Significance, analysis and occurrence of conjugated linoleic acid isomers (CLA) in foods

The group of conjugated linoleic acid isomers (CLA), which is formed in the rumen, has shown anticarcinogenic effects in numerous studies with animal models. By feeding of CLA, further positive physiological effects were achi eved on the immune system, on diabetes and especially on the fat metabolism . All mentioned effects were achieved with an isomeric mixture of CLA. An i dentification of the physiologically active isomer has so far only been rea ched for the reduction of fat deposits. For this effect the trans 10, cis12 -octadecadienoic acid was identified. It is obvious that an attribution of the physiological effects to single is omers requires an excellent analysis. Because the use of gas chromatography leads to coelution of single CLA-isomers, only a combination of different chromatographic techniques (GC-FID, Ag HPLC, RP-HPLC, GC-MS, GC-FTIR) can s olve this problem. Using Ag+-HPLC it is possible to separate about 20 peaks attributed to CLA isomers. Prior to an identification small amounts of CLA -isomers can be enriched with prep. HPLC on reversed phase. Gas chromatogra phy-mass spectroscopy (GC-MS) of the dimethyloxazoline derivatives is used to confirm the identity of the resolved positional CLA isomers. The double bond configuration of CLA isomers is done by gas chromatography-Fourier tra nsform infrared spectroscopy (GC-FTIR). Ln addition to the major 18:2 c9t11 isomer in natural products, several minor CLA isomers have been identified in beef, cheese, and human adipose tissue. On this basis it is possible to determine the CLA isomeric distribution. Furthermore only an accurate anal ysis can help to evaluate the intake of single isomers by humans.