p>Conjugated linoleic acid (CLA) is a collective term for a group of positional and geometrical conjugated dienoic isomers of linoleic acid that are found in dairy products and meat.3 The cis-9, trans-11 CLA is the principal dietary CLA form, but lower levels of the other isomers (trans-10, cis-12 CLA, trans-9, trans-11 CLA, and trans-10, trans-12-CLA) are present in food CLA sources. Naturally, CLA is produced in the rumen of ruminant animals by the fermentative bacteria that isomerize linoleic acid into CLA.

Mechanisms of action of CLA include: enhanced thermogenesis, increased satiety, augmented fat oxidation, reduced fat cell size as well as fat deposition, increased apoptosis of adipocytes and altered preadipocyte differentiation. Potentially, the combination or additive effects of all these mechanisms of action of CLA may lead to changes in weight and body fat, as no single mechanism fully explains CLA action.

Studies have shown that CLA, specifically the trans-10, cis-12 isomer, can reduce body weight and fat mass. Most animal studies associated with feeding CLA have shown that CLA lowers body fat and energy retention as well as increases energy expenditure, thereby decreasing weight(7-10); yet, others have shown no effects on weight.(11-14) This may be due to the dose or the CLA isomers used in animal studies. Results demonstrate that body weight and/or fat mass of animals were not affected when they were supplemented with low amounts of CLA mixture (0.5% in the diet), which contained about equal amounts of the trans-10, cis-12 isomer and cis-9, trans-11 isomer. Yet, weight gain was similar to control when high amounts of CLA mixture with mostly the cis-9, trans-11 isomer were given. However, most human studies have not been able to replicate the magnitude of weight lost. Only a few human studies suggest that CLA supplementation has reduced body fat and other studies did not show any effect. The variety of species used in studies may also explain the discrepancy of results obtained.

In animals, CLA supplements appear to have some undesirable side effects such as induced insulin resistance as well as fatty liver and spleen. These animal studies also demonstrate that CLA may have detrimental effects on plasma lipids. Human studies also show evidence that CLA may adversely influence health, in particular insulin sensitivity and blood lipids, but the results are conflicting. CLA is widely available in capsule form that improves its oxidative stability, therefore having an appropriate matrix, cost, and sensory quality for consumers. However, the efficacy of CLA is questionable because the animal evidence is more convincing than the human data. The lack of clarity on the mechanism of action can explain the inconsistencies in the research results. In addition, human studies should be carried out to determine the long-term effects of CLA and whether any adverse outcomes occur. In summary, the data available from literature demonstrates a poor AECES model. More research is needed to investigate the efficacy and the safety aspects before CLA will have optimal consumer acceptance.