Interactive effects between dietary fat and a vegetables-fruit mixture on colorectal carcinogenesis

Several dietary compounds are associated with colorectal cancer risk. These include the amount of dietary fat, which is positively associated with colorectal cancer, and a variety of vegetables and fruit, which are suggested to possess anticarcinogenic potential. Because diet is complex and dietary factors most probably interact, it is likely that these interactive effects between dietary components, rather then the effects of individual components, may account for a large part in the risk for developing colorectal cancer.The results of the studies performed by Alink et al. (1993) demonstrated that it is of paramount importance to study interactive effects between dietary components in evaluating the effects of total diet in colorectal carcinogenesis. They showed that the effects of a vegetables-fruit mixture on DMH-induced colorectal carcinogenesis in rats maintained on complete human diets, was modulated by the presence of other dietary components, such as the amount of dietary fat or fried meat.To investigate these observations by Alink et al. in more detail, we performed three long-term animal studies, which have been described in detail in Part I of the present thesis. In these studies, interactive effects between dietary fat (20 and 40 energy%) and a vegetables-fruit mixture (19.5% wt/wt) were studied on 1,2-dimethylhydrazine (DMH)- and N-methyl-N'-nitro-N-nitroso-guanidine (MNNG)-induced colorectal carcinogenesis in rats, and in ApcMin mice, which are genetic susceptible for developing multiple intestinal neoplasia (Chapters 1, 2, and 3). The composition and amount of fat (40e%) and of the vegetables-fruit mixture were based on regular amounts consumed in The Netherlands.The animal diets used for these studies were balanced for protein and micronutrient content. The diets differed only in the amount of fat/carbohydrate and the presence of specific plant constituents when a vegetables-fruit mixture was included in the diets. Notwithstanding the use of the same experimental diets in all three studies, the results were remarkably different.A vegetables-fruit mixture added to high-fat (40e%) diets resulted in a distinct protection of colorectal carcinogenesis in the MNNG-study, whereas in the DMH study no effect of the vegetables-fruit mixture was observed. In ApcMin mice the vegetables-fruit mixture enhanced rather then inhibited tumor development. Furthermore, a diet high in fat (40e%) enhanced colorectal carcinogenesis in the DMH-study and, although less pronounced, in the MNNG-study, whereas in the ApcMin mice no effect was observed.Finally, a vegetables-fruit mixture added to low-fat diets did not result in protection against colorectal cancer development in the MNNG-study, whereas when added to a high-fat diet an inhibitory effect was found. In male ApcMin mice, the same mixture added to low-fat diets decreased the number of small intestinal tumors, whereas it increased the number of small intestinal neoplasia when added to high-fat diets.To examine the differences observed between the effects of the vegetables-fruit mixture observed in the DMH-model and those in the MNNG-model in more detail, an experiment was designed, in which the inhibitory potency of a vegetables-fruit mixture was investigated on either the initiation or the promotion phase, using azoxymethane (AOM) to induce colorectal carcinogenesis and using both low- and high-fat diets (Chapter 4). In this study no protection of this mixture on colorectal carcinogenesis was observed either when present during the initiation or during the promotion phase, irrespective of the fat content.In conclusion, the variation in the results of the aforementioned studies can most probably be ascribed to methodological differences, such as differences in DMH and MNNG metabolism, route of administration and type of DNA damage. Apart from these differences, genetic susceptibility may play a role. Taken these methodological differences into account the present results do not consistently show that dietary fat has modulated the tumor preventive properties of a vegetables-fruit mixture in colorectal carcinogenesis (Alink et al., 1993).The second part of the thesis describes the results of studies into the interactive effects between dietary fat and the vegetables-fruit mixture on (anti)carcinogenic mechanisms. Hepatic xenobiotic enzyme activities (ethoxyresorufine-O-deethylation (EROD), pentoxyresorufine-O-deethylation (PROD), N-nitrosodimethylamine-demethylase (NDMA-d), cytosolic glutathion-S-transferase (GST), UDP-glucuronyl transferase (UDP-GT)) and immune parameters in spleen and mesenteric lymph nodes (NK cell activity, lymphocyte stimulation test, mixed lymphocyte reaction) were measured in rats of the long-term DMH-study (Chapter 5).Overall, it was shown that both a vegetables-fruit mixture and dietary fat had no effect on the enzyme activities and immune function. In a short-term (seven weeks) animal study, DMH-treatment appeared to influence hepatic and colonic xenobiotic enzyme activities (EROD, PROD, NDMA-d, UDP-GT and GST) rather strongly (Chapter 6).Furthermore, interaction between dietary fat and the vegetables-fruit mixture was observed on colonic NDMA-d activity. Finally, interactive effects between stearic acid, indole-3-carbinol and crude extracts of the vegetables-fruit mixture were studied in vitro (Chapter 7). Both indole-3-carbinol, an isolated plant constituent, and vegetables-fruit extracts, a complex mixtures of plant constituents, modulated the effects of stearic acid on cytotoxicity, gap junctional intercellular communication, and cytochrome P450-IA (EROD) activity. The effects of indole-3-carbinol and extracts of the vegetables-fruit mixture were partly influenced by the order at which these components with stearic acid were added to the cells as well as by the type of cells used.In general, the results of the studies described in Part II of this thesis supports the hypothesis that interaction between dietary constituents may influence their modulating effect on colorectal carcinogenesis.