Conjugated linoleic acid (CLA) is a wide term which groups together a mixture of positional and geometric isomers of C 18:2 fatty acids having conjugated, or contiguous, double bounds. These fatty acids arise along the ruminal process which ends with full saturation into stearic acid, a stepped pathway carried out by rumen bacteria.
These naturally occurring groups of dienoic derivatives of linoleic acid are incorporated into the fat in beef and milk of ruminants before the saturation process has been completed. Numerous beneficial effects are attributed to CLA, as in slowing down or even preventing tumor development. CLA decreases body fat storage in animal models  and promotes cardiovascular protection against atheroesclerosis . A growing bulk of evidence shows that CLA, mainly as cis-9, trans-11, 18:2 n-6 derivatives, consistently produced antitumor effects, thus reducing the incidence, progression, number of metastases and tumor burden in rats and in murine models of mammary gland, colon, forestomach, skin and prostate tumorigenesis . The evidence became so convincing that the National Academy of Science advised in 1996 that "Conjugated linoleic acid (CLA) is the only fatty acid shown unequivocally to inhibit carcinogenesis in experimental animals" . Beside CLA, other polyunsaturated fatty acids (PUFA) belonging to the essential fatty acid (EFA) n-3 family, whose main source are fish and seafood, are generally believed to act as antipromoters for several cancers.
Interestingly enough for those populations having little o negligible fish intake as in the case of the mediterranean population of Argentina [5, 6], CLA remains the only source of beneficial fatty acids with respect to tumor prevention and cardiovascular protection.
The natural source of CLA are red meats and fatty dairy products, which are mainly bovine derivatives. However, these foods are also heavily suspected to be related to the higher risk of developing colorectal cancer (CRC) in humans. Indeed, as a consequence of a huge, and often contradictory, epidemiological, clinical and experimental amount of research, general advice has been sent to the population at large, suggesting that fatty beef, and to a lesser extent, whole fatty dairy derivatives, should be eaten less frequently, whereas the amount of dietary n-3 PUFAs might be increased.
CRC in humans continues to be a major challenge, since death rates due to colon carcinoma have not diminished appreciably within the past decades . CRC is a major disease in Western populations, and diet may account for approximately 35% of the cases. The Argentinean food pattern, rich in beef and fats and poor in fibres and fish, may be related to an increased CRC risk. Our previous case-control studies on Argentinian people were consistent with these mentioned beliefs, showing that a high frequency of heavily roasted meat was associate with increased CRC risk. Saturated fatty acids and cholesterol also seemed to play an important role in this risk . However, further characterization of meat consumption and risk for CRC by total intake as well as by different varieties of meat showed an unexpected dual behaviour related to the type of meats. Fatty meat derivatives, such as cold cuts and sausages, mainly prepared from fatty beef were consistently associated with higher risk, whereas high consumption of lean beef behaved as a protective dietary habit. High total meat consumption was not related to an increased risk of CRC in our subjects, even adjusting for energy and all macronutrients. Even though that the CRC risk has been attributed to other factors, as amount and varieties of nitrosocompounds  and heme ring of hemoglobin , several extensive reviews point to an increased risk linked somewhere to meat fat content. Indeed, different kinds of meat have similar levels of protein, so the mayor difference may lay in other components, for example, the amount and quality of lipid components. High fat diets, rich in cholesterol and saturated lipids, may favour cancer because of their high caloric content, or they could lead to increased levels of bile acids in the colonic rumen or a disbalance of the essential fatty acids metabolism .
On the other hand, fats from bovine milk and meat contain variable proportions of CLA. Interestingly, CLA is located in the interstitial, non-visible, fat evenly distributed along muscle fibres as well as in subcutaneous depots. Whereas the visible fat may be easily discarded and not consumed during the meal, interstitial fats will be eaten. The remaining invisible intramuscular total fat in lean meats range from 25 to 50 g/Kg (2.5–5%) . As a consequence, beneficial effects of minor amounts of CLA will be relatively enhanced in lean meat compared to fatty meat sub-products since the latter contain thus a substantial amount of saturated fatty acids and cholesterol, as in cold cuts and cow viscera, diluting CLA.
In conclusion, whatever the beneficial or deleterious effects of CLA and saturated fatty acids/cholesterol mechanisms are, in the ruminant meats mixed natural fats may modulate opposite effects with regard to the risk of development of CRC in humans, according to their relative concentrations.