. Jan 2004;95(1):65-71.

  7. Lee MM, Gomez SL, Chang JS, Wey M, Wang RT, Hsing AW. Soy and isoflavone consumption in relation to prostate cancer risk in China. Cancer Epidemiol Biomarkers Prev. Jul 2003;12(7):665-668.

  8. Strom SS, Yamamura Y, Duphorne CM, et al. Phytoestrogen intake and prostate cancer: a case-control study using a new database. Nutr Cancer. 1999;33(1):20-25.

  9. Villeneuve PJ, Johnson KC, Kreiger N, Mao Y. Risk factors for prostate cancer: results from the Canadian National Enhanced Cancer Surveillance System. The Canadian Cancer Registries Epidemiology Research Group. Cancer Causes Control. Oct 1999;10(5):355-367.

  10. Sung JF, Lin RS, Pu YS, Chen YC, Chang HC, Lai MK. Risk factors for prostate carcinoma in Taiwan: a case-control study in a Chinese population. Cancer. Aug 1 1999;86(3):484-491.

  11. Oishi K, Okada K, Yoshida O, et al. A case-control study of prostatic cancer with reference to dietary habits. Prostate. 1988;12(2):179-190.

  12. Severson RK, Nomura AM, Grove JS, Stemmermann GN. A prospective study of demographics, diet, and prostate cancer among men of Japanese ancestry in Hawaii. Cancer Res. Apr 1 1989;49(7):1857-1860.

  13. Mills PK, Beeson WL, Phillips RL, Fraser GE. Cohort study of diet, lifestyle, and prostate cancer in Adventist men. Cancer. Aug 1 1989;64(3):598-604.

  14. Jacobsen BK, Knutsen SF, Fraser GE. Does high soy milk intake reduce prostate cancer incidence? The Adventist Health Study (United States). Cancer Causes Control. Dec 1998;9(6):553-557.

  15. Kurahashi N, Iwasaki M, Sasazuki S, Otani T, Inoue M, Tsugane S. Soy product and isoflavone consumption in relation to prostate cancer in Japanese men. Cancer Epidemiol Biomarkers Prev. Mar 2007;16(3):538-545.

  16. Yan L, Spitznagel EL. Meta-analysis of soy food and risk of prostate cancer in men. Int J Cancer. Nov 20 2005;117(4):667-669.

  17. Dalais FS, Meliala A, Wattanapenpaiboon N, et al. Effects of a diet rich in phytoestrogens on prostate-specific antigen and sex hormones in men diagnosed with prostate cancer. Urology. Sep 2004;64(3):510-515.

  18. Ornish D, Weidner G, Fair WR, et al. Intensive lifestyle changes may affect the progression of prostate cancer. J Urol. Sep 2005;174(3):1065-1069; discussion 1069-1070.

  19. Messina M, Kucuk O, Lampe JW. An overview of the health effects of isoflavones with an emphasis on prostate cancer risk and prostate-specific antigen levels. J AOAC Int. Jul-Aug 2006;89(4):1121-1134.

  20. Hamilton-Reeves JM, Rebello, S.A., Thomas, W., Kurzer, M.S. and Slaton, J.W. . Effects of soy protein isolate consumption on prostate cancer biomarkers in men with HGPIN, ASAP, and low grade prostate cancer. Nutrition and Cancer. 2008;60(1):7-13.

  21. Pendleton JM, Tan WW, Anai S et al. Phase II Trial of Isoflavone in prostate specific antigen recurrent prostate cancer after previous local therapy. BMC Cancer 2008;8:132.
  22. Griifiths K. Estrogen and prostatic disease. International Prostate Health Council Study Group. Prostate 200; 45:87-100.