Trans-geometrical and positional isomers of linoleic acid including conjugated linoleic acid (CLA) in German milk and vegetable fats

A representative number of different milk fats based on a wide range of feeding and lactation conditions as well as 123 German margarines, shortenings, cooking and dietetic fats were analyzed for a variety of trans-C18:2 isomers (exhibiting at least one trans double bond) by means of gas chromatography on a 100m Sil 88 capillary column. In milk fats contents of trans Δ9,trans Δ12,cis Δ9,trans Δ13 (+ trans Δ8,cis Δ12),trans Δ8,cis Δ13,cis Δ9,trans Δ12,trans Δ9,cis Δ12 and trans Δ11,cis Δ15 amounted to 0.09%, 0.11%, 0.11%, 0.10%, 0.07% and 0.33% on average and the content of total trans-C18:2 isomers (without cis Δ9, trans Δ11) was 0.99% in the mean. The content of conjugated linoleic acid cis Δ9,trans Δ11 amounted to 0.81% on average in 238 milk fats. In margarine among others the isomers trans Δ9,trans Δ12,cis Δ9,trans Δ13 (+ trans Δ8,cis Δ12), cis Δ9,trans Δ12 and trans Δ9,cis Δ12 were determined and quantified to 0.03%, 0.04%, 0.29% and 0.23% on average. The mean total content of trans-C18:2 isomers in margarines was 0.61%. Moreover, for all trans-C18:2 isomers the frequency distributions as well as the correlation coefficients towards the trans-C18:1 isomers trans Δ6 to trans Δ16 were derived.     

Effect of dietary conjugated linoleic acid (CLA) on metabolism of isotope-labeled oleic,linoleic, and CLA isomers in women

The purpose of this study was to investigate the effect of dietary CLA on accretion of 9c-18∶1, 9c, 12c-18∶2, 10t, 12c-18∶2, and 9c, 11t-18∶2 and conversion of these FA to their desaturated, elongated, and chain-shortened metabolites. The subjects were six healthy adult women who had consumed normal diets supplemented with 6 g/d of sunflower oil or 3.9 g/d of CLA for 63 d. A mixture of 10t, 12c-18∶2-d ₄, 9c, 11t-18∶2-d ₆, 9c-18∶1-d ₈, and 9c, 12c-18∶2-d ₂, as their ethyl esters, was fed to each subject, and nine blood samples were drawn over a 48-h period. The results show that dietary CLA supplementation had no effect on the metabolism of the deuterium-labeled FA. These metabolic results were consistent with the general lack of a CLA diet effect on a variety of physiological responses previously reported for these women. The ²H-CLA isomers were metabolically different. The relative percent differences between the accumulation of 9c, 11t-18∶2-d ₆ and 10t, 12c-18∶2-d ₄ in plasma lipid classes ranged from 9 to 73%. The largest differences were a fourfold higher incorporation of 10t, 12c-18∶2-d ₄ than 9c, 11t-18∶2-d ₆ in 1-acyl PC and a two- to threefold higher incorporation of 9c, 11t-18∶2-d ₆ than 10t, 12c-18∶2-d ₄ in cholesterol esters. Compared to 9c-18∶1-d ₈ and 9c, 12c-18∶2-d ₂, the 10t, 12c-18∶2-d ₄ and 9c, 11t-18∶2-d ₆ isomers were 20–25% less well absorbed. Relative to 9c-18∶1, incorporation of the CLA isomers into 2-acyl PC and cholesterol ester was 39–84% lower and incorporation of 10t, 12c-18∶2 was 50% higher in 1-acyl PC. This pattern of selective incorporation and discrimination is similar to the pattern generally observed for trans and cis 18∶1 positional isomers. Elongated and desaturated CLA metabolites were detected. The concentration of 6c, 10t, 12c-18∶3-d ₄ in plasma TG was equal to 6.8% of the 10t, 12c-18∶2-d ₄ present, and TG was the only lipid fraction that contained a CLA metabolite present at concentrations sufficient for reliable quantification. In conclusion, no effect of dietary CLA was observed, absorption of CLA was less than that of 9c-18∶1, CLA positional isomers were metabolically different, and conversion of CLA isomers to desaturated and elongated metabolites was low.     

Analytical techniques for conjugated linoleic acid (CLA) analysis

Interest in conjugated linoleic acid (CLA) research has increased significantly in recent years because various CLA isomers have been reported to exhibit different beneficial physiological effects in laboratory animals. These findings are detailed in the first book on this subject entitled Advances in CLA Research, Volume 1, edited by M. P. Yurawecz, M. M. Mossoba, J. K. G. Kramer, M. W. Pariza, and G. J. Nelson, and published by the American Oil Chemists Society, Champaign, Illinois, USA, in 1999. Various mixtures of CLA isomers are sold as dietary supplements today, but the biological activities observed in animal models have yet to be demonstrated in humans. Hurdles to achieving this goal include the commercial unavailability of many individual CLA isomers, and the challenge of isolating, separating and confirming the identity of low levels of CLA isomers and their metabolites in complex chemical and/or biological matrices. Therefore, there has been a critical need for analytical methodologies that can meet these difficult objectives. This dossier is about a collection of useful analytical techniques and procedures some of which are well known, while others have only recently been developed and successfully applied to CLA research. These include GC, Ag⁺‐HPLC, RP‐HPLC, GC‐MS, GC‐IR, and NMR.     

Modulation of arachidonic acid distribution by conjugated linoleic acid isomers and linoleic acid in MCF-7 and SW480 cancer cells

The relationship between growth and alterations in arachidonic acid (AA) metabolism in human breast (MCF-7) and colon (SW480) cancer cells was studied. Four different fatty acid preparations were evaluated: a mixture of conjugated linoleic acid (CLA) isomers (c9,t11, t10,c12, c11,t13, and minor amounts of other isomers), the pure c9,t11-CLA isomer, the pure t10,c12-CLA isomer, and linoleic acid (LA) (all at a lipid concentration of 16 microg/mL). 14C-AA uptake into the monoglyceride fraction of MCF-7 cells was significantly increased following 24 h incubation with the CLA mixture (P < 0.05) and c9,t11-CLA (P < 0.02). In contrast to the MCF-7 cells, 14C-AA uptake into the triglyceride fraction of the SW480 cells was increased while uptake into the phospholipids was reduced following treatment with the CLA mixture (P < 0.02) and c9,t11-CLA (P < 0.05). Distribution of 14C-AA among phospholipid classes was altered by CLA treatments in both cell lines. The c9,t11-CLA isomer decreased (P < 0.05) uptake of 14C-AA into phosphatidylcholine while increasing (P < 0.05) uptake into phosphatidylethanolamine in both cell lines. Both the CLA mixture and the t10,c12-CLA isomer increased (P < 0.01) uptake of 14C-AA into phosphatidylserine in the SW480 cells but had no effect on this phospholipid in the MCF-7 cells. Release of 14C-AA derivatives was not altered by CLA treatments but was increased (P < 0.05) by LA in the SW480 cell line. The CLA mixture of isomers and c9,t11-CLA isomer inhibited 14C-AA conversion to 14C-prostaglandin E2 (PGE2) by 20-30% (P < 0.05) while increasing 14C-PGF2alpha by 17-44% relative to controls in both cell lines. LA significantly (P < 0.05) increased 14C-PGD2 by 13-19% in both cell lines and increased 14C-PGE2 by 20% in the SW480 cell line only. LA significantly (P < 0.05) increased 5-hydroperoxyeicosatetraenoate by 27% in the MCF-7 cell line. Lipid peroxidation, as determined by increased levels of 8-epi-prostaglandin F2alpha (8-epi-PGF2alpha), was observed following treatment with c9,t11-CLA isomer in both cell lines (P < 0.02) and with t10,c12-CLA isomer in the MCF-7 cell line only (P < 0.05). These data indicate that the growth-promoting effects of LA in the SW480 cell line may be associated with enhanced conversion of AA to PGE2 but that the growth-suppressing effects of CLA isomers in both cell lines may be due to changes in AA distribution among cellular lipids and an altered prostaglandin profile.     

Lipase-catalyzed fractionation of conjugated linoleic acid isomers

The abilities of lipases produced by the fungus Geotrichum candidum to selectively fractionate mixtures of conjugated linoleic acid (CLA) isomers during esterification of mixed CLA free fatty acids and during hydrolysis of mixed CLA methyl esters were examined. The enzymes were highly selective for cis-9,trans-11–18∶2. A commercial CLA methyl ester preparation, containing at least 12 species representing four positional CLA isomers, was incubated in aqueous solution with either a commercial G. candidum lipase preparation (Amano GC-4) or lipase produced from a cloned high-selectivity G. candidum lipase B gene. In both instances selective hydrolysis of the cis-9,trans-11–18∶2 methyl ester occurred, with negligible hydrolysis of other CLA isomers. The content of cis-9,trans-11–18∶2 in the resulting free fatty acid fraction was between 94 (lipase B reaction) and 77% (GC-4 reaction). The commercial CLA mixture contained only trace amounts of trans-9,cis-11–18∶2, and there was no evidence that this isomer was hydrolyzed by the enzyme. Analogous results were obtained with these enzymes in the esterification in organic solvent of a commercial preparation of CLA free fatty acids containing at least 12 CLA isomers. In this case, G. candidum lipase B generated a methyl ester fraction that contained >98% cis-9,trans-11–18∶2. Geotrichum candidum lipases B and GC-4 also demonstrated high selectivity in the esterification of CLA with ethanol, generating ethyl ester fractions containing 96 and 80%, respectively, of the cis-9,trans-11 isomer. In a second set of experiments, CLA synthesized from pure linoleic acid, composed essentially of two isomers, cis-9,trans-11 and trans-10,cis-12, was utilized. This was subjected to esterification with octanol in an aqueous reaction system using Amano GC-4 lipase as catalyst. The resulting ester fraction contained up to 97% of the cis-9,trans-11 isomer. After adjustment of the reaction conditions, a concentration of 85% trans-10,cis-12–18∶2 could be obtained in the unreacted free fatty acid fraction. These lipase-catalyzed reactions provide a means for the preparative-scale production of high-purity cis-9,trans-11–18∶2, and a corresponding CLA fraction depleted of this isomer.     

Safety of conjugated linoleic acid (CLA) in overweight or obese human volunteers

The main objective of the study was to investigate the safety of conjugated linoleic acid (CLA) in healthy volunteers. The effect of CLA on body composition was also investigated. The trial design was a randomized, double‐blind placebo controlled study including 60 overweight or obese volunteers (body mass index (BMI) 27.5—39.0 kg/m²). The subjects were divided into two groups receiving 3.4 g CLA or placebo (4.5 g olive oil) daily for 12 weeks. The safety was evaluated by analysis of blood parameters and by clinical examinations at baseline and week 12. Vital signs and adverse events were registered at baseline, week 6, and week 12. Bio Impedance Assessment was applied for body composition measurements. 55 subjects completed the study. Adverse events occurred in 10% of the subjects. No difference in adverse events or other safety parameters was found between the treatment groups. Small changes in the laboratory safety data were not regarded as clinically significant. Moreover, no clinically significant changes in vital signs were observed in any of the groups. In the CLA group, mean weight was reduced by 1.1 kg (paired t‐test p = 0.005), while mean BMI was reduced by 0.4 kg/m²(p = 0.007). However, the overall treatment effect of CLA on body weight and BMI was not significant. There were no differences found between the groups with regard to efficacy parameters. The results indicate that CLA in the given dose is a safe substance in healthy populations with regard to the safety parameters investigated.     

Distributions of conjugated linoleic acid (CLA) isomers in tissue lipid classes of pigs fed a commercial CLA mixture determined by gas chromatography and silver ion-high-performance liquid chromatography

Pigs were fed a commercial conjugated linoleic acid (CLA) mixture, prepared by alkali isomerization of sunflower oil, at 2% of the basal diet, from 61.5 to 106 kg live weight, and were compared to pigs fed the same basal diet with 2% added sunflower oil. The total lipids from liver, heart, inner back fat, and omental fat of pigs fed the CLA diet were analyzed for the incorporation of CLA isomers into all the tissue lipid classes. A total of 10 lipid classes were isolated by three-directional thin-layer chromatography and analyzed by gas chromatography (GC) on long capillary columns and by silver-ion high-performance liquid chromatography (Ag⁺-HPLC); cholesterol was determined spectrophotometrically. Only trace amounts (<0.1%; by GC) of the 9,11–18∶2 cis/trans and trans, trans isomers were observed in pigs fed the control diet. Ten and twelve CLA isomers in the diet and in pig tissue lipids were sepatated by GC and Ag⁺-HPLC, respectively. The relative concentration of all the CLA isomers in the different lipid classes ranged from 1 to 6% of the total fatty acids. The four major cis/trans isomers (18.9% 11 cis, 13 trans-18∶2; 26.3% 10 trans, 12 cis-18∶2; 20.4% 9 cis, 11 trans-18∶2; and 16.1% 8 trans, 10 cis-18∶2) constituted 82% of the total CLA isomers in the dietary CLA mixture, and smaller amounts of the corresponding cis,cis (7.4%) and trans,trans (10.1%) isomers were present. The distribution of CLA isomers in inner back fat and in omental fat of the pigs was similar to that found in the diet. The liver triacylglycerols (TAG), free fatty acids (FFA), and cholesteryl esters showed a similar patterns to that found in the diet. The major liver phospholipids showed a marked increase of 9 cis,11 trans-18∶2, ranging from 36 to 54%, compared to that present in the diet. However, liver diphosphatidylglycerol (DPG) showed a high incorporation of the 11 cis,13 trans-18∶2 isomer (43%). All heart lipid classes, except TAG, showed a high content of 11 cis,13 trans-18∶2, which was in marked contrast to results in the liver. The relative proportion of 11 cis,13 trans-18∶2 ranged from 30% in the FFA to 77% in DPG. The second major isomer in all heart lipids was 9 cis,11 trans-18∶2. In both liver and heart lipids the relative proportions of both 10 trans,12 cis-18∶2 and 8 trans,10 cis-18∶2 were significantly lower compared to that found in the diet. The FFA in liver and heart showed the highest content of trans,trans isomers (31 to 36%) among all the lipid classes. The preferential accumulation of the 11 cis,13 trans-18∶2 into cardiac lipids, and in particular the major phospholipid in the inner mitochondrial membrane, DPG, in both heart and liver, appears unique and may be of concern. The levels of 11 cis,13 trans-18∶2 naturally found in foods have not been established.     

Occurrence of conjugated linoleic acid isomers in beef

The amounts of Δ9,Δ11-conjugated linoleic acid (CLA) isomers were determined in loin-associated fat samples of bulls (n=6) and steers (n=7) by capillary gas chromatography of fatty acid methyl ester (FAME) derivatives. The main CLA-isomer—18:2 c9,t11—provided approximately 0.76 ± 0.15% and 0.86 ± 0.15% of total FAME in bulls and steers, respectively. No differences (P>0.05) were observed between the CLA isomer distribution of bulls (t9,c11, 0.026 ± 0.014%; c9,c11, 0.015 ± 0.008%; and t9,t11, 0.029 ± 0.003%) and steers (t9,c11, 0.027 ± 0.014%; c9,c11, 0.015 ± 0.005%; and t9,t11, 0.030 ± 0.007%).     

Effects of specific conjugated linoleic acid isomers on growth characteristics in obese Zucker rats

Growing female obese Zucker (fa/fa) rats were treated (via intragastric gavage) for 21 d with either a (i) vehicle [corn oil; 0.9 g/kg body weight (BW)], (ii) CLA mixture [50∶50; trans-10,cis-12 and cis-9,trans-11 CLA], (iii) cis-9,trans-11 CLA, or (iv) trans-10,cis-12 CLA (CLA treatments at 1.5 g CLA/kg BW). Compared with controls, average daily gain (g/d) was reduced 24 and 44% by the CLA mixture and trans-10,cis-12 CLA, respectively There was no treatment effect on average whole-body (minus heart and liver) composition (dry matter basis): fat (70.2%), protein (21.0%), and ash (4.3%). Compared with animals treated with cis-9,trans-11 CLA, obese Zucker rats treated with trans-10,cis-12 and the CLA mixture had 7.8% more carcass water. Treatment had no effect on heart or liver weights or on heart or liver weights as a percentage of body weight, but compared with the other treatments trans-10,cis-12 CLA increased liver lipid contentby 33%. Hepatic lipid ratios of 16∶1/16∶0 and 18∶1/18∶0 (a proxy for Δ⁹-desaturase capability) were not affected by treatment (0.1 and 0.6, respectively). Simlar to previous reports, CLA increased hepatic lipid content and altered both liver and carcass FA composition (i.e., reduced arachidonic acid content), but the ability of CLA to manipulate body composition in obese Zucker rats remains questionable.     

Thermally induced formation of conjugated isomers of linoleic acid

Chemical pathways responsible of the conjugation of linoleic acid during heat treatments such as refining (deodorization), frying or cooking processes have been investigated. For this purpose, methyl linoleate was submitted to oxidative and non‐oxidative thermal conditions. The resulting degradation products were mainly composed of geometrical and conjugated fatty acid isomers. Oxidative conditions were obtained using tert‐butyl hydroperoxide under inert atmosphere, and air. The obtained results from both thermal oxidative conditions were compared to non‐oxidative thermal treatment. Higher levels of conjugated linoleic acid were found when linoleate was heated under oxidative conditions. Two distinct mechanisms responsible for the formation of CLA isomers are proposed and discussed. Evidence of formation of 9,11‐C18:2 and 10,12‐C18:2 acids from 9,12‐C18:2 by a free‐radical chain reaction is provided. The first step consists in the formation of a free radical by abstraction of an active bis‐allylic hydrogen. By delocalization of the initial free radical, two allylic free radicals were stabilized and converted into the corresponding CLA isomers via the abstraction of a hydrogen radical from other linoleic acid or oxygenated species. Kinetic observations confirmed the significance of the bimolecular mechanism. Moreover, the proposed mechanism is supported by several pieces of information from the literature on peroxidation of linoleic acid. Under pure thermal conditions and/or for diluted samples, a second pathway to the formation of CLA from heat‐treated linoleic acid is proposed via an intramolecular rearrangement of the pentadienyl structure. This thermal [1,3]‐sigmatropic rearrangement results in a mixture of 9,11 and 10,12 CLA isomers. The formed cis/trans CLA isomers were readily rearranged by a [1,5]‐sigmatropic shift to yield trans‐8,cis‐10 and cis‐11,trans‐13 CLA isomers, respectively.     

Impact of novel methodologies on the analysis of conjugated linoleic acid (CLA). Implications of CLA feeding studies

Interest in conjugated linoleic acid (CLA) has increased in the past decade as a result of reports of several health benefits related to its consumption. Naturally occurring CLA isomers are found in milk, dairy, and meat products from ruminants. Detailed isomeric composition of CLA in different chemical and biological matrices had been hindered by the lack of adequate analytical techniques. New methodologies were developed and used to determine the distribution of major and minor geometric and positional CLA isomers in cheese, beef, cow milk, human adipose, and human milk. Base-catalyzed methylation was used. A novel silver ion high-performance liquid chromatographic procedure was developed, which successfully resolved up to 16 isomers. The double bond configuration and position for CLA isomers were confirmed by gas chromatography (GC)-direct deposition-Fourier transform infrared spectroscopy and GC-electron ionization mass spectrometry, respectively: The incorporation of CLA isomers in tissues of animals fed CLA diets was also determined. Currently available analytical data suggest the need to re-evaluate prior CLA studies and their nutritional and biological implications.     

Quantitative determination of conjugated linoleic acid isomers in beef fat

The amounts of 14 conjugated linoleic acid (CLA) isomers (t12t14, t11t13, t10t12, t9t11, t8t10, t7t9, t6t8; 12,14 c/t, t11c13, c11t13, t10c12, 9,11 c/t, t8c10, t7c9‐18:2) in 20 beef samples were determined by triple‐column silver‐ion high‐performance liquid chromatography (Ag⁺‐HPLC). Quantitation was performed using an external CLA reference standard consisting of cis9,trans11‐18:2,trans9,trans11‐18:2 and cis9,cis11‐18: 2. Linearity was checked as being r > 0.9999 between 0.02 × 10^‐3 to 2 mg/ml. The determination limit (5‐fold signal/noise ratio) of the CLA reference was estimated to be 0.25, 0.50, 1.0 ng/injection for the cis/trans, trans,trans and cis,cis isomers, respectively. As expected, cis9,trans11‐18:2 was the predominant isomer (1.95 ± 0.54 mg/g fat) in beef, followed by trans7,cis9‐18:2 (0.19 ± 0.04 mg/g fat); cis,cis isomers were below the determination limit in most beef samples. Total CLA amounts determined by Ag⁺‐HPLC were compared to total CLAs determined by gas chromatography (GC, 100 m CPSil^TM 88 column). The amounts obtained by GC were generally higher than those determined by Ag⁺ ‐HPLC due to co‐eluting compounds.     

Analysis of conjugated linoleic acid isomers and content in french cheeses

Conjugated linoleic acid (CLA) occurs in food as a result of microbial enzymatic reactions, free radical-type oxidation, and heat treatment. CLA is found in animal products, such as meat and dairy products, especially in cheeses. The CLA composition of 12 different French cheeses was determined by a combination of different analytical methods: reversed-phase high-performance liquid chromatography (RP-HPLC), gas chromatography-mass spectrometry (GC-MS), GC-Fourier transform infrared (GC-FTIR), and silver nitrate thin-layer chromatography (AgNO₃-TLC). New isomers (Δ8,10- and Δ11,13-octadecadienoic acids with all possible cis and trans configurations) that co-eluted with previously identified isomers (Δ9c,11t-; Δ9t,11c-; Δ10c,12t-; Δ10t,12c-; Δ11c,13c-; Δ9c,11c-; Δ10c,12c-; Δ9t,11t-; Δ10t12t-octadecadienoic acids) were detected. Δ9c,11t-Octadecadienoic acid was the major CLA isomer in these cheeses. All isomers were present in each product, whatever the production process. However, CLA content in the cheeses varied from 5.3 to 15.80 mg/g of cheese fat, which depended primarily on the origin of the milk (season, geography) and somewhat on the production process.     

Enzymatic enrichment of conjugated linoleic acid isomers and incorporation into triglycerides

A method was developed for the enrichment of either the cis9,trans11 or the trans10,cis12 isomer of conjugated linoleic acid (CLA) from a synthetic CLA mixture consisting predominantly of these isomers in equal amounts. Lipases were screened for their ability to selectively esterify one isomer at a significantly greater rate than the other isomer. An immobilized lipase from Rhizomucor miehei was nonselective, but a lipase from Geotrichum candidum esterified the cis9,trans11 isomer more rapidly than the trans10,cis12 isomer. This selectivity was exploited at the kilogram scale to prepare an ester fraction with a content of 91% cis9,trans11 CLA and an unreacted free fatty acid fraction consisting of 82% trans10,cis12 CLA, based on total CLA content. The components of the reaction mixture were separated by molecular distillation. Each enriched fraction was then incorporated into palm oil triglycerides by interesterification with the non-selective lipase from R. miehei. Two triglyceride fats resulted, which were enriched in either cis9,trans11 CLA (26.5% cis9,trans11 and 1.7% trans10,cis12) or trans10,cis12 CLA (3.5% cis9,trans11 and 22.9% trans10,cis12).     

Production of conjugated linoleic acid isomers by dehydration and isomerization of castor bean oil

The possibility of combining dehydration and isomerization of castor bean oil as a means to obtain CLA as TAG forms was studied. First, dehydration was carried out using various catalysts and reaction parameters. Best results were obtained using phosphoric acid (0.1% w/w) at 280°C for 5 h. Under such conditions, satisfactory proportions of CLA were obtained (54% of total FA) with a majority of 9-cis, 11-trans isomer (61% of total CLA). Other catalysts such as bisulfate-bisulfite, sulfuric acid, tungstic and phosphotungstic acids, or resins and zeolites were also tested. With the exception of resins and zeolites, these catalysts also led to CLA production but in limited amounts in comparison with phosphoric acid. In a second step, an isomerization reaction was carried out to transform the residual nonconjugated linoleic acid also produced during dehydration into CLA. Using Wilkinson catalyst [RhCl(PPh₃)₃] in ethanol solvent, dehydrated castor bean oil was isomerized in high yields (>98%), allowing a complete disappearance of nonconjugated linoleic acids. The resulting dehydrated/isomerized oil contained more than 87% CLA with the 9-cis, 11-trans isomer being predominant (40% of CLA fraction). Finally, urea fractionation was also applied on dehydrated/isomerized castor bean oil FA to obtain FFA products containing about 93% CLA.     

Analysis,occurrence, and physiological properties of trans fatty acids (TFA) with particular emphasis on conjugated linoleic acid isomers (CLA) – a review

The present review provides an outline of the current knowledge of trans fatty acids (TFA) including their structure and formation, analysis, occurrence in foods, estimation and evaluation of daily intake, contents in human adipose tissues and fluids, and physiological properties. Special emphasis is put on conjugated linoleic acids (CLA), which are related to unique beneficial physiological properties. The effects of CLA on carcinogenesis in in vivo and human cancer cell culture studies, on cancer inhibition via the immune system, and further physiological properties are briefly reported. Ways to affect the CLA contents in foods, e.g. influence of feeding regimens or processing conditions, are also discussed.     

Conjugated linoleic acid (CLA) isomers: formation,analysis, amounts in foods,and dietary intake

The analysis, content, and daily intake of conjugated linoleic acid isomers (CLA) are presented in the following review. Modern analytical techniques such as capillary gas chromatography (GC), silverion high performance liquid chromatography (Ag⁺-HPLC) combined with different detection methods (flame ionisation, mass spectroscopic, ultra violet) are mandatory for the unequivocal determination of geometric and positional CLA isomers. An overview is given on the CLA contents in 139 German foods, e. g. milk/dairy products, meat/meat products, edible oils, margarines, fish, and deep fried products. The dietary intake for men and women is estimated using consumption data.     

Silver-ion high-performance liquid chromatographic separation and identification of conjugated linoleic acid isomers

This is the first report of the application of silverion impregnated high-performance liquid chromatography (Ag⁺-HPLC) to the separation of complex mixtures of conjugated linolenic acid (CLA) isomers present in commercial CLA sources and foods and in biological specimens. This method showed a clear separation of CLA isomers into three groups related to their trans,trans, cis,trans or trans,cis, and cis,cis configuration of the conjugated double-bound system. In addition, this method separated within each geometrical isomeric group. Following Ag⁺-HPLC isolation, gas chromatography (GC)-electron impact mass spectrometry, and GC-direct deposition-Fourier transformed infrared spectroscopy were used to confirm the identity of two major positional isomers in the cis/trans region, i.e., Δ8,10- and Δ11,13-octadecadienoic acid, which had not been chromatographically resolved previously, Furthermore, the potential of this method was demonstrated by showing different Ag⁺-HPLC profiles exhibiting patterns of isomeric distributions for biological specimens from animals fed a diet containing a commerical CLA preparation, as well as for a commerical cheese product.