Conjugated Linoleic Acid. 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.
The dairy and beef industries have taken note of CLA’s healthy benefits and are researching the natural enrichement of CLA in foods.
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.
Some studies of CLA in human diets show that it may reduce body fat, especially abdominal fat. A maximum reduction in body fat was achieved with a daily dose of 3.4g. However, some experts do not recommend taking CLA supplements. CLA supplements contain high levels of the t10,c12 CLA isomer, which has been linked to multiple side effects. Supplementation with this form of CLA has been shown to increase C-reactive protein levels, possibly to induce oxidative stress, to reduce insulin sensitivity, and to increase lipid peroxidation. In contrast, the cis-9,trans-11 CLA isomer, the primary form of CLA found naturally in beef and dairy products, has been shown to protect cells from oxidative damage by increasing glutathione levels without inducing lipid peroxidation. It is possible, however, that the observation of markers of increased lipid oxidation may indicate potentially desirable lipolytic effects. Further studies are necessary to establish the clinical significance of such observations.
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 acids 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.