Effects of soy or rye supplementation of high-fat diets on colon tumour development in azoxymethane-treated rats

Evidence is accumulating that a diet high in plant-derived foods may be pro tective against cancer. One class of plant component under increasing inves tigation is the phytoestrogens of which there are two main groups: the isof lavones, found mainly in soy products, and the lignans, which are more ubiq uitous and are found in fruit, vegetables and cereals with high levels bein g found in flaxseed, In this study, we have used carefully balanced high-fa t (40% energy) diets: a control diet (containing low isoflavone soy protein as the sole protein source), a rye diet (the control diet supplemented wit h rye bran) and a soy diet (containing as protein source a high isoflavone soy protein). The effect of these diets on the development of colonic cance r was studied in F-344 rats treated with the carcinogen, azoxymethane (two doses of 15 mg/kg given 1 week apart). Colons from treated animals were exa mined for aberrant crypt foci (ACF) and tumours after 12 and 31 weeks. Resu lts after 12 weeks showed no differences in the total number of ACF in the control, soy or rye bran groups. However, the soy group had increased numbe rs of small ACF (less than four crypts/focus) while the rye group had decre ased numbers of large ACF (greater than six crypts/focus). Examination of c olons after 31 weeks gave similar low numbers of ACF in each group with no differences in multiplicity. There were no differences in the number of tum ours between the control (1.36 tumours/rat) and soy (1.38 tumours/rat) grou ps. However, there was a significant decrease in the number of tumours in t he rye group (0.17 tumours/rat), These results suggest that soy isoflavones have no effect on the frequency of colonic tumours in this model while rye bran supplementation decreases the frequency of colon cancer. This effect is due not to a decrease in early lesions but in their progression to large r multi-crypt ACF. The study also supports the hypothesis that larger ACF a re more predictive of subsequent tumorigenicity.