Conjugated linoleic acid
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.
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. Unlike most trans fatty acids found in the human diet, CLA occurs naturally, produced by microorganisms in the fore-stomach of ruminants. Non-ruminants, such as humans, may be able to produce some isomers of CLA from non-conjugated ruminant fats. One such example is vaccenic acid, which could be converted to CLA by delta-9-desaturase.
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.
Many 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 and to induce oxidative stress and to reduce insulin sensitivity and increase lipid peroxidation.
CAS registry number: 2420-56-6, Molecular Formula: C18H32O2
CLA may be beneficial in other aspects, including prevention of breast and colon cancer. title = Dairy Fat Gets a Reprieve|url=http://www.newhope.com/nutritionsciencenews/NSN_backs/Feb_00/natnews_1.cfm|accessdate = 2007-08-11
Possible Side Effects of Purified Isomers of Dietary CLA
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. 
CLA is available commercially in doses of about 500mg to 1300mg (or .5 g to 1.3 g). The optimal dosage is about 3.4 g, with larger doses showing little or no improvement.
CLA should be taken with protein-rich food to prevent side effects, including diarrhea, nausea, and stomachache.
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. 
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- "Fatty Acid Profiles of Liver, Adipose Tissue, Speen, and Heart of Mice Fed Diets Containing T10, C-12-, and C9, T11-Conjugated Linoleic Adic".
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