Our results indicate that whole cooked navy beans provide protection against colon carcinogenesis in male AOM-induced ob/ob
mice, specifically against earlier stages of colon carcino-genesis. This is evidenced by the lower incidence of focal hyperplasia (), the lack of lesion multiplicity, and the lower incidence of any type of lesions (55.9% of inhibition from control; ) compared to mice fed the control diet. However, the incidence of adenocarcinomas was not affected by feeding whole dry beans () as it was in studies by Hangen and Bennink (20
) and Hughes et al. (21
). Hughes et al. (21
) reported that diets rich in cooked pinto beans were associated with significantly lower incidence rates of any adenocarcinomas (50%), neoplastic lesions (52.4%), and any type of lesions (49.2%) in colons of AOM-induced F344 rats. Similarly, Hangen and Bennink (20
) reported that AOM-induced F344 rats fed diets rich in cooked black beans or cooked navy beans had significantly lower incidence rates of adenocarcinomas (75.0% or 61.1%, respectively) and neoplastic colon lesions (53.7% or 58.8%, respectively). Furthermore, Murillo et al. (50
) reported that a diet containing 10% flour of garbanzo beans, another commonly consumed dry bean market class (44
), was associated with a significant 64% decrease in colonic aberrant crypt foci, a surrogate marker for early stages of colon carcinogenesis, in AOM-induced CF-1 mice. In addition, all 3 bean diets (whole bean: 55.9%; bean residue: 67.1%; bean extract: 87.2%) significantly lowered the incidence of any colon lesions compared to mice on the control diet (). Therefore, although we did not observe a protective effect of the whole bean diet against formation of adenocarcinomas, results of this and other animal studies suggest a protective effect of dry bean consumption against colon carcinogenesis.
It is unclear to us why the number of adenocarcinomas was not affected by feeding whole cooked navy beans. One potential explanation is that the whole bean diet inhibited early but not invasive steps of colon carcinogenesis. However, this explanation is difficult to reconcile with the protective effect of whole bean diets against adenocarcinomas in other animal studies (20
). Furthermore, both bean fractions nearly completely inhibited formation of adenocarcinomas in this study (bean residue: 1 adenocarcinoma; bean extract: 0 adenocarcinomas; ). One potential explanation that could account for the differences in results is that the whole navy bean diet had a lower bioavail-ability of cancer-preventive compounds than previous studies and that the 60% ethanol extraction procedure improved the bioavailability of these compounds. However, statistically significant differences in incidence rates were observed only for incidence rates of adenocarcinomas between the whole bean and bean extract diet (). One has to also consider the amount of whole beans consumed on the bean fraction diets relative to the amount consumed on the whole bean diet. Assuming equal food intake in the 3 bean groups, mice on the bean residue diet consumed 14% more whole bean equivalents, whereas mice on the bean extract diet consumed 47% more whole bean equivalents.
A potential limitation of other AOM-induced rat studies was that the bean diets might have been limiting in tryptophan content, which could have caused the lower body weights of the bean-fed rats compared to the rats fed the control diet (20
). Therefore, we adjusted the diets for tryptophan content, which is the second limiting amino acid in dry beans. We did not observe decreased body weights in mice receiving cooked navy beans or its fractions, but rather, we observed the opposite, that is, increased body weights (whole beans: 8.3%, bean residue: 5.0%, bean extract: 1.5%; ). We conclude that even high concentrations of cooked navy beans or their fractions do not have detrimental effects on growth.
Several components of dry beans, such as phenolic compounds, saponins, phytic acid, residual activity of protease inhibitors, resistant starches, and nonstarch polysaccharides, have been proposed to account for the colon cancer-protective effect of dry beans (11
). Most of the cancer-protective compounds, except for resistant starches and nonstarch polysaccha-rides, are soluble in aqueous alcohol (37
). Therefore, our hypothesis was that the diet containing the bean extract might be more efficacious than the diet containing the bean residue. Consistent with our hypothesis, the lowest incidence rates of neoplastic lesions, neoplastic or preneoplastic lesions, or any type of lesions in colons were observed in mice fed the bean extract diet (–). However, we did not observe significant differences in incidence rates of various types of colon lesions between mice fed diets containing bean residue or bean extract (; –). We conclude that both the 60% ethanol-soluble fraction and the insoluble fraction of dry beans are efficacious in inhibiting colon carcinogenesis and contribute to the cancer-preventive effect of cooked dry beans.
The human relevance of our results is suggested by results from human intervention and epidemiological studies (13
). Results of a human intervention study suggest that daily consumption of at least 31 g of cooked dry beans can prevent recurrence of advanced adenomatous polyps (19
). Therefore, we chose for this study diets with large differences in dry bean contents. We are aware that diets containing 74% of dry beans () are not feasible in humans; however, results by Murillo et al. (50
) in AOM-induced CF-1 mice suggest that diets with lower dry bean contents might be also effective in preventing at least early steps of colon carcinogenesis. Furthermore, the bean extract might provide an opportunity to receive the same health benefits as from high dry bean intakes at much lower consumption levels. Dose-response studies of the bean extract are required to determine the lowest efficacious dose for cancer prevention.
mice have not been previously used as a colon cancer model. The ob/ob
mouse has a mutation in the leptin gene resulting in hyperphagia, obesity, hyperinsulinemia, hyperglycemia, and increased inflammatory response to liposaccharides (9
). We chose the ob/ob
mouse as the animal model to examine the protective effects of dry beans against CRC in humans because subjects of the Polyp Prevention Trial in which increased dry bean consumption was associated with decreased advanced adenoma recurrence were on average over-weight (19
). In comparison to well-established models of colon cancer (20
), the AOM-induced ob/ob
mice in our study had a much lower incidence of neoplastic lesions (). We used a comparatively mild AOM induction protocol (2 dosages of 7 mg AOM/kg of body weight 1 wk apart) to achieve human-relevant low incidence rates, as the lifetime risk in humans to develop adenocarcinomas is low, with 5.8% for men and 5.4% for women (2
). However, the low incidence rates create difficulties to reach statistical significances for dietary colon cancer prevention studies. Further studies that use higher AOM dosages, more AOM cycles, or both are warranted to evaluate whether higher tumor incidences, similar to that of other colon cancer models, can be achieved in AOM-induced ob/ob
In conclusion, our study confirms the protective effect of cooked dry bean consumption against colon carcinogenesis in a new colon cancer animal model, the AOM-induced ob/ob mouse. Furthermore, we demonstrated that both the 60% ethanol-soluble and the insoluble fraction of cooked navy beans are efficacious in inhibiting colon carcinogenesis. This suggests that components in the soluble fraction as well as the insoluble fraction of the ethanol extract have preventive properties against colon carcinogenesis. Further studies are warranted to determine which specific components in each fraction contribute to the cancer-protective effect so that even more potent dietary components for colon cancer prevention can be developed.