Conjugated linoleic acid
Conjugated linoleic acids (CLA) are a family of at least 28 isomers of linoleic acid found mostly in the meat and dairy products derived from ruminants. Their two double bonds are conjugated (i.e., separated by a single bond). CLAs can be either cis-fats or trans-fats. CLA is marketed as a dietary supplement on the basis of its supposed health benefits. Biochemistry. CLA is a mixture of isomers of octadecadienoic fatty acids. Most studies of CLAs have used a mixture of isomers wherein the isomers c9,t11-CLA (rumenic acid) and t10,c12-CLA were the most abundant . More recent studies using individual isomers indicate that the two isomers have very different health effects. Conjugated linoleic acid is both a "trans" fatty acid and a "cis" fatty acid. The "cis" bond causes a lower melting point and, ostensibly, also the observed beneficial health effects. Unlike other "trans" fatty acids, it may have beneficial effects on human health. CLA is conjugated, and in the United States, "trans" linkages in a conjugated system are not counted as "trans" fats for the purposes of nutritional regulations and labeling. CLA and some "trans" isomers of oleic acid are produced by microorganisms in  the rumens of ruminants. Non-ruminants, including humans, produce certain isomers of CLA from "trans" isomers of oleic acid, such as vaccenic acid, which is converted to CLA by delta-9-desaturase. In healthy humans, CLA and the related conjugated linolenic acid (CLNA) isomers are bioconverted from linoleic acid and alpha-linolenic acid, respectively, mainly by "Bifidobacterium" bacteria strains inhabiting the gastrointestinal tract. However, this bioconversion may not occur at any significant level in those with a digestive disease, gluten sensitivity, or dysbiosis. Health effects. CLA is marketed in dietary supplement form for its supposed anti-cancer benefit (for which there  is no strong evidence or known mechanism, and very few studies conducted so far) and as a bodybuilding aid. A 2004 review of the evidence said that while CLA seemed to benefit animals, there was a lack of good evidence of human health benefits despite the many claims made for it. Likewise, there is insufficient evidence that CLA has a useful benefit for overweight or obese people as it has no long-term effect on body composition. Although CLA has shown an effect on insulin response in diabetic rats, there is no evidence of this effect in humans. A 2017  study found CLA supplementation has been associated with increased plasma C-reactive protein concentrations and a reduction in serum adiponectin concentrations, which indicates that CLA supplements have a pro-inflammatory effect. Another study suggests that CLA consumption reduces the risk of breast or colon cancer. This is similar to the results of reduced milk intake and reduced breast or colon cancer. Dietary sources. Food products from grass-fed ruminants (e.g. mutton and beef) are good sources of CLA and contain much more of it than those from grain-fed animals. Eggs from chickens that have been fed CLA are also  rich in CLA, and CLA in egg yolks has been shown to survive the temperatures encountered during frying. Some mushrooms, such as "Agaricus bisporus" and "Agaricus subrufescens", are rare non-animal sources of CLA. However, dietary punicic acid—which is abundant in pomegranate seeds—is converted to the CLA rumenic acid upon absorption in rats, suggesting that non-animal sources can still effectively provide dietary CLA. History. The biological activity of CLA was noted by researchers in 1979 who found it to inhibit chemically-induced cancer in mice and research on its biological activity has continued. In 2008, the United  States Food and Drug Administration categorized CLA as generally recognized as safe (GRAS).