Introduction Conjugated Linoleic Acid (CLA) is a naturally occurring free fatty acid found mainly in meat and dairy products, in small amounts. CLA was discovered by accident in 1978 by Michael W. Pariza at the University of Wisconsin while looking for mutagen formations in meat during cooking. As implied by the name, the double bonds of CLAs are conjugated. Biochemistry Most studies of CLA have used a mixture of isomers wherein the c9, t11-CLA and t10, c12-CLA isomers were the most abundant. Conjugated linoleic acid is a trans fat, though some researchers claim that it is not harmful in the same fashion as other trans fatty acids, but rather is beneficial. CLA is a conjugated system, and in the United States, trans linkages in a conjugated system are not counted as trans fat for the purposes of nutritional regulations and labeling. CLA, as well as some trans isomers of oleic acid, is produced by microorganisms in the rumen of ruminants. Non-ruminants, such as humans, are able to produce some isomers of CLA from some trans isomers of oleic acids, such as vaccenic acid, which is converted to CLA by delta-9-desaturase. Diet and health Various antioxidant and anti-tumor properties have been attributed to CLA, and studies on mice and rats show promising results in reducing mammary, skin, and colon tumor growth. A European team led by the Swiss scientist Lukas Rist has found that mothers consuming mostly organic milk and meat products have about 50 percent higher levels of rumenic acid in their breast milk. Studies on CLA in humans show a tendency for reduced body fat particularly abdominal fat, changes in serum total lipids and decreased whole-body glucose uptake. The maximum reduction in body fat mass was achieved with a 3.4 g daily dose. CLA supplementation has, however, been shown to increase C-reactive protein levels, possibly induce oxidative stress, reduce insulin sensitivity and increase lipid peroxidation. However, the significance of these findings is unknown, and other studies suggest that CLA may protect cells from oxidative damage by increasing glutathione levels without inducing lipid peroxidation. It remains possible, however, that observations of increased markers of lipid oxidation may be related to potentially desirable lipolytic effects. Further studies are necessary to establish clinical significance. Dietary Sources Kangaroo meat may have the highest concentration of CLA when compared with other foods. Food products of grass-fed ruminants (e.g. lamb, beef) are good sources, and contain much more CLA than those from grain-fed animals. In fact, products of grass-fed animals can produce 300-500% more CLA than cows fed the typical diet of 50% hay and silage, with 50% grain. Eggs are also rich in CLA, and it has been shown that the CLA in eggs survives the high temperatures encountered during frying. Possible side effects of CLA in humans There are concerns that the use of CLA by overweight people may increase insulin resistance, possibly increasing the risk of developing diabetes. However, this evidence is controversial, and other studies report no changes in insulin sensitivity. A recent study (2006) by the US Department of Agricult