Most studies have focused on the effects of isoflavones on breast cancer, endometrial cancer and prostate cancer. Numerous studies have also investigated its effects on other kinds of cancer, including leukaemia, ovarian, colon, thyroid, bladder, skin and pancreatic cancer [1].
Ovarian cancer
Several epidemiological studies show that intake of isoflavones is linked with reduced risk of ovarian cancer. The Japan Collaborative Cohort Study, consisting of 64,327 women, found that the intake of tofu may have preventive action against ovarian cancer [2]. A study with American women also found that the group of women with highest isoflavones intake showed lower risk of ovarian cancer [1]. A meta-analysis found that women with highest soy intake showed a 48% lower risk of ovarian cancer than women with the lowest intake [3]. In contrast, outcomes of some animal studies cause concern about the safety of isoflavones [1]. These studies, often using very high levels of isoflavones, showed that isoflavones increased the production of ovarian cell tumors in female mice and piglets.
However, one study demonstrated that isoflavones protected rat from the formation of chemically induced ovarian cancer. Because of these conflicting results, more studies are required to determine the safety of isoflavones on ovarian tissue.
Thyroid cancer
The incidence of thyroid cancer is a lot higher in women than in men, suggesting that estrogenic effects may be involved in its pathogenesis. In vitro studies demonstrated that low levels of genistein were able to induce the proliferation of thyroid cancer cells, whereas high levels inhibited their growth. Animal studies showed that high levels of soy intake, but not purified isoflavones, increased thyroid weight in rats with iodine deficiency, indicating that not isoflavones but other soy ingredients may have goitrogenic effects. Isoflavones intake has no negative effect on the thyroid of healthy humans, as long as iodine intake is adequate. A study by the Northern California Cancer Center demonstrated that consumption of isoflavones was linked with a reduced risk of thyroid cancer [4].
Leukemia
Raynal et al evaluated the in vitro and in vivo antileukemic activity of genistein. They found that this isoflavone inhibited the growth of culturted leukemic cells and produced a moderate antileukemic effect in mice [5]. Asians have lower incidences of leukaemia than populations from Western countries, but incidence of infant leukaemia is higher in some Asian cities. Isoflavones may act as inhibitors of topoisomerase II and their intake during pregnancy may induce infant leukaemia. More studies are required to confirm and quantify this risk [1].
Colon cancer
One study using low levels of daidzein demonstrated that this isoflavone stimulated the growth of colon cancer cells, whereas other in vitro studies demonstrated a growth inhibition of genistein at daidzein in higher concentrations [1]. Although results of animal studies are conflicting, many of them showed a protective effect of isoflavones on colon cancer. Two meta-analysis demonstrated a potential beneficial effect of isoflavones on colon cancer. One meta-analysis published in 2005 by Badger et al. concluded that epidemiological and animal studies suggest that consumption of soy protein is associated with lower incidence of breast, colon and prostate cancer [6]. They found that a daily intake of as little as 5 g of soy protein was associated with significant reduction in the risks of these cancers. A more recent 2010 study by Yan et al showed that soy consumption reduced the risk of colon cancer by 21%, but only in women [7].
Bladder cancer
Some studies link soy milk consumption with lower risk of bladder cancer, but there is some evidence that isoflavones may promote the development of bladder cancer in Asians. On the other hand, in vitro and in vivo studies are able to demonstrate a protective role of isoflavones. More studies are required to confirm the affect of isoflavones on bladder cancer risk [1].
Gastric cancer
There is contradictory evidence regarding the relation between the intake isoflavones and gastric cancer. Lead author Azusa Hara from the National Cancer Center, Tokyo, wrote in the American Journal of Clinical Nutrition that isoflavones do no prevent gastric cancer [8]. She came to that conclusion after investigating data from a large, population-based prospective study of 39,569 men and 45,312 women. A recent study conducted by Kweon and co-workers evaluated the preventive effect of non-fermented soy foods on gastric cancer risk in the Shanghai Women’s and Men’s Health Studies, involving 128,687 participants [9]. They found that non-fermented soy foods had a low protective effect, but the association was not statistically significant. But among the separate soy food groups, intake of tofu was linked to a significant reduction in risk of distal gastric cancer in men and intake of dry soybeans was inversely associated with decreased risk of gastric cancer in postmenopausal women.
References
[1] Andres et al. Crit Rev Toxicol. Risks and benefits of dietary isoflavones for cancer. Crit Rev Toxicol. 2011 Jul;41(6):463-506.
[2] Sakauchi et al. Dietary habits and risk of ovarian cancer death in a large-scale cohort study (JACC study) in Japan. Nutr Cancer. 2007;57(2):138-45.
[3] Chang et al. Diet and risk of ovarian cancer in the California Teachers Study cohort. Am J Epidemiol. 2007 Apr 1;165(7):802-13.
[4] Haselkorn et al. Why are thyroid cancer rates so high in southeast asian women living in the United States? The bay area thyroid cancer study. Cancer Epidemiol Biomarkers Prev. 2003 Feb;12(2):144-50.
[5] Raynal et al. Antileukemic activity of genistein, a major isoflavone present in soy products.
J Nat Prod. 2008 Jan;71(1):3-7.
[6] Badger et al. Soy protein isolate and protection against cancer. J Am Coll Nutr. 2005 Apr;24(2):146S-149S.
[7] Yan et al. Soy consumption and colorectal cancer risk in humans: a meta-analysis.
Cancer Epidemiol Biomarkers Prev. 2010 Jan;19(1):148-58.
[8] Isoflavone intake and risk of gastric cancer: a population-based prospective cohort study in Japan. Am J Clin Nutr. 2011 Dec 14.
[9] Kweon SS et al. Intake of specific nonfermented soy foods may be inversely associated with risk of distal gastric cancer in a chinese population. J Nutr. 2013 Nov;143(11):1736-42.