Conjugated linoleic acid (CLA) refers to a family of many isomers of linoleic acid (at least 13 are reported), which are found primarily in the meat and dairy products of ruminants. As implied by the name, the double bonds of CLAs are conjugated.
Additional recommended knowledge
CLA were first isolated by Pariza in 1983 from ground meat but their structure was determined later.
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 acid, 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 ; however, it is suspected that sufficient concentrations to achieve anti-inflammatory effects within human tissues may not be attainable via oral consumption.
A European team lead 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 shows 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 and to induce oxidative stress and to reduce insulin sensitivity and increase lipid peroxidation.
Possible side effects of CLA in humans
There are concerns that the use of CLA by overweight people may actually cause insulin resistance, leading to an increased risk for developing diabetes.
A recent study (2006) conducted in mice by the US Department of Agriculture gives account of some highly concerning effects of CLA: it can dramatically induce essential fatty acid redistribution (DHA and AA) in various organ tissues.
The same study raises the concern that it might pose significant risks, especially regarding cardiovascular health and inflammatory diseases. DHA content in heart tissue for instance was found to be reduced by no less than 25% by certain CLA isomers, while spleen DHA increased 6-fold and spleen AA was reduced to only 5% of its normal levels in that tissue.
. Another study (2005) of CLA supplementation of hatching chicks showed high mortality and low hatchability rate among the CLA-supplemented groups, and also a decrease in brain DHA levels of CLA-incubated chicks. 
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.
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