Conjugated linoleic acids (CLA) are a family of at least 28 isomers of linoleic acid found especially in the meat and dairy products derived from ruminants. As the name implies, the double bonds of CLAs are conjugated.
History
CLAs were discovered accidentally by researchers looking for mutagens in beef. In 1979, researchers from the University of Wisconsin applied a beef extract to mice skin. The mice were then exposed to a strong carcinogen. When the researchers counted the number of tumors developed by the mice 16 weeks later, they found, to their surprise, that the mice exposed to the beef extract had 20% fewer tumors. The identity of this anti-carcinogen was not discovered till almost a decade later in 1987. Micheal Pariza, the scientist who discovered CLA, later remarked that "few anticarcinogens, and certainly no other known fatty acids, are as effective as CLA in inhibiting carcinogenesis in these models." Although CLA is best known for its anti-cancer properties, researchers have also found that the cis-9, trans-11 form of CLA can reduce the risk for cardiovascular disease and help fight inflammation.
CLA is also known for its body weight management properties, which include reducing body fat and increasing lean muscle mass. Over 30 clinical studies have been published investigating the effect of CLA on weight management. The trials have quite variable designs, which leads to inconsistency. However a meta-analysis conducted in 2007 clearly shows that CLA does indeed have a small impact on fat mass.
In July 2008, CLA received a no objection letter from the FDA on it GRAS (Generally Regarded as Safe) status for certain food categories including fluid milk, yogurt, meal replacement shakes, nutritional bars, fruit juices and soy milk. With GRAS status, food companies are now able to add CLA to products in these food categories.
Biochemistry
Most studies of CLAs have used a mixture of isomers wherein the isomers c9,t11-CLA 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 is not harmful, but beneficial. 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.
Diet and health
Antioxidant and anti-cancer properties have been attributed to CLA, and studies on mice and rats show encouraging results in hindering the growth of tumors in mammary, skin, and colon tissues. It has been reported that CLA can up-regulate the tumor suppressor gene PTPRG, and may have anti-cancer properties.
A European team led by the Swiss scientist Lukas Rist has found that mothers consuming mostly organic milk and meat products have about 50 percent higher levels of rumenic acid in their breast milk.
Thirty four intervention studies using CLA in humans investigate the effects of CLA to reduce body fat, especially abdominal fat. The effect of CLA on fat mass is so small; therefore, it is not the answer to the prevalence of obesity, but it can be used as an additional tool for those individuals with a healthy lifestyle and exercise program to achieve and maintain a healthy body weight. A maximum reduction in body fat was achieved with a daily dose of 3.4g.
Possible adverse effects of CLA in humans
There are concerns that the use of CLA supplements by overweight people may tend to cause or to aggravate insulin resistance, which may increase their risk of developing diabetes. Most supplements contain mixtures of two CLA isomers: the cis-9, trans-11 isomer (the predominant CLA isomer in milk and beef) as well as the trans-10, cis-12 (t10c12) isomer. It is the trans-10, cis-12 isomer that is linked to many adverse side effects. Research indicates that supplementation with t10c12 CLA dramatically increases rates of oxidative stress, to levels considerably higher than that observed in heavy smokers. However, the evidence is controversial, and some studies using a mixture of c9t11 and t10c12 CLA showed no changes in insulin sensitivity. Although researchers are still not sure of the long term health effects of consuming CLA from supplements, foods naturally enriched in CLA, such as beef and dairy, are a safe alternative.
In one study t10c12 CLA produced a 32% increase in biliary cholesterol concentration which increases the chance of gallstone formation.
In 2006, a study by the US Department of Agriculture suggested that CLA can induce essential fatty acid redistribution in mice. Changes in docosahexaenoic acid (DHA) and arachidonic acid (AA) levels were observed in some organs. For instance, the t10,c12 CLA reduced the DHA content of heart tissue by 25%, while in the spleen, DHA content rose, and AA fell. DHA is an omega-3 fatty acid important to cardiovascular health and the dramatic reduction of DHA in heart tissue can have serious health consequences. In contrast, c9,t11 CLA did not alter DHA content in the heart, but did reduce spleen DHA slightly. A study of CLA supplementation (equal amounts of c9,t11 and t10,c12) in hatchling chicks (2005) showed high mortality and low hatchability rates among CLA-supplemented groups, and also a decrease in brain DHA levels of CLA-treated chicks. These studies raise the question of whether CLA may increase the risk of cardiovascular and inflammatory diseases, but it has yet to be established whether such changes occur in humans, and whether they are clinically relevant.
Dietary Sources
Of all foods, kangaroo meat may have the highest concentration of CLA. Food products (e.g. mutton and beef) from grass-fed ruminants are good sources of CLA, and contain much more of it than those from grain-fed animals. In fact, meat and dairy products from grass-fed animals can produce 300-500% more CLA than those of cattle fed the usual diet of 50% hay and silage, and 50% grain.
Eggs are also rich in CLA, and it has been shown that CLA in eggs survives the temperatures encountered during frying.
Some mushrooms like Agaricus bisporus and Agaricus blazei , are a rare vegetable source of CLA.
See also
- Conjugated fatty acids
References
- ^ Banni S (June 2002). "Conjugated linoleic acid metabolism". Curr. Opin. Lipidol. 13 (3): 261–6. doi: 10.1097/00041433-200206000-00005 . PMID 12045395 . http://meta.wkhealth.com/pt/pt-core/template-journal/lwwgateway/media/landingpage.htm?issn=0957-9672&volume=13&issue=3&spage=261 .
- ^ Ha YL, Grimm NK, Pariza MW (1987). "Anticarcinogens from fried ground beef: heat-altered derivatives of linoleic acid". Carcinogenesis 8 (12): 1881–7. doi: 10.1093/carcin/8.12.1881 . PMID 3119246.
- ^ "CLA: Conjugated Linoleic Acid - Google Book Search" . http://books.google.com/books?hl=en&id=su_k_WkP0KgC&dq=Conjugated+linoleic+acid+Williams&printsec=frontcover&source=web&ots=W-aRO2HX46&sig=20WMkVLUeV-AXmYtcIZwEgF14GU&sa=X&oi=book_result&resnum=10&ct=result#PPP1,M1 .
- ^ Tricon S, Burdge GC, Kew S, et al. (September 2004). "Opposing effects of cis-9,trans-11 and trans-10,cis-12 conjugated linoleic acid on blood lipids in healthy humans". Am. J. Clin. Nutr. 80 (3): 614–20. PMID 15321800. ...
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