Conjugated linoleic acid
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 "typical" polyunsaturated fatty acids where double bonds are separated by two single bonds. 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 naturally present in milkfat and beef, has shown some potentially beneficial effects on rodent models of cardiometabolic disease, and is also available as a dietary supplement.<ref name=TalbottHughes2007/>
BiochemistryEdit
CLA describes a variety of isomers of octadecadienoic fatty acids.<ref>Template:Cite journal</ref>
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.<ref name="usda">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Studies show however that individual isomers have distinct health effects.<ref name="Tricon S, Burdge GC, Kew S, et al. 2004 614–20">Template:Cite journal</ref><ref name="Risérus"> Template:Cite journal </ref>
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.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> 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.Template: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.<ref name=Kuhnt2006>Template:Cite journal</ref><ref name=Banni>Template:Cite journal</ref>
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.Template:Citation needed However, this bioconversion may not occur at any significant level in those with a digestive disease, gluten sensitivity, or dysbiosis.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>
Health effectsEdit
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)<ref>Template:Cite journal</ref> and as a bodybuilding aid.<ref name="TalbottHughes2007">Template:Cite book</ref> 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.<ref name=Rainer2004>Template:Cite journal</ref>
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.<ref name=sr2012>Template:Cite journal</ref> Although CLA has shown an effect on insulin response in diabetic rats, there is currently no evidence of this effect in humans.<ref name=Davi2010>Template:Cite journal</ref>
Dietary sourcesEdit
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.<ref>Template:Cite journal</ref> 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.<ref>Template:Cite journal</ref> Some mushrooms, such as Agaricus bisporus and Agaricus subrufescens, are rare non-animal sources of CLA.<ref name="Chen.CityofHope">Template:Cite journal</ref><ref name="CLAblazei">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>
However, dietary punicic acid—which is abundant in pomegranate seeds—is converted to the rumenic acid (which is 9Z11E-CLA) upon absorption in rats,<ref name="Tsuzuki">Template:Cite journal</ref> suggesting that non-animal sources can still effectively provide dietary CLA.
HistoryEdit
In 1979 CLAs were found to inhibit chemically-induced cancer in mice <ref name="pmid3119246">Template:Cite journal</ref> and research on its biological activity has continued.<ref>Template:Cite journal</ref>
In 2008, the United States Food and Drug Administration categorized CLA as generally recognized as safe (GRAS).<ref>Template:Cite news</ref>