Conjugated linoleic acid (CLA) is a mixture of isomers of the essential fatty acid linoleic acid (LA, C18:2 (n-6)), containing two conjugated double bonds. Diary products contain the CLA isomer cis-9, trans-11, but CLA is also produced industrially with the main CLA forms resulting from this process being cis-9, trans-11 CLA and trans-10, cis-12 CLA in various proportions. Other CLA isomers may be present in industrially produced CLA in much smaller quantities. In animal models, CLA has been shown to be beneficial in weight loss. Human studies are controversial, but there is evidence that consumption of CLA might induce weight loss in humans. Many studies have investigated the effects of CLA by using olive oil as the control. However, olive oil contains predominately the monounsaturated fatty acid oleic acid. Because CLA is an isomer of linoleic acid more suitable control might be safflower oil, which contains predominately linoleic acid.
In order to understand the metabolism of CLA in more detail, researchers have investigated the effects of an isomeric CLA mixture (4.5g/d), safflower oil, heated oxidised safflower oil or olive oil in a 4 week randomised study involving 85 overweight men. The results showed that CLA significantly reduced the body weight of the subjects when compared to the other oils. However, there was no significant reduction in BMI or waist to hip ratio. There was also no significant difference in endothelial function, total cholesterol, low density lipoprotein, high density lipoprotein, triglycerides, insulin sensitivity, C-reactive protein, platelet activating factor acetylhydrolase, oxidised LDL, soluble adhesion proteins, lipoprotein a or paraoxonase between treatments. However CLA did significantly reduce arylesterase activity and increased concentrations of F2-isoprostane 8-iso-prostaglandin F (PGF)2α. These results suggest that CLA is not damaging to endothelial function, but support the role for CLA in weight loss in humans.
Oxidative stress can increase the release of D2, E2 and F2 isoprostane species from cell membranes via soluble phospholipase enzymes such as paraoxinase (PON) and platelet activating factor acetylhydrolase (PAF-AH). The resulting isoprostanes can then disrupt membrane integrity and cause lipid peroxidation. Previous studies have consistently shown increases in urinary 8-iso PGF2α a biomarker of lipid peroxidation. However, increased systemic oxidative stress usually cause increases in both 8-iso PGF2α and malonaldehyde, which is not the case with CLA. Other studies have shown that CLA competes with 8-iso PGF2α for peroxisomal β-oxidation and by this mechanisms can block its degradation. It has been shown for example, that children with peroxisomal β-oxidation deficiency exhibit decreased metabolic clearance of 8-iso PGF2α. Therefore rather that increasing oxidative stress and lipid perioxidation, as would be suggested by both increases in malonaldehyde and 8-iso PGF2α, CLA may simply inhibit the degradation of the latter.
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