Conjugated linoleic acid
Group of compounds found in meat and dairy from ruminants
From Wikipedia, the free encyclopedia
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.[1] Fatty acids with a trans double bond in a conjugated system are therefore exempt from trans fat labeling in the United States[2] and the Codex Alimentarius.[3]
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.[4]
Chemistry
Chemical nomenclature
CLA describes a variety of isomers of octadecadienoic fatty acids.[5] 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 E–Z 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).[6] Studies show however that individual isomers have distinct health effects.[7][8]
Chemical properties
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.[1] 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.[2]
Endogenous production
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.[9][10]
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.[11][12][13][14]
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.[15] 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.[16] Some mushrooms, such as Agaricus bisporus and Agaricus subrufescens, are rare non-animal sources of CLA.[17][18]
However, dietary punicic acid—which is abundant in pomegranate seeds—is converted to the rumenic acid (which is (9Z,11E)-CLA) upon absorption in rats,[19] suggesting that non-animal sources can still effectively provide dietary CLA.
Health effects
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)[20] and as a bodybuilding aid.[4] 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.[21]
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.[22] Although CLA has shown an effect on insulin response in diabetic rats, there is currently no evidence of this effect in humans.[23]
History
In 1979 CLAs were found to inhibit chemically induced cancer in mice [24] and research on its biological activity has continued.[25]
In 2008, the United States Food and Drug Administration categorized CLA as generally recognized as safe (GRAS).[26]