Conjugated linoleic acids (CLA)
are a family of at least 13 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
CLAs were first isolated in 1983, by M. W. Pariza, from ground meat. Their structure was determined later.
Most studies of CLAs have used a mixture of isomers wherein the isomers c9, t11-CLA and t10, c12-CLA were the most abundant ones.
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
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.
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.
Studies of CLA in human diets show that it tends to reduce body fat, particularly abdominal fat, improves serum lipid profiles, and decreases whole-body glucose uptake. The maximum reduction in body fat was achieved with a daily dose of 3.4g. CLA supplementation has, however, been shown to increase C-reactive protein levels, possibly to induce oxidative stress, to reduce insulin sensitivity, and to increase lipid peroxidation. However, the significance of these findings is unknown, and other studies suggest that CLA may 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 side effects of CLA in humans
There are concerns that the use of CLA by overweight people may tend to cause or to aggravate insulin resistance, which may increase their risk of developing diabetes
. However, the evidence is controversial, and some studies showed no changes in insulin sensitivity.
In one study 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, certain CLA isomers reduced the DHA content of heart tissue by 25%, while in the spleen, DHA content rose, and AA fell. A study of CLA supplementation 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.
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.
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