The 2007 World Cancer Research Report into diet and cancer risk concluded there was suggestive but limited evidence that legumes reduced the risk of stomach and prostate cancer. Other reviews since then have suggested the evidence for prostate cancer protection is stronger and that legumes are also protective against breast and colorectal cancers, although much of the evidence is limited to the effect of soy intake. Two meta-analyses concluded that there is a 14-25% reduced risk of breast cancer with high soy intakes.
An analysis of the Nurses Health Study, involving 34,467 US women, found that those who consumed four or more servings of legumes a week had a lower incidence of colorectal adenomas than women who reported consuming one serving or less. This is supported by results from the Shanghai Women’s Health Study, which found women in the highest tertile of soy consumption had a 33% lower colorectal cancer risk.
Suggested Mechanisms:
The mechanisms of cancer protection are not clearly understood. Legumes contain several phenolic compounds, in addition to glutathione, soluble proteins and tocopherols which are considered to be natural antioxidants and may provide some cancer protective effects.
Legumes are also significant sources of resistant starches, which are fermented by colonic bacteria to short chain fatty acids, thus improving colonic health.
Researchers have attributed the anticarcinogenic effects to other various components present in legumes, including dietary folate, selenium, zinc, saponins, protease inhibitors, lectins, phytates and isoflavones. Isoflavones and coumestans are the main classes of phytoestrogens that occur in pulses. Phytoestrogens have been detected in a range of legumes, but soybeans contain the highest concentration. Due to their similarity in structure to the human hormone estrogen, phytoestrogens may have weak estrogenic activity when consumed and compete with oestrodiol, the most active estrogen, for estrogen receptors and hence prevent the growth of tumour cells. By preventing the production of oestrone, they deny the tumour a source of endogenous estrogen.
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