Chemopreventive effects of dietary flaxseed on colon tumor development.

Experimental studies have demonstrated that fatty acid composition of dietary fat plays a vital role in the development of colon tumors.  Fats containing high levels of n6 fatty acids have been reported to enhance chemically induced colon carcinogenesis in rats, while an inhibitory effect has been observed for n3 fatty acids.  Flaxseed oil, which contains ALA, has recently been reported to exert chemo preventative effects on induced colon tumor development in rats.  Flaxseed is also a rich source of lignans, which have demonstrated a protective effect against both breast and colon tumors.  Thus, the objective of the present study was to investigate the chemoprevative effects of dietary flaxseed meal, rich in both ALA and lignans, on azoxymethane-induced colon tumor development in rats, and to compare these effects to those of dietary corn meal rich in n6 fatty acids. 

Forty-eight male Fischer rats were divided into two groups of 24 and fed an AIN-93M diet supplemented with either 15% corn meal (Group 1) or 15% flaxseed meal (Group 2).  A subcutaneous injection of azoxymethane (15mg/kg) was given once a week for 3 consecutive weeks to initiate intestinal tumor development.  After 36 weeks of initiation, rats were anesthetized and blood collected by cardiac puncture.  Rats were then sacrificed and the gastrointestinal tract removed.  The site, size, and number of colonic tumors were determined, and serum samples analyzed for fatty acid composition and lignan levels.  Cyclooxygenase-1 (COX-2) and COX-2 expression was also determined from colonic microsome samples.

Incidence of colon tumors in the flax meal group was significantly lower (29.4%) compared to the cornmeal group (82.6%).  The mean number of tumors per rat was significantly lower in the flax meal group (0.3 tumors/rat) compared to the cornmeal fed rats (1.3 tumors/rat).  Tumor size in the flax meal group was also significantly lower (average = 5.2mm2) versus the corn meal group (44.4mm2).  Serum levels of n6 fatty acids (linoleic and arachidonic acids) were higher in the cornmeal fed group, while levels of n3 fatty acids (ALA, EPA, DHA) were significantly higher in the flax meal group.  Expression of COX-1 and COX-2 was significantly lower in the flax meal group as compared to the cornmeal group. 

These results indicate that dietary flaxseed may be effective in reducing risk of developing colon cancer.  The chemopreventative effect of flaxseed may be attributed to both its high levels of ALA and lignans with a concurrent decrease in levels of COX-1 and COX-2.