Conjugated linoleic acids: Physiological effects in animal and man with special regard to body composition

In the last decade, conjugated linoleic acids (CLA) have been shown to have some beneficial (but also unfavourable) effects: anticarcinogenic properti es, immune modulation, reduction of body fat and increase of lean body mass , normalisation of impaired glucose tolerance, promotion of fatty streak fo rmation, and isomer-specific effects. The research base on CLA has been der ived almost exclusively from animal models, while some of the biological pr operties have been fairly well-documented, others are still open to questio n. For about 5 years a lot of commercial CLA mixtures have been offered. These mixtures produced from linoleic acid-rich oil like sunflower or safflower oil by alkali isomerization contained, besides cis-9, trans-11 and trans-10 ,cis-12 CLA isomers (about 20-40% of each), parts of cis,cis and trans,tran s isomers as well. The quality of the recent products is significantly impr oved and they contain only two CLA isomers: cis-9,trans-11 and trans-10,cis -12. CLA play apparently a key role in regulating body composition. Several stud ies showed a reduction in body fat mass and a slight increase in lean body mass depending on the species. A possible explanation for the decrease of b ody fat may be a stimulation of lipolysis and a reduction of lipoprotein li pase activity in adipocytes. In adipose and muscle tissue a CLA-stimulated increase of carnitine palmitoyltransferase activity resulting in an enhance d fatty acid oxidation was shown. There is evidence that CLA provide protec tion against cytokine-induced (Tumour necrosis factor-alpha, interleukin-1) skeletal-muscle catabolism (anabolic effect). The body composition modulat ing effects are most impressive in rodents and seem to become smaller in pi gs and in humans. Data on humans are insufficient. Further research is essential to characterize the multifunctionality of CLA in humans, in order to identify the specific physiological mechanism of th e biologically active isomers and to determine the optimal level of these i somers for beneficial effects.