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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). CLAs are simultaneously both cis-fats and trans-fats, as one of the two C=C bonds is cis and the other trans.
CLA is marketed as a dietary supplement on the basis of its claimed health benefits.[1]
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CLA describes a variety of isomers of octadecadienoic fatty acids.[2]
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.[3] Studies show however that individual isomers have distinct health effects.[4][5]
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.[6] 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.[citation needed] 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.[7][8]
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.[citation needed] However, this bioconversion may not occur at any significant level in those with a digestive disease, gluten sensitivity, or dysbiosis.[9][10][11][12]
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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)[13] and as a bodybuilding aid.[1] 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.[14]
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.[15] Although CLA has shown an effect on insulin response in diabetic rats, there is no evidence of this effect in humans.[16]
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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.[17] 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.[18] Some mushrooms, such as Agaricus bisporus and Agaricus subrufescens, are rare non-animal sources of CLA.[19][20]
However, dietary punicic acid—which is abundant in pomegranate seeds—is converted to the CLA rumenic acid upon absorption in rats,[21] suggesting that non-animal sources can still effectively provide dietary CLA.
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In 1979 CLAs were found to inhibit chemically-induced cancer in mice [22] and research on its biological activity has continued.[23]
In 2008, the United States Food and Drug Administration categorized CLA as generally recognized as safe (GRAS).[24]
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