Enhancement of 1,2-dimethylhydrazine-induced colon carcinogenesis in mice by dietary agar

The effect of dietary agar on colon carcinogenesis was investigated. The frequency and number of tumours of the colon induced by subcutaneous injections of 1,2-dimethylhydrazine (DMH) was examined in male CF₁ mice fed different levels of agar and fat in the diet. The mice were fed one of four diets: a low-fat, low-fibre control diet, the control diet with added agar, a high-fat diet, or a high-fat, agar-containing diet. The mice were injected with 20 mg DMH/kg body weight weekly for 20 wk. Controls were injected with saline. The mice fed agar had significantly more colon tumours per animal than those fed diets that did not contain agar and there was a slightly increased frequency of malignant tumours in agar-fed mice compared with that in mice given the control diet. Feeding high-fat diets only slightly increased the incidence of malignant tumours and did not significantly affect the number of tumours per animal. No tumours occurred in mice injected with saline. Since it has been suggested that faecal steroids have a co-carcinogenic role in colon carcinogenesis, faecal bile acids and neutral sterols were determined. Feeding agar decreased the faecal neutral sterol and bile-acid concentrations, whereas feeding high-fat diets did not affect the faecal bile-acid concentrations and increased faecal neutral sterol concentrations only when agar was also in the diet. The results of this study show that dietary agar enhances DMH-induced colon carcinogenesis in mice even though the level of faecal steroids is lowered.