Chemopreventive Effects of Dietary Flaxseed Oil on Colon Tumor Development.

Fatty acid composition of dietary fat, primarily the levels of n-3 and n-6 polyunsaturated fatty acids, has shown profound effect on colon tumor development in animal studies.  Fats containing n-6 fatty acids (for example, corn oil) enhanced and n-3 fatty acids (for example, fish oil and mustard oil) reduced chemically induced colon tumors in rats.  The purpose of this study was to investigate the effects of dietary flaxseed oil (containing alpha-linolenic acid ALA, an n-3 polyunsaturated fatty acid) on azoxymethane-induced colon tumor in rats and how it compared with the dietary corn oil�treated group. Male Fischer rats, separated into 2 groups of 30, were assigned to the AIN-93M diet, which was supplemented with either 15% corn oil or 15% flaxseed oil.  Carcinogenesis was initiated with subcutaneous injections of azoxymethane (15 mg/kg) once a week for three consecutive weeks. Thirty-five weeks after initiation, the rats were sacrificed under ether anesthesia. Blood was collected by cardiac puncture. The gastrointestinal tract was isolated and flushed with ice-cold normal saline. The site, size, and number of tumors were recorded. The incidence and multiplicity of the tumors in the colon were determined. The fatty acid composition in the serum, colon, and tumors was estimated by using gas chromatography-flame ionization detection. Colon tumor incidence was found to be 100% and 54%, whereas multiplicity was found to be 3.1 and 0.7 tumors per rat in corn oil� and flaxseed oil�treated groups, respectively. Tumor size was significantly larger in the corn oil�treated group than in the flaxseed oil group. Colon and serum samples of the corn oil group showed an increase in the n-6 fatty acid levels, whereas the flaxseed oil group exhibited an increase in the n-3 fatty acid levels. The results indicate that dietary flaxseed oil, containing high levels of n-3 fatty acids, is effective in preventing colon tumor development when compared with dietary corn oil containing n-6 fatty acids in rats. (Author's abstract)
Laboratory studies conducted in animal models show high-fat diets containing corn oil, safflower oil, beef fat, or lard increased chemically induced colon tumors compared with low-fat diets. Dietary fish and mustard oil, rich in n-3 fatty acid, reduce azoxymethane-induced colon tumorigenesis in rats. The purpose of the present investigation was to determine the chemopreventive effects of dietary flaxseed oil rich in n-3 fatty acids on azoxymethane-induced colon tumor development and to compare them with the effects of dietary corn oil containing n-6 fatty acids on colon tumor development. The results show colon tumor incidence in the corn oil�treated group was significantly higher than the flaxseed oil�treated group. Tumors developed in only half of the rats in the flaxseed oil group, whereas in the corn oil�treated group the incidence was 100%. The average number of colon tumors per rat was about three in the corn oil�treated groups, whereas it was less than one in the flaxseed oil�treated group. The average tumor size in the corn oil group was significantly larger when compared with the flaxseed oil group. An increase in n-6 fatty acids were noted in the tumor microsomal fractions of both corn oil and flaxseed oil�treated groups and indicates that n-6 fatty acids are associated with tumor development and may actually promote tumor development. The authors conclude that moderate amounts of dietary flaxseed oil could be effective in reducing the risk of developing colon cancer. Further studies on the effects of dietary flaxseed oil on prostaglandin synthesis and COX expressions are needed to elucidate the mechanism of action. (Editor's comments)