Cancers of the breast, endometrium, ovary, and prostate fall into the hormone-dependent classification. An association between hormonal status and cancer risk arose from observations of oestrogen deprivation and breast cancer and testosterone deprivation and prostate cancer. Nutritional influences on breast cancer have been studied extensively and several (but not all) studies show diminished risk with greater intakes of dietary fiber. The situation for other cancers, especially prostate cancer, appears to be rather unclear, but given the commonality of the proposed protective mechanisms, it is reasonable to expect that some linkage may be found. Male vegetarians have been reported to have lower testosterone and oestradiol plasma concentrations compared to omnivores, and inverse correlations of testosterone and oestradiol with fiber intake have been reported.
Potential Mechanisms Indicating a Role in the Etiology of Hormone-Dependent Cancers
There are many published studies that have produced mixed and inconsistent results on the potential mechanisms involved. Dietary fiber could act by reducing circulating concentrations of oestrogen and testosterone. Such an effect would not be unexpected in view of the fact that soluble NSPs can increase bile acid and neutral steroid excretion and fecal steroid outputs are higher in vegetarians than in omnivores. However, one anomaly is the finding that wheat bran (which does not enhance biliary steroid excretion) lowers circulating and urinary oestrogens. It is possible that fiber acts rather differently on hormones than on bile acids and neutral sterols. For example, the colonic flora may be modified so as to increase deconjugation of the sex hormone precursors or their conversion to other metabolites. Direct binding of sex hormones is possible but is subject to the same concerns as were raised for cholesterol reduction. In addition, it is possible that other components in, or associated with, fiber (phytooestrogens or antioxidants) may be responsible for any observed protective effect. Soy phytooestrogens are believed to play a role in lowering the risk of breast cancer in Asian populations. Lycopenes are antioxidant carotenoids from tomatoes, and their intake has been correlated with a lower risk of prostate cancer.
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