The main objective of the present study was to test the hypothesis that exogenous insulin would enhance colon carcinogenesis. Thirty-six female Fischer 344 rats were fed ad libitum a low-fat rodent chow and given a single azoxymethane injection (20 mg/kg); one week later, they were randomized into two groups. Control rats were given a subcutaneous saline injection, 5 days/wk, and experimental rats were given Ultralente bovine insulin (20 U/kg). The promoting effect of insulin injections was assessed by the multiplicity (number of crypts) of aberrant crypt foci after 100 days of treatment (72 injections). The rats given insulin ate more and were heavier than controls (215 +/- 11 vs. 182 +/- 7 g, p < 0.001). Insulin injections also increased the amount of abdominal fat, plasma triglycerides, and insulinemia and decreased blood glucose (all p < 0.05). The number of aberrant crypt foci was the same in both groups, but their multiplicity was significantly increased by the insulin injections (2.8 +/- 0.3 vs. 2.5 +/- 0.2 crypt/focus in controls, p = 0.007). In addition, the proportion of sialomucin-producing foci was higher in insulin-injected rats than in controls (p = 0.04). These data show that exogenous insulin can promote colon carcinogenesis in rats and suggest that life-style and diets leading to low blood insulin might protect humans against colorectal cancer.

High intake of red meat or processed meat is associated with increased risk of colon cancer. In contrast, consumption of white meat (chicken) is not associated with risk and might even reduce the occurrence of colorectal cancer. We speculated that a diet containing beef or bacon would increase and a diet containing chicken would decrease colon carcinogenesis in rats. One hundred female Fischer 344 rats were given a single injection of azoxymethane (20 mg/kg i.p.), then randomized to 10 different AIN-76-based diets. Five diets were adjusted to 14% fat and 23% protein and five other diets to 28% fat and 40% protein. Fat and protein were supplied by 1) lard and casein, 2) olive oil and casein, 3) beef, 4) chicken with skin, and 5) bacon. Meat diets contained 30% or 60% freeze-dried fried meat. The diets were given ad libitum for 100 days, then colon tumor promotion was assessed by the multiplicity of aberrant crypt foci [number of crypts per aberrant crypt focus (ACF)]. The ACF multiplicity was nearly the same in all groups, except bacon-fed rats, with no effect of fat and protein level or source (p = 0.7 between 8 groups by analysis of variance). In contrast, compared with lard- and casein-fed controls, the ACF multiplicity was reduced by 12% in rats fed a diet with 30% bacon and by 20% in rats fed a diet with 60% bacon (p < 0.001). The water intake was higher in bacon-fed rats than in controls (p < 0.0001). The concentrations of iron and bile acids in fecal water and total fatty acids in feces changed with diet, but there was no correlation between these concentrations and the ACF multiplicity. Thus the hypothesis that colonic iron, bile acids, or total fatty acids can promote colon tumors is not supported by this study. The results suggest that, in rats, beef does not promote the growth of ACF and chicken does not protect against colon carcinogenesis. A bacon-based diet appears to protect against carcinogenesis, perhaps because bacon contains 5% NaCl and increased the rats' water intake.

Carrageenans (CAR) are sulfated polymers from seaweed used as gelling agents in foods. Chemical carcinogen induction of tumors in the colon of rats is enhanced by CAR. We speculated that gut microflora is involved in this effect. We thus studied the initiating and promoting effects of undegraded CAR-kappa (345,000 mol wt) in conventional rats and in germ-free rats associated with a human fecal flora. The initiating effect of CAR was studied by scoring aberrant crypt foci (ACF) in the colon of Fischer 344 rats given CAR (10% in water). The promoting effect of CAR was studied by comparing the multiplicity of ACF (number of crypts/focus) in rats receiving pure water or CAR (0.25% and 2.5% in water) for 100 days, starting 7 days after azoxymethane initiation (1 dose of 20 mg/kg i.p.). Duplicate studies were conducted in conventional rats and in human flora-associated rats maintained in isolators. Results show that CAR did not initiate ACF. In conventional rats, the 2.5% CAR gel promoted the growth of ACF: 2.98 +/- 0.29 and 3.44 +/- 0.48 crypts/ACF in control and treated rats, respectively (p < 0.02). The 0.25% CAR gel did not promote ACF. CAR can thus enhance intestinal tumors in this rat model, but only at a high dose level. In contrast, we did not observe any promoting effect of the administration of the 2.5% CAR gel in human flora-associated rats: 2.81 +/- 0.18 and 2.78 +/- 0.38 crypts/ACF in control and treated rats, respectively (p = 0.80). The specific microflora of rats, but not the human gut flora, might be involved in colon tumor enhancement by CAR.

Processed meat intake is associated with increased risk of colorectal cancer. This association may be explained by the endogenous formation of N-nitroso compounds (NOC). The hypothesis that meat intake can increase fecal NOC levels and colon carcinogenesis was tested in 175 Fischer 344 rats. Initiation was assessed by the number of aberrant crypt foci (ACF) in the colon of rats 45 days after the start of a high-fat bacon-based diet. Promotion was assessed by the multiplicity of ACF (crypts per ACF) in rats given experimental diets for 100 days starting 7 days after an azoxymethane injection. Three promotion studies were done, each in 5 groups of 10 rats, whose diets contained 7%, 14%, or 28% fat. Tested meats were bacon, pork, chicken, and beef. Fecal and dietary NOC were assayed by thermal energy analysis. Results show that feces from rats fed bacon-based diets contained 10-20 times more NOC than feces from control rats fed a casein-based diet (all p < 0.0001 in 4 studies). In bacon-fed rats, the amount of NOC input (diet) and output (feces) was similar. Rats fed a diet based on beef, pork, or chicken meat had less fecal NOC than controls (most p < 0.01). No ACF were detected in the colon of bacon-fed uninitiated rats. After azoxymethane injection, unprocessed but cooked meat-based diets did not change the number of ACF or the ACF multiplicity compared with control rats. In contrast, the bacon-based diet consistently reduced the number of large ACF per rat and the ACF multiplicity in the three promotion studies by 12%, 17%, and 20% (all p < 0.01). Results suggest that NOC from dietary bacon would not enhance colon carcinogenesis in rats.

Promotion would suppose the selection of initiated cells. We tested the selection of aberrant crypt cells by cholic acid, a colon cancer promoter, and the effect of protectors, phytate and food restriction. After an azoxymethane injection, rats were allocated to a control diet, or to supplements of cholic acid, sodium phytate, or to a 50% food restriction. The proliferation and apoptosis of 1200 crypts were assessed, after immuno-staining for BrdU. Cholic acid increased the proliferation of aberrant crypts but not of normal crypts. Phytate and food restriction decreased the proliferation of normal crypts, but not of aberrant crypts. Apoptosis was not affected by diets. Results support the hypothesis that cholic acid can select initiated cells in the colon.