A fish oil diet inhibits colon cancer in mice.

High-fat, high-cholesterol diets have been linked to colon cancer in both epidemiological and animal studies. Saturated and N-6 polyunsaturated fats have both been implicated as playing causative roles. Epidemiological studies have shown a reduced colon cancer incidence in populations consuming a large quantity of N-3 polyunsaturated fat. This study asked whether N-3 polyunsaturated fat found in fish oil would also be associated with reduced colon cancer in animal studies. 1,2-Dimethylhydrazine was used to induce colon cancer in mice fed three high-fat, high-cholesterol diets (beef tallow, safflower oil, and fish oil) and one low-fat, cholesterol-free diet (soybean oil). Colon adenocarcinomas developed in 55% of mice fed tallow, 48% of those fed low-fat diets, 33% of those fed safflower oil, and 18% of those fed fish oil (p less than 0.05). Tumors per animal were also greatest in mice fed tallow and fewest in those fed fish oil (p less than 0.05). Plasma cholesterol levels were significantly higher in mice fed tallow than in mice fed fish oil, but this did not show any association with tumors. Plasma triglyceride levels were not significantly different among groups but were strongly correlated with colon cancers (r = 0.90, p less than 0.025). In both plasma and colon mucosa cells, saturated fat levels were similar. Monounsaturated fat was highest in plasma of mice fed tallow and correlated strongly with colon cancers (r = 0.84, p less than 0.005). N-6 polyunsaturated fat was highest in plasma and colon mucosa cells of both mice fed safflower oil and those fed low-fat (soybean oil) diet, but there was no association with tumors. N-3 polyunsaturated fat was highest in plasma and colon mucosa cells of mice fed fish oil and showed a negative association with tumors (r = -0.57, p less than 0.05). Thus, in this model of colon cancer, a diet high in fish oil was associated with less colon cancer. Monounsaturated fat was most strongly correlated with tumors. This study suggests monounsaturated fat promotes colon tumors and N-3 polyunsaturated fat inhibits colon tumors.