Conjugated linoleic acids (CLA) are a family of isomers of linoleic acid. In principle, 28 isomers are possible. CLA is found mostly in the meat and dairy products derived from ruminants. The two C=C double bonds are conjugated (i.e., separated by a single bond) as opposed by more typical polyunsaturated fatty acids where double bonds are separated by two single bonds.
In many natural CLAs, one of the two C=C bonds is cis and the other trans, making it technically a kind of trans fat. Conjugated trans fatty acids do not, however, appear to have the harmful effects of typical trans fats. Fatty acids with a trans double bond in a conjugated system are therefore exempt from trans fat labeling in the United States and the Codex Alimentarius.
CLA (especially ones with a trans bond) is naturally present in milkfat and beef. CLA has shown some potentially beneficial effects on rodent models of cardiometabolic disease, and is also available as a dietary supplement.
CLA describes a variety of isomers of octadecadienoic fatty acids. To uniquely identify a CLA molecule, a nomenclature resembling "cis-9-trans-11-CLA" is used, identifying the location and direction of the double bonds. This may be informally abbreviated as "c9,t11-CLA". Another equivalent is "(9Z,11E)-CLA", deriving from the EâÂÂZ notation.
Commonly, CLAs are studied as some mixture of isomers wherein the isomers c9,t11-CLA (rumenic acid) and t10,c12-CLA were the most abundant (which is typical of ruminant sources). Studies show however that individual isomers have distinct 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 c9,t11-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.
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 acidâÂÂwhich is abundant in pomegranate seedsâÂÂis converted to the rumenic acid (which is (9Z,11E)-CLA) upon absorption in rats, suggesting that non-animal sources can still effectively provide dietary CLA.
CLA is marketed in dietary supplement form for its supposed anti-cancer benefit (for which there is a lack of robust evidence or well-accepted mechanism, and very few studies conducted so far) and as a bodybuilding aid. A 2004 review of the evidence said that while CLA seemed beneficial in animal models, there was a lack of good evidence for health benefits in humans despite the many claims made for CLA supplementation.
Likewise, there is insufficient evidence that CLA has a useful benefit for overweight or obese people, as it has been found to have no long-term effect on body composition. Although CLA has shown an effect on insulin response in diabetic rats, there is currently no evidence of this effect in humans.
In 1979 CLAs were found 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).