Evidence that the trans-10,cis-12 isomer of conjugated linoleic acid induces body composition changes in mice

Infant plasma trans, n-6, and n-3 fatty acids and conjugated linoleic acids are related to maternal plasma fatty acids, length of gestation, and birth weight and length

BACKGROUND

Arachidonic acid (AA) and docosahexaenoic acid (DHA) are important for growth and neural development. trans Fatty acids (TFAs) may inhibit desaturation of linoleic acid (LA) and alpha-linolenic acid (ALA) to AA and DHA, respectively. Conjugated linoleic acids (CLAs) also alter lipid metabolism and body fat.

OBJECTIVE

We determined the associations of birth outcome with maternal and infant plasma concentrations of TFAs, CLAs, AA, and DHA.

DESIGN

In healthy women, we sampled maternal blood at 35 wk gestation (n = 58) and umbilical cord blood at birth (n = 70).

RESULTS

Mean (+/- SEM) TFA concentrations (% by wt) in infant plasma were as follows: triacylglycerol, 2.83 +/- 0.19 (range: 0.63-12.79); phospholipid, 0.67 +/- 0.03 (0.11-1.33); and cholesteryl ester, 2.04 +/- 0.01 (0.86-4.24). LA, AA, DHA, TFA, and CLA concentrations in infant phospholipids correlated with the same fatty acid in maternal plasma phospholipids (n = 44; P < 0.05). Infant plasma cholesteryl ester and triacylglycerol TFAs and cholesteryl ester CLAs (r = -0.33, -0.42, and -0.49, respectively) were significantly inversely related to length of gestation. Triacylglycerol and cholesteryl ester AA were positively related to length of gestation (r = 0.41 and 0.37, respectively) and birth weight (r = 0.27 and 0.23, respectively). Inverse correlations occurred between infant plasma TFA and DHA concentrations in triacylglycerols (r = -0.33) and between TFA and AA concentrations in cholesteryl esters (r = -0.23).

CONCLUSION

The results suggest possible important effects of TFAs and of AA on fetal growth and length of gestation.

The effect of conjugated linoleic acid on platelet function, platelet fatty acid composition, and blood coagulation in humans

Despite extensive research on conjugated linoleic acid (CLA) showing multiple beneficial effects in animal models, little is known about the role of dietary CLA in human health. To investigate if the beneficial effects of CLA seen in animal models are relevant to humans, we conducted a study with 17 healthy female volunteers who lived in the Metabolic Research Unit of the Western Human Nutrition Research Center for 93 d. This paper reports only the results from this study that are related to the effects of CLA supplementation on blood coagulation, platelet function, and platelet fatty acid composition. Throughout the study, the subjects were fed a low-fat diet (30 en% fat, 19 en% protein, and 51 en% carbohydrate) consisting of natural foods with the recommended dietary allowances for all known nutrients. After a 30-d stabilization period, subjects were randomly assigned to either an intervention group (n = 10) whose diet was supplemented with 3.9 g/d of CLA or a control group (n = 7) who received an equivalent amount of sunflower oil consisting of 72.6% linoleic acid with no detectable CLA. Platelet aggregation was measured in platelet-rich plasma using adenosine diphosphate, collagen, and arachidonic acid agonists. No statistical difference was detected between the amount of agonist required to produce 50% aggregation of platelet-rich plasma before and after the subjects consumed the CLA, with the exception of a decrease in response to collagen. This decrease was found in both control and intervention groups with no significant difference between the groups, suggesting that both linoleic acid (sunflower oil) and CLA might have similar effects on platelet function. The prothrombin time, activated partial thromboplastin time, and the antithrombin III levels in the subjects were determined. Again, there was no statistically significant difference in these three parameters when pre- and post-CLA consumption values were compared. The in vivo bleeding times were also unaffected by CLA supplementation (10.4 + 2.8 min pre- and 10.2 + 1.6 min postconsumption). Platelet fatty acid composition was not markedly influenced by the consumption of dietary CLA, although there was a small increase in the amount of the 9 cis,11 trans-18:2 isomer normally present in platelets after feeding CLA for 63 days. In addition, small amounts of the 8 trans,10 cis-18:2 and the 10 trans,12 cis-18:2 isomers were detected in the platelets along with traces of some of the other isomers. Thus, when compared to sunflower oil, the blood-clotting parameters and in vitro platelet aggregation showed that adding 3.9 g/d of dietary CLA to a typical Western diet for 63 d produces no observable physiological change in blood coagulation and platelet function in healthy adult females. Short-term consumption of CLA does not seem to exhibit antithrombotic properties in humans.

CLA reduces antigen-induced histamine and PGE(2) release from sensitized guinea pig tracheae

Conjugated linoleic acid (CLA) has been shown to enhance immune reactions such as lymphocyte blastogenesis and delayed-type hypersensitivity. We investigated the role of CLA in type I (immediate) hypersensitivity, using a guinea pig tracheal superfusion model for measuring antigen-induced airway smooth muscle contraction and inflammatory mediator release. Female Hartley guinea pigs were fed a diet supplemented with 0.25 g corn oil or linoleic acid/100 g of diet (control) or 0.25 g CLA/100 g of diet for at least 1 wk before and during active sensitization to ovalbumin antigen. Tracheae from sensitized guinea pigs were suspended in air-filled water-jacketed (37 degrees C) tissue chambers in a superfusion apparatus. Tracheae were superfused with buffer containing antigen, and tissue contraction was recorded. Superfusate was collected at 90-s intervals for evaluation of histamine and PGE(2) release. CLA did not affect antigen-induced tracheal contractions when expressed as gram contraction per gram tissue. CLA significantly reduced antigen-induced histamine and PGE(2) release. CLA appears to decrease release of some inflammatory mediators during type I hypersensitivity reactions.

Different effects of conjugated linoleic acid isomers on lipoprotein lipase activity in 3T3-L1 adipocytes

Conjugated linoleic acids (CLAs) are the positional and geometric isomers of linoleic acid. In the present study the effects of cis-9, trans-11 CLA (c9,t11 CLA) and trans-10, cis-12 CLA (t10,c12 CLA ) on intracellular and heparin-releasable (HR-) lipoprotein lipase (LPL) activity in 3T3-L1 adipocytes were investigated. Cells were exposed to the two CLA isomers and linoleic acid, which were bound to bovine serum albumin (BSA). In the adipocytes insulin up-regulated and tumor necrosis factor alpha (TNFalpha) down-regulated HR-LPL activity, which corresponds with the findings in vivo. The experimental fatty acids at low concentrations (<30 µmol/L) moderately increased intracellular and HR-LPL activity. At a concentration of 100 µmol/L, c9,t11 CLA and t10,c12 CLA suppressed HR-LPL activity to 20 and 24% below the BSA control level, respectively, while linoleic acid had no effect unless its concentration was as high as 1000 µmol/L. Insulin abolished the inhibitory effect of c9,t11 CLA, but not of t10,c12 CLA. In the presence of insulin, t10,c12 CLA inhibited HR-LPL activity by 41% compared to BSA control. In contrast to TNFalpha, which suppressed both intracellular LPL and HR-LPL activity, CLAs suppressed HR-LPL activity without decreasing intracellular LPL activity. Additionally, t10,c12 CLA (100 µmol/L) partially prevented TNFalpha-induced decrease of intracellular LPL activity. These results indicate that CLAs differ from linoleic acid in regulating HR-LPL activity, and t10,c12 CLA appeared to be more effective than c9,t11 CLA.

Changes in conjugated linoleic acid composition within samples obtained from a single source

Conjugated linoleic acid (CLA; 9c,11t-18:2) and CLA isomers have been reported, in animals, to exhibit a variety of health-related benefits. Silver ion high-performance liquid chromatography (Ag-HPLC) was found to provide better resolution of the isomers than gas chromatography. Most commercially available samples of CLA, prepared by base-catalyzed isomerization of linoleic acid (9c,12c-18:2), are composed of mixtures of four major isomers. While these isomers have been characterized, we found significant changes in CLA isomer rations within samples obtained from the same producer/commercial supplier over a period of 1.5 yr. In the first sample, the four cis/trans isomers (8t,10c-18:2, 9c,11t-18:2, 10t,12c-18:2 and 11c,13t-18:2) were present in a ratio of approximately 1:2:2:1, while in the second sample they were present in almost equal proportions. If indeed certain daily levels of CLA intake are required to produce suggested health benefits in humans, changes in concentrations of specific CLA isomers could significantly impact these effects. Care must be taken to analyze the CLA used in human and animal studies.

The growth inhibitory effect of conjugated linoleic acid on a human hepatoma cell line, HepG2, is induced by a change in fatty acid metabolism, but not the facilitation of lipid peroxidation in the cells

We investigated the growth inhibitory effect of conjugated linoleic acid (CLA) on HepG2 (human hepatoma cell line), exploring whether the inhibitory action occurs via lipid peroxidation in the cells. When the cells were incubated up to 72 h with 5-40 microM of CLA (a mixture of 9c,11t-18:2 and 10t,12c-18:2), cell proliferation was clearly inhibited in a dose and time dependent manner but such an inhibition was not confirmed with linoleic acid (LA). In order to evaluate the possible contribution of lipid peroxidation exerted by CLA to cell growth inhibition, alpha-tocopherol (5-20 microM) and BHT (1-10 microM) as potent antioxidants were added to the medium with CLA (20 microM), which did not restore cell growth at all. Furthermore, after 72 h incubation, the membranous phospholipid hydroperoxide formation in the CLA-supplemented cells was suppressed respectively to 25% and 50% of that in LA-supplemented cells and control cells. No difference was observed by a conventional lipid peroxide assay, the TBA test, between CLA-supplemented cells and LA-supplemented cells. Although the cellular lipid peroxidation was not stimulated, lipid contents (triacylglycerol, total cholesterol and free cholesterol) and fatty acid contents (palmitic acid, palmitoleic acid and stearic acid) markedly increased in CLA-supplemented cells compared with LA-supplemented and control cells. Moreover, supplementation with 20 microM LA and 20 microM arachidonic acid profoundly interfered with the inhibitory effect of CLA in HepG2. These results suggest that the growth inhibitory effect of CLA on HepG2 is due to changes in fatty acid metabolism but not to lipid peroxidation.

Conjugated linoleic acid: effects on plasma lipids and cardiovascular function

Conjugated linoleic acid is a collective name for mixtures of several positional and geometric conjugated dienoic isomers of linoleic acid, which have been shown to impact favorably on several biological processes, particularly carcinogenesis. Recent studies have clearly established that the c9, t11 and t10, c12 isomers have distinct biological effects. The latter may be of particular importance in affecting blood lipids. Because conjugated linoleic acid has been suggested to be anti-atherogenic, this review is focused on its effects on cardiovascular function. Careful scrutiny of the literature suggests that at present it is premature to assign any beneficial role to conjugated linoleic acid in terms of its ability to impact either blood lipids or atherogenesis.

Conjugated linoleic acid reduces body fat mass in overweight and obese humans

Conjugated linoleic acid (CLA) has been shown to reduce body fat mass (BFM) in animals. To investigate the dose-response relationships of conjugated linoleic acid with regard to BFM in humans, a randomized, double-blind study including 60 overweight or obese volunteers (body mass index 25-35 kg/m(2)) was performed. The subjects were divided into five groups receiving placebo (9 g olive oil), 1.7, 3.4, 5.1 or 6.8 g conjugated linoleic acid per day for 12 wk, respectively. Dual-energy X-ray absorptiometry was used to measure body composition [measurements at wk 0 (baseline), 6 and 12]. Of the 60 subjects, 47 completed the study. Eight subjects withdrew from the study due to adverse events; however, no differences among treatment groups were found regarding adverse events. Repeated-measures analysis showed that a significantly higher reduction in BFM was found in the conjugated linoleic acid groups compared with the placebo group (P: = 0.03). The reduction of body fat within the groups was significant for the 3.4 and 6.8 g CLA groups (P: = 0.05 and P: = 0.02, respectively). No significant differences among the groups were observed in lean body mass, body mass index, blood safety variables or blood lipids. The data suggest that conjugated linoleic acid may reduce BFM in humans and that no additional effect on BFM is achieved with doses > 3.4 g CLA/d.

Dietary conjugated linoleic acid consumption during pregnancy and lactation influences growth and tissue composition in weaned pigs

We evaluated the effects of conjugated linoleic acid (CLA) on growth performance, tissue fatty acid composition and ex vivo lipogenic enzyme activity in piglets (n = 40) reared on sows fed diets supplemented with CLA or linoleic acid (LA). Weaned offspring of both sow groups were offered either a CLA- or LA-enriched starter diet for 35 d. The starter diets were formulated to contain 2 g CLA (containing 58.9 g CLA/100 g total fatty acids) or LA per 100 g feed. All piglets were slaughtered at 70 d of age and tissue samples of the back fat, omental fat and longissimus dorsi were collected. Irrespective of the dietary fat supplied in the starter period, piglets reared on the CLA sows had greater final body and warm carcass weights (P: < 0.01), and greater feed intake (P: = 0.02) than piglets reared on the LA sows. The dietary effect on the fatty acid composition was similar for the adipose and muscle tissues. Compared with the LA-enriched diets, CLA increased the level of total saturated fatty acids (P: < 0.05), whereas that of monounsaturated fatty acids was decreased (P: < 0.05). Dietary CLA increased glucose-6-phosphate dehydrogenase (P: < 0.01) and malic enzyme activities (P: < 0.06) in the fat tissues, but did not affect fatty acid synthase activity. The shift toward a higher deposition of saturated fatty acids and a lower deposition of monounsaturated fatty acids is the result of down-regulation of Delta9-desaturase activity that was induced by CLA rather than an altered rate of de novo synthesis.

Conjugated linoleic acid: implications for human health

Conjugated linoleic acid (CLA) is being sold as a panacea that has the capability of reducing or eliminating cancer, preventing heart disease, improving immune function, and altering body composition to treat obesity or build lean body mass. Unfortunately, there has been very little published human research on CLA. This review will examine the literature on CLA and discuss the animal research on which the above claims are made. The limited human studies will be presented with an evaluation of the potential uses of CLA for human health and disease.

Changes in conjugated linoleic acid and its metabolites in patients with chronic renal failure

BACKGROUND

Conjugated linoleic acid (CLA) is a mixture of isomers of linoleic acid with conjugated double bonds that constitutes the most abundant fatty acid with conjugated dienes (CDs) in humans. CLA, erroneously considered in the past as a product of lipoperoxidation, has a dietary origin and has shown to possess anticarcinogenic and anti-atherogenic activity, mainly in animal studies. CLA can be metabolized to conjugated linolenic acid (CD18:3) and to conjugated eicosatrienoic acid (CD20:3) and these metabolites may be implicated in CLA activity. Because of the presence of dyslipidemia and the high incidence of cardiovascular and neoplastic diseases in uremic patients, we evaluated CLA and its metabolites in these patients in order to evaluate their metabolism and site distribution.

METHODS

We measured CLA, CD18:3, CD20:3, CD fatty acid hydroperoxides (lipoperoxidation products), and linoleic acid in the plasma, adipose tissue, and red blood cell (RBC) membranes by using high-pressure liquid chromatography in the following groups: (1) 23 chronic renal failure (CRF) patients with creatine clearance (CCr)> 10 mL/min (26.2 +/- 16.7); (2) 21 end-stage CRF patients in conservative treatment with CCr <10 mL/min (6.8 +/- 1.8); (3) 30 hemodialysis (HD) patients; and (4) 30 healthy controls.

RESULTS

The incorporation of CLA, CD18:3, and CD20:3 in RBC membranes was significantly reduced in group 1 and was even more reduced in groups 2 and 3. CLA significantly increased both in the plasma and adipose tissue of end-stage CRF patients only. CD18:3 and CD20:3 did not change in the plasma and adipose tissue of any group. No significant changes in linoleic acid and CD fatty acid hydroperoxides were found.

CONCLUSIONS

The alterations of CD in CRF patients are not due to lipoperoxidation. The increased levels of CLA in plasma and adipose tissue of end-stage CRF patients may be due either to a reduced metabolization of CLA to CD18:3 and CD20:3, or to an altered site distribution with reduced incorporation in cellular membranes and accumulation in the plasma and adipose tissue. The clinical significance of these changes remains to be investigated.

Conjugated linoleic acid persistently increases total energy expenditure in AKR/J mice without increasing uncoupling protein gene expression

AKR/J mice fed a high fat diet were treated with a 1% (1 g/100 g) admixture of conjugated linoleic acids (CLA) for 5 wk and compared with control mice. Body weights, energy intakes and energy expenditure (EE) determined by indirect calorimetry were measured weekly. CLA treatment reduced adipose depot weights by approximately 50% but had no significant effects on either body weight or energy intake. CLA increased EE persistently by an average of 7.7% throughout the 5-wk experiment. This greater EE, despite no difference in energy intake, was sufficient to account for the lower body fat stores in the CLA-treated mice. De novo fatty acid biosynthesis in adipose tissue, measured by incorporation of deuterium-labeled water, was not decreased by CLA treatment and therefore did not explain the lower adipose lipid in these mice. Expression of uncoupling protein (UCP) in skeletal muscle, white adipose tissue and kidney was not affected by CLA treatment. In brown adipose tissue, UCP1 expression was not affected by CLA treatment. However, UCP2 expression, although quite low, was significantly greater in CLA-fed mice. We conclude that CLA acts to reduce body fat stores by chronically increasing metabolic rate. This effect on metabolic rate is likely not due to increased UCP gene expression. Furthermore, the reduced body fat is not due to decreased de novo fatty acid synthesis in white adipose tissue.

Comparison of silver-ion high-performance liquid chromatographic quantification of free and methylated conjugated linoleic acids

Silver-ion high-performance liquid chromatography was used to fractionate a mixture of conjugated linoleic acid (CLA) isomers (as the free fatty acids, CLAFFA) in commercial CLA mixtures and biological samples. Due to the unchanged retention mechanism, it was assumed that the elution order of the isomers remained the same as that of methyl esters separated on the same column. The most abundant isomers, cis/trans 10, 12-18:2 and cis/trans 9,11-18:2, were separated better as free acids on a single column than in the methyl ester form. Quantification of the CLA standard was used as the reference profile to evaluate different methylation methods commonly used to prepare CLA methyl esters for quantitation. Acid-and base-catalyzed derivatization methods resulted in CLA intraisomerization and losses in total conjugated dienes content. Acid (HCl and BF3) methylations significantly elevated the level of trans,trans isomers and significantly reduced the cis/trans isomers. Base methylation, tetramethylguanidine/methanol, resulted in loss of trans,trans isomers, and a substantial loss of total underivatized conjugated dienes. Other catalysts such as the trimethylsilyldiazomethane produced additional peaks of unidentified artifacts. The analysis of CLAFFA appears to provide more accurate quantification of CLA isomers in commercial and biological samples.

Acute reduction of serum leptin level by dietary conjugated linoleic acid in Sprague-Dawley rats

The purpose of this study was to clarify the effect of conjugated linoleic acid on lipid accumulation in adipose tissue. Sprague-Dawley rats were fed a diet containing 2% conjugated linoleic acid for 1, 3, 6, and 12 weeks. In rats fed 2% conjugated linoleic acid, the weight of perirenal white adipose tissue was comparable with that of rats fed a conjugated linoleic acid-free diet. For fatty acid composition of perirenal white adipose tissue, both 16:1/16:0 and 18:1/18:0 ratios were significantly lower in the conjugated linoleic acid-fed group than the control group. Although there was no remarkable difference in serum triglyceride, total cholesterol, and phospholipid levels between dietary groups, serum leptin level was significantly lower than the control group, and lipid content in the perirenal white adipose tissue exerted a tendency toward low compared to the control value at 1-week feeding. On the other hand, leptin level in perirenal white adipose tissue was significantly lower in the conjugated linoleic acid-fed group than the control group at 12-week feeding. In conclusion, these observations suggest dietary conjugated linoleic acid is an acute reducer of serum leptin level. This may afford an explanation of the mechanism of anti-obesity effect in conjugated linoleic acid.

The trans-10,cis-12 isomer of conjugated linoleic acid downregulates stearoyl-CoA desaturase 1 gene expression in 3T3-L1 adipocytes

Conjugated linoleic acids (CLA) are a group of positional and geometric conjugated dienoic isomers of linoleic acid. The objective of this study was to determine the effects of the cis-9,trans-11 and trans-10,cis-12 isomers of conjugated linoleic acid on lipid composition and gene expression during the differentiation of mouse 3T3-L1 preadipocytes. Treatment of differentiating 3T3-L1 preadipocytes with trans-10,cis-12 conjugated linoleic acid (CLA) resulted in a dose-dependent decrease in the expression of the stearoyl-CoA desaturase 1 gene (SCD1). The expression of other adipocyte genes such as adipose P2 (aP2), fatty acid synthase (FAS), SCD2 and the key adipogenic transcription factors, peroxisome proliferator-activated receptor gamma2 (PPARgamma2) and CCAAT enhancer binding protein alpha (C/EBPalpha), remained elevated. Cells treated with trans-10,cis-12 CLA exhibited smaller lipid droplets, with reduced levels of the major monounsaturated fatty acids, palmitoleate and oleate. By contrast, the cis-9,trans-11 isomer did not alter adipocyte gene expression. Repression of the stearoyl-CoA desaturase gene expression in adipocytes by the trans-10,cis-12 isomer may contribute to the mechanisms by which CLA reduces body fat in mice.

Linoleic acid conjugation by human intestinal microorganisms is inhibited by glucose and other substrates in vitro and in gnotobiotic rats

The anticarcinogen conjugated linoleic acid (CLA) is a product of bacterial activity that isomerizes linoleic acid (LA) in the rumen of herbivores. Therefore, fatty dairy products in the human diet are enriched with CLA. Although bacteria capable of in vitro LA conjugation were detected in the human intestinal tract, CLA synthesis from dietary sunflower seed oil was not observed in gnotobiotic rats associated with these intestinal bacteria. The objective of the study was to investigate variables that affect LA conjugation. In vitro, LA conjugation was strongly inhibited by glucose and other substrates. Concentrations of 1.5 mmol glucose/L inhibited LA conjugation by 50%. Methyl-alpha-D-glucoside was a less effective inhibitor than glucose, and 2-deoxy-D-glucose did not inhibit LA conjugation at all. To analyze the concentration of carbohydrates in intestinal contents, the LA-conjugating bacterial mixed culture and human fecal microorganisms were introduced into germ-free rats. Samples of feces and cecum and colon contents of both groups exhibited in vitro LA-conjugating activity. Rats associated with human intestinal microorganisms contained 5.7 +/- 1. 3 mmol glucose/L in the cecal contents and 6.6 +/- 1.0 mmol glucose/L in the colonic contents. Rats associated with CLA-producing bacterial culture contained 3.4 +/- 1.3 mmol glucose/L in the cecal contents and 4.2 +/- 1.0 mmol glucose/L in the colonic contents. These values are within a range that may explain the observed inhibition of LA conjugation in vivo.

Conjugated linoleic acid suppresses triglyceride accumulation and induces apoptosis in 3T3-L1 preadipocytes

Four sets of experiments were conducted to examine the influence of conjugated linoleic acid (CLA) isomers during proliferation and differentiation of cultures of 3T3-L1 preadipocytes using physiological culturing conditions. Cultures treated with either albumin [bovine serum albumin (BSA) vehicle] or linoleic acid (LA) served as controls. For the proliferation study (Expt.1), cells were cultured in media containing a crude mixture of CLA isomers or pure LA at 0, 10, 50, or 200 microM for 4 d. Preadipocyte proliferation (cell number, 3H-thymidine incorporation into DNA) decreased as the level of CLA increased in the cultures. In contrast, LA had no impact on DNA synthesis. In Experiment 2a, postconfluent cultures were grown in media containing a crude mixture of CLA isomers or LA at 0, 10, 50, or 200 microM for the next 6 d. Postconfluent cultures supplemented with 50-200 microM CLA had less triglyceride (TG) and were smaller in size than cultures supplemented with similar amounts of LA. In Experiment 2b, postconfluent cultures supplemented with 200 microM of a crude mixture of CLA isomers or LA were harvested on days 1, 3, 6, or 9. Differences in TG content of cultures supplemented with 200 microM CLA compared to control and LA-supplemented cultures became apparent after 3 d of culture. Experiments 3a and 3b examined whether the fatty acid vehicle (BSA vs. ethanol) or the vitamin E status (+/-0.2 mM alpha-tocopherol) of the cultures altered CLA's impact on preadipocyte TG content. In Experiment 3a, ethanol-treated cultures had more TG than non-ethanol-treated cultures regardless of the fatty acid treatment. In Experiment 3b, cultures treated with 100 microM of either a crude mixture of CLA or the trans-10,cis-12 CLA isomer without supplemental vitamin E for 6 d had less TG than CLA-treated cultures containing vitamin E. In Experiment 4, postconfluent cultures were grown in media containing 100 microM LA or either a crude mixture of CLA isomers or the trans-10,cis-12 CLA isomer for 24-96 h to assess CLA's influence on the cell cycle and indices of apoptosis. Cultures treated with 100 microM CLA for 24-96 h had more apoptotic cells than BSA- or LA-treated cultures. Furthermore, cultures treated for 48 h with CLA had fewer cells in the S-phase than control cultures. The effects of the trans-10,cis-12 CLA isomer were more pronounced than those of the crude mixture of CLA isomers. These data suggest that CLA may exert its antiobesity effects by inhibiting proliferation, attenuating TG content, and/or inducing apoptosis in (pre)adipocytes.

Changes in body composition with conjugated linoleic acid

Conjugated linoleic acid has been shown to reduce body fat accumulation in several animal models. We have conducted several studies in AKR/J mice showing that CLA reduces body fat accumulation whether animals are fed a high-fat or low-fat diet, with no effect on food intake. One mechanism by which CLA reduces body fat is by increased energy expenditure, which is observed within one week of CLA feeding and is sustained for at least six weeks. The increased energy expenditure is sufficient to account for the decreased fat accumulation. Increased uncoupling protein gene expression does not appear to be involved in the increased energy expenditure. We have observed increased fat oxidation but no decrease in de novo fat biosynthesis with CLA feeding. We have also observed increased liver weights and plasma insulin levels with higher doses of CLA. In all of the studies we have conducted to date we have used a CLA preparation that contains several isomers, primarily c9,t11 and t10,c12. It was assumed that the active form was c9,t11, as CLA was identified as an anticarcinogenic compound from cooked beef, of which the c9,t11 form accounts for 60% to 80% of the CLA. Most of the studies conducted so far must be repeated using the purified isomers in order to determine which isomers are responsible for each of the identified actions of CLA.

Inhibition of hepatic stearoyl-CoA desaturase activity by trans-10, cis-12 conjugated linoleic acid and its derivatives

Conjugated linoleic acid (CLA) has been reported to decrease stearoyl-CoA desaturase (SCD) activity by decreasing mRNA expression. This investigation was designed to determine whether structurally related compounds of CLA have a direct inhibitory effect on SCD activity. Trans-10,cis-12 CLA had strong inhibitory activity on SCD while cis-9,trans-11, and trans-9,trans-11 isomers had no effect. Trans-10 octadecenoate was not inhibitory, whereas cis-12 octadecenate was inhibitory, but not as effective as trans-10,cis-12 CLA. Of the oxygenated derivatives, 9-peroxy-cis/trans-10, trans-12 octadecadienoate was a more effective inhibitor than trans-10,cis-12 CLA, whereas 9-hydroxy-trans-10, cis-12 octadecadienoate was less effective. Interestingly, cis-11 octadecadienoate and cis-12 octadecen-10-ynoate were slightly inhibitory. However, trans-9 and trans-11 octadecenoates, and trans-9,cis-12 octadecadienoate were all inactive under test condition, as were linoleate, oleate, and arachidonate. Derivatives of CLA acid modified to alcohol, amide or chloride were all inactive. A cis-12 double bond appears to be a key structural feature for inhibiting SCD activity, especially when coupled with a trans-10 double, whereas a cis-11 double bond is less effective.

Conjugated linoleic acid supplementation in humans: effects on body composition and energy expenditure

Recent animal studies have demonstrated that dietary conjugated linoleic acid (CLA) reduces body fat and that this decrease may be due to a change in energy expenditure. The present study examined the effect of CLA supplementation on body composition and energy expenditure in healthy, adult women. Seventeen women were fed either a CLA capsule (3 g/d) or a sunflower oil placebo for 64 d following a baseline period of 30 d. The subjects were confined to a metabolic suite for the entire 94 d study where diet and activity were controlled and held constant. Change in fat-free mass, fat mass, and percentage body fat were unaffected by CLA supplementation (0.18+/-0.43 vs. 0.09+/-0.35 kg; 0.01+/-0.64 vs. -0.19+/-0.53 kg; 0.05+/-0.62 vs. -0.67+/-0.51%, placebo vs. CLA, respectively). Likewise, body weight was not significantly different in the placebo vs. the CLA group (0.48+/-0.55 vs. -0.24+/-0.46 kg change). Energy expenditure (kcal/min), fat oxidation, and respiratory exchange ratio were measured once during the baseline period and during weeks 4 and 8 of the intervention period. At all three times, measurements were taken while resting and walking. CLA had no significant effect on energy expenditure, fat oxidation, or respiratory exchange ratio at rest or during exercise. When dietary intake was controlled, 64 d of CLA supplementation at 3 g/d had no significant effect on body composition or energy expenditure in adult women, which contrasts with previous findings in animals.

Dietary conjugated linoleic acid reduces rat adipose tissue cell size rather than cell number

We investigated the basis for the reduction in fat pad size in rats fed conjugated linoleic acid (CLA). In the first study, growing female Sprague-Dawley rats (initial weight150 g) were fed diets containing 0, 0.25 and 0.5 g/100 g diet of a purified (97% CLA) and 0.5% of a feed-grade (55% CLA) source of CLA for 5 wk to determine the effects on growth performance and fat mass. There was no effect of CLA on growth rate or food intake. Dietary CLA reduced retroperitoneal fat pad weight 13, 25 and 32% in rats fed 0.25 and 0. 5% of the pure CLA and 0.5% of the feed-grade CLA, respectively (P < 0.05). Similar effects were observed in the parametrial fat pad. The reduced pad size was due to smaller adipocyte size rather than a reduced cell number. Relative to the control group, mean cell volume was 15, 28 and 29% lower in tissue from rats fed 0.25 and 0.5% of the pure CLA and 0.5% of the feed-grade CLA, respectively (P < 0.01). In the second study, rats were fed CLA (0 vs. 0.5%) for 7 or 49 d. Reductions in fat pad weight were observed within 7 d. In addition, the effects of CLA on energy metabolism were studied in the chronically fed rats. There were no significant effects of CLA on oxygen consumption, CO(2) or heat production. During wk 4 of feeding, but not at other times, there was a 5% lower respiratory quotient in CLA-fed rats (P < 0.05). There was a time-dependent accumulation of CLA in adipose tissue and a decrease in monounsaturated fatty acids. These results suggest that the reduction in fat mass in rats fed CLA can be accounted for by a reduction in cell size rather than a change in cell number.

High dietary levels of a conjugated linoleic acid mixture alter hepatic glycerophospholipid class profile and cholesterol-carrying serum lipoproteins of rats

To investigate the body composition, hepatic lipids, and serum lipoproteins in response to graded levels of a conjugated linoleic acid (CLA) mixture added to a high linoleate diet, adult male Sprague-Dawley rats were randomly assigned into four dietary groups of 10 rats each and fed for 5 weeks controlled amounts of diets containing 0%, 1%, 3%, or 5% of a CLA mixture in exchange for sunflower oil. The various dietary lipid treatments did not significantly influence growth and body partitioning, although there was a trend toward decreased contents of extractable lipids in carcass (whole bled body without liver and gut) with increasing CLA. When carcass lipids of CLA-treated rats were extracted, a distinct accumulation of total CLA was observed. A dietary level of 1% CLA mixture exhibited only weak effects on hepatic glycerophospholipid levels. CLA levels of 3% and 5% caused distinct changes in phospholipid subclass distribution. These changes were reduced levels of lysophosphatidylethanolamine (LPE) and ethanolamine plasmalogen (EPL) and increased levels of phosphatidylcholine (PC). Further, a 5% level of CLA increased the hepatic concentration of phosphatidylserine (PS) compared with the other treatments. The incorporation of total CLA into individual phospholipids followed a dose-responsive manner. The extent of incorporation of CLA was not the same among the glycerophospholipid species analyzed, the order being cardiolipin > phosphatidylethanolamine and PC > LPE/EPL > phosphatidylinositol > PS. Further, CLA increased the proportions of n-3 fatty acids in the individual glycerophospholipids. High CLA diets containing 3% and 5% of a CLA mixture were associated with increased activity of catalase in the peroxisome-enriched cell fraction of liver and exhibited marked reductions of cholesterol in the low and high density lipoproteins relative to rats receiving no CLA.

Trans-vaccenic acid is desaturated to conjugated linoleic acid in mice

Mice were fed pure trans11 octadecenoic acid (trans-vaccenic acid; TVA) to determine whether it is desaturated to cis9, trans11 octadecadienoic acid, a predominant isomer of conjugated linoleic acid (CLA). In a preliminary trial, 12% of the TVA consumed during a 2-wk feeding period was recovered in the carcass as CLA. As a proportion of TVA in the tissues available for bioconversion, 48.8% was desaturated. We tested whether desaturation could be modified by supplementing no modifier, 0.5% clofibric acid to stimulate desaturation, or increasing the polyunsaturated fatty acids (PUFA) (10% corn oil vs. 4% corn oil) to inhibit desaturation in diets with or without 1% TVA. These diets were fed to six groups of mice in a 3x2 factorial arrangement of treatments. Feeding 1% TVA with 10% corn oil decreased feed intake (2.70 vs. 3.73 g/d, SEM 0.23; P<0.05). Bioconversion of dietary TVA was 12.0, 7.5 and 5.1% for mice fed no modifier of desaturation, clofibrate and increased PUFA, respectively. Conversion based on TVA available for desaturation was 52.6, 55.5 and 37.0%, respectively. Thus, clofibrate did not increase bioconversion, but increasing PUFA decreased conversion by 30%. To test whether TVA decreases food intake directly or after conversion to CLA, four groups of mice were fed diets containing 1% stearic, TVA, elaidic or conjugated linoleic acid. Dietary CLA decreased food intake and body fat, but did not change body protein. CLA was found in the carcass only when TVA or CLA was fed. CLA was found in both triacylglycerol and phospholipids when CLA was fed, but only in triacylglycerol when TVA was fed, suggesting that bioconversion occurred in the adipose tissue. In three trials, conversion of dietary TVA to CLA was 11.4+/-1.25%; conversion of stored TVA was 50.8+/-1.91%. Similar bioconversion of TVA in humans would increase current estimates of CLA available for the general population by 6- to 10-fold.

Effects of conjugated linoleic acid isomers on lipid-metabolizing enzymes in male rats

Male weanling Wistar rats (n = 15), weighing 200-220 g, were allocated for 6 wk to diets containing 1% (by weight) of conjugated linoleic acid (CLA), either as the 9c,11 t-isomer, the 10t,12c-isomer, or as a mixture containing 45% of each of these isomers. The five rats of the control group received 1% of oleic acid instead. Selected enzyme activities were determined in different tissues after cellular subfractionation. None of the CLA-diet induced a hepatic peroxisome-proliferation response, as evidenced by a lack of change in the activity of some characteristic enzymes [i.e., acyl-CoA oxidase, CYP4A1, but also carnitine palmitoyltransferase-I (CPT-I)] or enzyme affected by peroxisome-proliferators (glutathione S-transferase). In addition to the liver, the activity of the rate-limiting beta-oxidation enzyme in mitochondria, CPT-I, did not change either in skeletal muscle or in heart. Conversely, its activity increased more than 30% in the control value in epididymal adipose tissue of the animals fed the CLA-diets containing the 10t,12c-isomer. Conversely, the activity of phosphatidate phosphohydrolase, a rate-limiting enzyme in glycerolipid neosynthesis, remained unchanged in adipose tissue. Kinetic studies conducted on hepatic CPT-I and peroxisomal acyl-CoA oxidase with CoA derivatives predicted a different channeling of CLA isomers through the mitochondrial or the peroxisomal oxidation pathways. In conclusion, the 10t,12c-CLA isomer seems to be more efficiently utilized by the cells than its 9c,11t homolog, though the Wistar rat species appeared to be poorly responsive to CLA diets for the effects measured.

Mechanisms of action of conjugated linoleic acid: evidence and speculation

Conjugated linoleic acid (CLA) has been shown to inhibit carcinogenesis and atherosclerosis, enhance immunologic function while protecting against the catabolic effects of immune stimulation, affect body composition change (reducing body fat gain while enhancing lean body mass gain), and stimulate the growth of young rats. We discuss possible biochemical mechanisms that underlie these physiological effects. We emphasize the importance of considering the effects, both individually and combined, of the two CLA isomers (cis-9, trans-11 CLA and trans-10, cis-12 CLA) that have been shown to exhibit biological activity and which appear to exert their effects via different biochemical mechanisms.

An isomeric mixture of conjugated linoleic acids but not pure cis-9, trans-11-octadecadienoic acid affects body weight gain and plasma lipids in hamsters

We report the effect of an atherogenic diet supplemented with cis-9, trans-11-octadecadienoic acid (c9t11), linoleic acid (LA) or an isomeric mixture of conjugated linoleic acids (CLA) on plasma lipids, weight gain and food intake of male Golden Syrian hamsters. Animals were assigned to three diet groups (n = 10), and fed nonpurified diet, supplemented with 10% hydrogenated coconut oil and 0.05% cholesterol for 6 wk. The first diet group was further supplemented with 1% CLA (CLA group), the second diet group with 0.2% c9t11 (c9t11 group) and the third group with 0.2% LA (LA group). The diets were designed to have equivalent levels of c9t11 in the CLA and c9t11 groups. At 2 and 6 wk of feeding, the CLA group had significantly lower plasma triglyceride and total cholesterol concentrations than either the c9t11 or the LA groups. HDL-cholesterol did not differ among diet groups. The CLA group had significantly lower weight gain but greater food intake than either the c9t11 or the LA groups. There were no significant differences between the c9t11 and the LA groups in any of the variables measured. We conclude that under our experimental conditions of short-term feeding, c9t11, thought to be the active compound in CLA, does not produce the same effect as the isomer mixture.

Identification of the conjugated linoleic acid isomer that inhibits milk fat synthesis

Conjugated linoleic acids (CLA) are octadecadienoic fatty acids that have profound effects on lipid metabolism. Our previous work showed that CLA (mixture of isomers) markedly reduced milk fat synthesis. In this study, our objective was to evaluate the effects of specific CLA isomers. Multiparous Holstein cows were used in a 3x3 Latin square design, and treatments were 4-day abomasal infusions of 1) skim milk (control), 2) 9,11 CLA supplement, and 3) 10,12 CLA supplement. CLA supplements provided 10 g/day of the specific CLA isomer (cis-9,trans-11 or trans-10,cis-12). Treatments had no effect on intake, milk yield, or milk protein yield. Only the 10,12 CLA supplement affected milk fat, causing a 42 and 44% reduction in milk fat percentage and yield, respectively. Milk fat composition revealed that de novo synthesized fatty acids were extensively reduced. Increases in ratios of C(14:0) to C(14:1) and C(18:0) to C(18:1) indicated the 10,12 CLA supplement also altered Delta(9)-desaturase. Treatments had minimal effects on plasma concentrations of glucose, nonesterified fatty acids, insulin, or insulin-like growth factor-I. Overall, results demonstrate that trans-10,cis-12 CLA is the isomer responsible for inhibition of milk fat synthesis.

Conjugated linoleic acid-enriched butter fat alters mammary gland morphogenesis and reduces cancer risk in rats

Conjugated linoleic acid (CLA) is a potent cancer preventive agent in animal models. To date, all of the in vivo work with CLA has been done with a commercial free fatty acid preparation containing a mixture of c9,t11-, t10,c12- and c11,t13-isomers, although CLA in food is predominantly (80-90%) the c9,t11-isomer present in triacylglycerols. The objective of this study was to determine whether a high CLA butter fat has biological activities similar to those of the mixture of free fatty acid CLA isomers. The following four different endpoints were evaluated in rat mammary gland: 1) digitized image analysis of epithelial mass in mammary whole mount; 2) terminal end bud (TEB) density; 3) proliferative activity of TEB cells as determined by proliferating cell nuclear antigen immunohistochemistry; and 4) mammary cancer prevention bioassay in the methylnitrosourea model. It should be noted that TEB cells are the target cells for mammary chemical carcinogenesis. Feeding butter fat CLA to rats during the time of pubescent mammary gland development reduced mammary epithelial mass by 22%, decreased the size of the TEB population by 30%, suppressed the proliferation of TEB cells by 30% and inhibited mammary tumor yield by 53% (P < 0.05). Furthermore, all of the above variables responded with the same magnitude of change to both butter fat CLA and the mixture of CLA isomers at the level of CLA (0.8%) present in the diet. Interestingly, there appeared to be some selectivity in the uptake or incorporation of c9,t11-CLA over t10,c12-CLA in the tissues of rats given the mixture of CLA isomers. Rats consuming the CLA-enriched butter fat also consistently accumulated more total CLA in the mammary gland and other tissues (four- to sixfold increases) compared with those consuming free fatty acid CLA (threefold increases) at the same dietary level of intake. We hypothesize that the availability of vaccenic acid (t11-18:1) in butter fat may serve as the precursor for the endogenous synthesis of CLA via the Delta9-desaturase reaction. Further studies will be conducted to investigate other attributes of this novel dairy product.

Effect of dietary conjugated linoleic acid on the composition of egg yolk lipids

Forty-eight 27-wk-old White Leghorn hens were assigned randomly to four diets containing 0, 1.25, 2.5, or 5.0% conjugated linoleic acid (CLA). Hens were fed the CLA diets for 2 wk before eggs were collected for the study. Classes of egg yolk lipids were separated, and fatty acid concentrations in total lipid, triglyceride (TG), phosphatidylethanolamine (PE), and phosphatidylcholine (PC) were analyzed to determine the incorporation of dietary CLA isomers into different classes of egg yolk lipids. The amounts of CLA incorporated into lipid, PC, PE, and TG of egg yolk were proportional to the levels of CLA in the diet. However, more CLA was incorporated in TG than in PC and PE. The incorporation rates of different CLA isomers into different classes of lipids also were significantly different: cis-9, trans-11 and cis-10, trans-12 CLA were deposited more in TG, but cis-11, trans-13 CLA deposition in TG was significantly less. There were large differences in the concentrations of cis-8, trans-10 CLA in PC and PE. The inclusion of CLA into the diet influenced the metabolism of polyunsaturated fatty acids. The contents of 5,8,11,14-eicosatetraenoic, 9,12-octadecadienoic, and 9,12,15-octadecatrienoic acids were decreased as dietary CLA increased. Three isomers of hexadecadienoic acid were found in egg yolk lipids from hens fed 5% dietary CLA. The detection of hexadecadienoic acid isomers in lipid indicates that the utilization of CLA as an energy source after the first round of beta-oxidation may be less favorable than that of 9,12-octadecadienoic acid.

Dietary conjugated linoleic acids increase lean tissue and decrease fat deposition in growing pigs

Conjugated linoleic acids (CLA) decrease the body fat content of rodents; the aim of this study was to determine whether dietary CLA altered carcass composition of pigs. Female Large White x Landrace pigs (n = 66) were used in this study. To obtain initial body composition, six pigs were slaughtered at 57 kg live weight, whereas the remaining pigs were allocated to one of six dietary treatments (0, 1.25, 2.5, 5.0, 7.5 and 10.0 g/kg CLA, containing 55% of CLA isomers). The diets, containing 14.3 MJ digestible energy (DE) and 9. 3 g available lysine per kg, were fed ad libitum for 8 wk. Dietary CLA had no significant effect on average daily gain (861 vs. 911 g/d for pigs fed diets with and without CLA, P = 0.15) or feed intake (2. 83 vs. 2.80 kg/d, P = 0.74). The gain to feed ratio was increased by dietary CLA by 6.3% (0.328 vs. 0.348, P = 0.009). Fat deposition decreased linearly (-8.2 +/- 2.09 g/d for each gram per kilogram increase in CLA concentration; P < 0.001) with increasing inclusion of CLA. At the highest level of CLA inclusion, fat deposition was decreased by 88 g/d (-31%). Similarly, the ratio of fat to lean tissue deposition decreased linearly (-0.093 +/- 0.0216 for each gram per kilogram increase in CLA concentration; P < 0.001) with increasing dietary CLA. The carcass lean tissue deposition response to dietary CLA was quadratic in nature and was maximized (+25%) at 5. 0 g/kg dietary CLA. Overall, dietary CLA increased the gain to feed ratio and lean tissue deposition and decreased fat deposition in finisher pigs.

Recent advances in conjugated linoleic acid research

New results on the physiological properties of conjugated linoleic acid have been published by several working groups, especially showing the effects of single conjugated linoleic acid isomers on carcinogenesis and body composition. Recently, other studies have shown that conjugated linoleic acid has an influence on diabetes mellitus, platelet aggregation and the immune system. Conjugated linoleic acid was found to modify prostaglandin metabolism and delta9-desaturase activity and influence apoptosis. Furthermore, improved analytical methods using 13C nuclear magnetic resonance and silver ion high performance liquid chromatography are available to investigate the composition of conjugated linoleic acid mixtures and the exact structure of separated isomers. Also, the synthesis of isolated isomers is described, as published by different authors, in order to determine further the effects of each single conjugated linoleic acid isomer. In addition, new data on the contents of conjugated linoleic acid in foods, human adipose tissue and fluids are given in this review. More data need to be obtained using isolated isomers, with particular emphasis on studies in humans.

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.