Role of the Conjugated Linoleic Acid in the Prevention of Cancer

Healthier meat products as functional foods

A promising approach to improving health care would be to produce a healthier food supply as a preventive health care strategy. The food supply could be improved by producing functional foods that have nutritional profiles that are healthier than conventional products. However, production of functional foods is not always easily accomplished since they must also taste good, be convenient and reasonably priced so that consumers will regularly purchase and use the products. Meats have great potential for delivering important nutrients such as fatty acids, minerals, dietary fiber, antioxidants and bioactive peptides into the diet. However, to produce successful products with these ingredients, technologies must be developed to increase their stability and decrease their flavor impact on muscle foods. In addition, many regulatory hurdles must be overcome for the commercial production of meats with added nutrients. These include redefinition of standard of identities and policies that allow front of the package nutritional claims. Without these regulatory changes, production of healthier meat products won't become a reality since these products would not have a competitive advantage over unfortified meats.

Effect of diet supplementation on the expression of bovine genes associated with Fatty Acid synthesis and metabolism

Conjugated linoleic acids (CLA) are of important nutritional and health benefit to human. Food products of animal origin are their major dietary source and their concentration increases with high concentrate diets fed to animals. To examine the effects of diet supplementation on the expression of genes related to lipid metabolism, 28 Angus steers were fed either pasture only, pasture with soybean hulls and corn oil, pasture with corn grain, or high concentrate diet. At slaughter, samples of subcutaneous adipose tissue were collected, from which RNA was extracted. Relative abundance of gene expression was measured using Affymetrix GeneChip Bovine Genome array. An ANOVA model nested within gene was used to analyze the background adjusted, normalized average difference of probe-level intensities. To control experiment wise error, a false discovery rate of 0.01 was imposed on all contrasts. Expression of several genes involved in the synthesis of enzymes related to fatty acid metabolism and lipogenesis such as stearoyl-CoA desaturase (SCD), fatty acid synthetase (FASN), lipoprotein lipase (LPL), fatty-acyl elongase (LCE) along with several trancription factors and co-activators involved in lipogenesis were found to be differentially expressed. Confirmatory RT-qPCR was done to validate the microarray results, which showed satisfactory correspondence between the two platforms. Results show that changes in diet by increasing dietary energy intake by supplementing high concentrate diet have effects on the transcription of genes encoding enzymes involved in fat metabolism which in turn has effects on fatty acid content in the carcass tissue as well as carcass quality. Corn supplementation either as oil or grain appeared to significantly alter the expression of genes directly associated with fatty acid synthesis.

Variations in conjugated linoleic acid (CLA) content of processed cheese by lactation time, feeding regimen, and ripening

Dairy products are major sources of conjugated linoleic acid (CLA); thus, an increase in CLA content can improve the quality value of dairy products. The objective of this work was to determine the effects of lactation time, feeding regimen, and ripening period on the level of CLA in processed cheese. CLA content in milk varied with the period of lactation; high in spring (April and May, about 6.8 mg CLA/g fat) and relatively low in mid summer and winter (about 4.3 mg CLA/g fat). The effects of dietary regimen and ripening period were determined in milk, which was obtained from March to May. After aging for 4 months, the cheese made from milk obtained from cows fed on pasture contained relatively higher levels of CLA compared to cheese made from milk obtained from cows fed indoors (8.12 mg CLA/g fat vs 6.76 mg CLA/g fat), but there was no difference in 7 month-aged cheeses. In both pasture and indoor feeding, 7 month-aged cheeses showed higher CLA content than 4 month-aged cheeses. The contents of stearic acid (C18:0) and linolenic acid (C18:3) were significantly higher in cheese from pasture fed cows compared to those in cows fed indoors. These findings should be helpful for the efficient production of functional dairy products with high CLA contents.

Effect of an oil byproduct from conjugated linoleic acid (CLA) purification on CLA accumulation and lipogenic gene expression in broilers

A previous study showed that supplementing broilers with an oil byproduct obtained during the purification process of conjugated linoleic acid (CLA) from safflower oil could result in CLA-enriched egg yolks more efficiently than feeding purified CLA (free fatty acid form). On this basis, this study evaluated whether dietary CLA byproduct (CBP) supplementation would enhance CLA accumulation in broiler muscle and its lipogenic mRNA expression in the liver. A total of 456 1-day-old male broiler chicks were randomly assigned to four groups, each of which was given one of the following 2% dietary supplements for 4 weeks: soybean oil (control), safflower oil (SAF), purified CLA, and CBP. During the feeding trial, little alteration in broiler performance was observed among the test groups. CLA accumulation efficiency in the breast muscle did not differ significantly between the CLA- and CBP-fed groups after feeding of the test diet for 3 weeks. CLA supplementation also induced lipogenesis in the livers of the broilers, and it significantly increased the relative mRNA levels of sterol regulatory element binding protein 1 (SREBP1), as well as its target genes: fatty acid synthase (FAS) and acetyl coenzyme A carboxylase (ACC) (p < 0.05). However, in the CBP-fed group, SREBP1 and ACC mRNA levels were not significantly different from the controls (p > 0.05). These results suggest that CBP could be an efficient dietary source that promotes CLA accumulation in broiler muscle without inducing lipogenesis in the liver or compromising performance and meat quality in the birds.

Differences between human subjects in the composition of the faecal bacterial community and faecal metabolism of linoleic acid

Conjugated linoleic acid (CLA) is formed from linoleic acid (LA; cis-9,cis-12-18:2) by intestinal bacteria. Different CLA isomers have different implications for human health. The aim of this study was to investigate LA metabolism and the CLA isomers formed in two individuals (V1 and V2) with different faecal metabolic characteristics, and to compare fatty acid metabolism with the microbial community composition. LA incubated with faecal samples was metabolized at similar rates with both subjects, but the products were different. LA was metabolized extensively to stearic acid (SA; 18:0) in V1, with minor accumulation of CLA and more rapid accumulation of vaccenic acid (VA; trans-11-18:1). CLA accumulation at 4 h was almost tenfold higher with V2, and little SA was formed. At least 12 different isomers of CLA were produced from LA by the colonic bacteria from the two individuals. The predominant (>75%) CLA isomer in V1 was rumenic acid (RA; cis-9,trans-11-18:2), whereas the concentrations of RA and trans-10,cis-12-18:2 were similar with V2. Propionate and butyrate proportions in short-chain fatty acids were higher in V1. A 16S rRNA clone library from V1 contained mainly Bacteroidetes (54% of clones), whereas Firmicutes (66% of clones) predominated in V2. Both samples were devoid of bacteria related to Clostridium proteoclasticum, the only gut bacterium known to metabolize VA to SA. Thus, the CLA formed in the intestine of different individuals may differ according to their resident microbiota, with possibly important implications with respect to gut health.

Extensive analysis of long-chain polyunsaturated fatty acids, CLA, trans-18:1 isomers, and plasmalogenic lipids in different retail beef types

The objective of this investigation was to provide a comprehensive analysis of the total lipid composition of present-day retail beef meat available at the consumer level and to evaluate the total lipid composition with special emphasis on the nutritional value. For this purpose, 40 beef cuts were obtained from four cattle farms based on either a natural grazing system (NGS) or an intensive production system (IPS). The total lipid composition was analyzed using complementary chemical and chromatographic procedures. The content of n-3 LC-PUFA, CLA, total trans-18:1, and branched-chain fatty acids was significantly higher in NGS beef than in IPS beef. The trans-18:1 and CLA profiles were affected by the different production systems, whereby they can be utilized empirically to differentiate between feeding regimen and production management. Fatty acid ratios that have health implications ( n-6/ n-3, LA/alphaLNA, and AA/EPA) were remarkably beneficial for NGS beef compared with IPS beef. In conclusion, from the human health perspective, beef raised on NGS is clearly superior with regard to a more favorable fatty acid profile in comparison to IPS beef.

9trans,11trans conjugated linoleic acid inhibits the development of azoxymethane-induced colonic aberrant crypt foci in rats

We investigated the effects of 9trans,11trans (9t,11t)-conjugated linoleic acid (CLA) isomer on azoxymethane (AOM)-induced colonic aberrant crypt foci (ACF) in rats. Male F344 rats were given 2 weekly subcutaneous injections of AOM (20 mg/kg bw) to induce colonic ACF. They also were fed a diet containing either 0.01%, 0.1%, or 1% 9t,11t-CLA for 4 wk starting 1 wk before the first dosing of AOM. The group that received a diet supplemented with 9t,11t-CLA had a significantly lower number of ACF/colon in comparison to the AOM alone group in a dose-dependent manner up to 0.1%. Furthermore, treatment with 9t,11t-CLA induced apoptosis and suppressed cell proliferation activity in the non-lesional crypts. The downregulation of cyclooxygenase-2 and cyclin D1 and the activation of peroxisome proliferators activated receptor gamma were observed in the colonic mucosa of rats fed a diet supplemented with 9t,11t-CLA. Our findings thus provide some novel insight into the chemopreventive effect of 9t,11t-CLA against preinitiation as well as postinitiation stages of colorectal carcinogenesis.

Conjugated linoleic acid isomers and cancer

We reviewed the literature regarding the effects of conjugated linoleic acid (CLA) preparations enriched in specific isomers, cis9, trans11-CLA (c9, t11-CLA) or trans10, cis12-CLA (t10, c12-CLA), on tumorigenesis in vivo and growth of tumor cell lines in vitro. We also examined the potential mechanisms by which CLA isomers may alter the incidence of cancer. We found no published reports that examined the effects of purified CLA isomers on human cancer in vivo. Incidence of rat mammary tumors induced by methylnitrosourea was decreased by c9, t11-CLA in all studies and by t10, c12-CLA in just a few that included it. Those 2 isomers decreased the incidence of forestomach tumors induced by benzo (a) pyrene in mice. Both isomers reduced breast and forestomach tumorigenesis. The c9, t11-CLA isomer did not affect the development of spontaneous tumors of the intestine or mammary gland, whereas t10, c12-CLA increased development of genetically induced mammary and intestinal tumors. In vitro, t10, c12-CLA inhibited the growth of mammary, colon, colorectal, gastric, prostate, and hepatoma cell lines. These 2 CLA isomers may regulate tumor growth through different mechanisms, because they have markedly different effects on lipid metabolism and regulation of oncogenes. In addition, c9, t11-CLA inhibited the cyclooxygenase-2 pathway and t10, c12-CLA inhibited the lipooxygenase pathway. The t10, c12-CLA isomer induced the expression of apoptotic genes, whereas c9, t11-CLA did not increase apoptosis in most of the studies that assessed it. Several minor isomers including t9, t11-CLA; c11, t13-CLA; c9, c11-CLA; and t7, c11-CLA were more effective than c9, t11-CLA or t10, c12-CLA in inhibiting cell growth in vitro. Additional studies with purified isomers are needed to establish the health benefit and risk ratios of each isomer in humans.

Effect of Dietary Supplementation with Conjugated Linoleic Acid, with Oleic, Linoleic, or Linolenic Acid, on Egg Quality Characteristics and Fat Accumulation in the Egg Yolk

The effects of conjugated linoleic acid (CLA) with other fatty acids on the fatty acid composition of egg yolk and on egg quality characteristics were studied in 5 groups: 1) CLA 0% (control), 2) CLA 2%, 3) CLA 2%+oleic acid (OA) 2% (CLA+OA), 4) CLA 2%+linoleic acid (LA) 2% (CLA+LA), and 5) CLA 2%+alpha-linolenic acid (LNA) 2% (CLA+LNA). Some parameters of egg quality such as shell thickness, shell strength, yolk color, yolk index, egg diameter, and Haugh units were aggravated when CLA was fed alone, but the quality was improved when CLA was combined with some other fatty acids. The egg production rate, which was decreased by feeding CLA alone, was improved by co-supplementation with LA or OA. An increase in CLA content was observed in all the dietary groups fed CLA for 2 wk. Feeding hens with CLA+LNA led to a linear increase in CLA content in the egg yolk after the fourth week of the feeding trial. Egg yolks from hens given CLA had considerably higher amounts of saturated fatty acids and lower amounts of monounsaturated fatty acids than egg yolks from the control group. The pattern of change in CLA concentration during the feeding trial was similar to the level of C18:0, which was inversely correlated with the level of C18:1. The unsaturated fatty acid co-supplementation strategy applied in this study offers insight into the mechanism of CLA accumulation in the egg yolk without apparent adverse effects on egg quality and egg production.

The conjugated linoleic acid ester of estrone induces the mobilisation of fat in male Wistar rats

We investigated whether the substitution of the fatty acid moiety in oleoyl-estrone (OE) by conjugated linoleic acid, i.e. conjugated linoleoyl-estrone (cLE) may help improve the antiobesity effects of OE. Overweight (17% fat) male rats were treated for 10 days with oral OE or cLE (10 nmol/g per day) and compared with controls receiving only the oily vehicle. Rat weight and food intake were measured daily. After killing by decapitation, body composition and main plasma parameters were analysed. cLE induced marked decreases in body weight, energy intake, carcass energy and body lipid, whilst sparing protein; the effects were not significantly different from those obtained with OE. Energy expenditure was unchanged, but energy intake decreased to 46% (OE) or 55% (cLE) of controls; whole body energy decreased by 29% (OE) or 24% (cLE) in the 10-day period studied. Plasma composition showed almost identical decreases in glucose and cholesterol elicited by OE and cLE, with a more marked decrease in triacylglycerols by OE and no effect of either on NEFA. OE decreased leptin and insulin levels, but the effects of cLE were more marked on both, with similar decreases in adiponectin. It can be concluded that cLE is a new drug of the OE family; its overall effects on energy were akin to those of OE, albeit fractionally less effective at the single dose tested. However, this lower potency on lipid mobilisation does not affect other effects, such as powerful hypercholesterolemic effects or the modulation of adiponectin. And last, but not least, cLE seems to produce a more marked decrease in leptin and insulin than OE, which may reflect a coordinate action of the conjugated linoleic acid moiety and the "OE effect" on target tissues. If that were the case, cLE may constitute an improvement over OE in its action on insulin resistance.

On the supposed influence of milk homogenization on the risk of CVD, diabetes and allergy

Commercial milk is homogenized for the purpose of physical stability, thereby reducing fat droplet size and including caseins and some whey proteins at the droplet interface. This seems to result in a better digestibility than untreated milk. Various casein peptides and milk fat globule membrane (MFGM) proteins are reported to present either harmful (e.g. atherogenic) or beneficial bioactivity (e.g. hypotensive, anticarcinogenic and others). Homogenization might enhance either of these effects, but this remains controversial. The effect of homogenization has not been studied regarding the link between early cow's milk consumption and occurrence of type I diabetes in children prone to the disease and no link appears in the general population. Homogenization does not influence milk allergy and intolerance in allergic children and lactose-intolerant or milk-hypersensitive adults. The impact of homogenization, as well as heating and other treatments such as cheesemaking processes, on the health properties of milk and dairy products remains to be fully elucidated.

Trans-11–18: 1 is effectively δ9-desaturated compared withTrans-12–18: 1 in humans

The aim of this human intervention study was to evaluate the Δ9-desaturation oftrans-11–18:1 (trans-vaccenic acid;tVA) tocis-9,trans-11–18:2 (c9,t11 conjugated linoleic acid; CLA) and oftrans-12–18:1 (t12) tocis-9,trans-12–18:2 after a short-term (7d) and a long-term (42d) supplementation period. The conversion rates of bothtrans-18:1 isomers were estimated by lipid analysis of serum and red blood cell membranes (RBCM). Subjects started with a 2-week adaptation period without supplements. During the 42d intervention period, the diet of the test group was supplemented with 3g/d oftVA and 3g/d oft12. The diet of the control group was supplemented with a control oil. SerumtVA andt12 levels in the test group increased by fivefold and ninefold after 7d, respectively, and by eight- and 12-fold after 42d, respectively, when compared with the adaptation period (p≤0·002). The serumc9,t11CLA levels increased by 1·7- and 2·0-fold after 7d and 42d, respectively (p≤0·001). After 42d, the test group's RBCMc9,t11CLA content was elevated by 20% (p=0·021), whereas in the control group it was decreased by 50% (p=0·002). The conversion rate oftVA was estimated at 24% by serum and 19% by RBCM. No increase inc9,t12–18:2 was observed in the serum and RBCM, and thus no conversion oft12 could be determined. In conclusion, the endogenous conversion of dietarytVA toc9,t11CLA contributes approximately one quarter to the human CLA pool and should be considered when determining the CLA supply.

[Linoleic conjugated acid: current interest in human nutrition]

Conjugated linoleic acid (CLA) is a group of positional and geometric isomers of linoleic acid. The major dietary sources of CLA are meat from rumiants and dairy products. There are also foods enriched in CLA (by modification animal feed or addition of commercial CLA from cartamo oil). However, the major isomer of CLA in natural food is cis-9,trans-11 and commercial CLA contains approximately equal amounts of cis-9,trans-11 and trans-10,cis-12 isomers. Although CLA is minor component of the diet, at present CLA has provoked intense scientific research accompanied by an advertising boom. This is due to the potential health effects demonstrated in animal and cell culture experiments including effects on body composition, specially a reduction in body fat mass and anticarcinogenic, antidiabetogenic and immune modulating effects. The results obtained in human volunteers are limited and even sometimes contradictory. This review focuses on recently studies in humans. To draw conclusions on CLA's potential interest as a functional ingredient, additional studies are needed to clearly define optimal level of CLA intake, short and long term effects and side effects of each individual CLA isomer in order to determine its safety and efficacy.

trans-10,cis-12 Conjugated Linoleic Acid Inhibits the G1–S Cell Cycle Progression in DU145 Human Prostate Carcinoma Cells

Conjugated linoleic acid (CLA) is a group of positional and geometric isomers of linoleic acid, and has evidenced anti-cancer activities in experimental animal cancer models and in vitro studies. The two predominant isomers of CLA are cis-9,trans-11 CLA (c9t11) and trans-10,cis-12 CLA (t10c12). The present study was performed to study the effect of the individual CLA isomers on DU145 cell growth. The cells were incubated in serum-free medium with different concentrations of the fatty acids. Treatment of cells with t10c12 (at 2.5-10 micromol/L) resulted in a dose-dependent reduction in the numbers of viable cells, whereas c9t11 CLA at a concentration of 5 micromol/L slightly increased viable cell numbers at 3 days (P < .05). DNA flow cytometric analysis revealed that the treatment of DU145 cells with t10c12 for 24 hours induced a small but significant increase in the number of cells in the G1 phase, accompanied by a complementary decrease in cells in the S phase. c9t, however, had no effect on cell cycle progression. To determine the molecular mechanisms underlying t10c12-induced G1 arrest, the levels of cell cycle regulatory proteins were estimated by western blot analyses. t10c12 induced a marked increase in p21(CIP1/WAF1) protein levels in a dose-dependent manner. p27(KIP1) was not affected by t10c12. t10c12 moderately decreased cyclin A and cyclin D1 protein levels (P > .05). However, t10c12 did not affect the expression of cyclin-dependent kinase (CDK) 2, CDK4, or cyclin E. t10c12 increased p21(CIP1/WAF1) bound to CDK2 and attenuated CDK2 activity. These results indicate that t10c12-induced p21(CIP1/WAF1) binds to CDK, and inhibits the activity of this enzyme, which results in the observed decrease in the G1-S progression in DU145 cells.

Endogenous versus exogenous fatty acid availability affects lysosomal acidity and MHC class II expression

Although the immune system, inflammation, and cellular metabolism are linked to diseases associated with dyslipidemias, the mechanism(s) remain unclear. To determine whether there is a mechanistic link between lipid availability and inflammation/immune activation, we evaluated macrophage cell lines incubated under conditions of altered exogenous and endogenous lipid availability. Limiting exogenous lipids results in decreased lysosomal acidity and decreased lysosomal enzymatic activity. Both lysosomal parameters are restored with the addition of oleoyl-CoA, suggesting that fatty acids play a role in the regulation of lysosomal function. Cell surface expression of major histocompatibility complex (MHC)-encoded molecules is also decreased in the absence of exogenous lipids. Additionally, we observe decreased gamma-interferon stimulation of cell surface MHC class II. Using cerulenin to limit the endogenous synthesis of fatty acids results in decreased cell surface expression of MHC class II but does not appear to alter lysosomal acidity, suggesting that lysosomal acidity is dependent on exogenous, but not endogenous, fatty acid availability. Testing these conclusions in an in vivo mouse model, we observed statistically significant, diet-dependent differences in lysosomal acidity and MHC class II cell surface expression. Collectively, these data demonstrate a mechanistic link between lipid availability and early events in the immune response.

Bioproduction of conjugated linoleic acid by probiotic bacteria occurs in vitro and in vivo in mice

Probiotics have been shown to reduce the incidence of colon cancer in animal models. The mechanisms responsible for this activity are poorly defined. Conjugated linoleic acids (CLA) are a group of isomers of linoleic acid (LA) possessing anti-inflammatory and anticarcinogenic properties, which can be produced from LA by certain bacterial strains. In this study, the ability of probiotic bacteria to exert anticarcinogenic effects through the production of CLA was assessed. Incubation of probiotic bacteria (VSL3, Lactobacillus acidophilus, L. bulgaricus, L. casei, L. plantarum, Bifidobacterium breve, B. infantis, B. longum, and Streptococcus thermophilus) in the presence of LA yielded CLA production as measured by gas chromatography. Conditioned medium, containing probiotic-produced CLA, reduced viability and induced apoptosis of HT-29 and Caco-2 cells, as assessed by MTT assay and DNA laddering, respectively. Western blotting demonstrated an increased expression of PPARgamma in cells treated with conditioned medium compared with LA alone. Incubation of murine feces with LA after administering VSL3 yielded 100-fold more CLA than feces collected prior to VSL3 feeding. This study supports a role for supplemental probiotics as a strategy both for attenuating inflammation and for preventing colon cancer.

Simultaneous determination of saponins and fatty acids in Ziziphus jujuba (Suanzaoren) by high performance liquid chromatography-evaporative light scattering detection and pressurized liquid extraction

The seed of Ziziphus jujube Mill. var. spinosa (Bunge) Hu ex H. F. Chou, Suanzaoren in Chinese, is one of commonly used Chinese medicines. Saponins and fatty oil contains several fatty acids in Suanzaoren are responsible for its therapeutic activities. In this study, a new HPLC coupled with evaporative light scattering detection (ELSD) and pressurized liquid extraction (PLE) method was developed for the simultaneous quantitative determination of 11 major components of 2 saponins and 9 fatty acids, namely jujuboside A, jujuboside B, lauric acid, myristic acid, palmitic acid, palmitoleic acid, stearic acid, oleic acid, linoleic acid, arachidic acid and docosanoic acid in Suanzaoren. Simultaneous separation of these eleven compounds was achieved on a C18 analytical column. The mobile phase consisted of (A) 0.1% aqueous acetic acid and (B) methanol with 0.1% acetic acid using a gradient elution. The drift tube temperature of ELSD was set at 75 degrees C, and nitrogen flow-rate was 1.8l/min. All calibration curves showed good linearity (r(2)>0.9955) within test ranges. This method showed good reproducibility for the quantification of these eleven components in Suanzaoren with intra- and inter-day variations of less than 3.41 and 4.37%, respectively. The validated method was successfully applied to quantify 11 investigated components in nine commercial samples of Suanzaoren.

Conjugated Linoleic Acid Impairs Endothelial Function

OBJECTIVE

To determine the effect of dietary supplementation with conjugated linoleic acid (CLA) on body mass index (BMI), body fat distribution, endothelial function, and markers of cardiovascular risk.

METHODS AND RESULTS

Forty healthy volunteers with BMI >27 kg/m2 were randomized to receive a CLA isomeric mixture or olive oil in a 12-week double-blind study. Subcutaneous body fat and abdominal/hepatic fat content were assessed using skin-fold thicknesses and computed tomography scanning, respectively. Endothelial function was assessed by brachial artery flow-mediated dilatation (FMD). Plasma isoprostanes were measured as an index of oxidative stress. CLA supplementation did not result in a significant change in BMI index or total body fat. There was a significant decrease in limb (-7.8 mm, P<0.001), but not torso skin-fold thicknesses or abdominal or liver fat content. Brachial artery FMD declined (-1.3%, P=0.013), and plasma F2-isoprostanes increased (+91 pg/mL, P=0.042).

CONCLUSIONS

A CLA isomeric mixture had at most modest effects on adiposity and worsened endothelial function. On the basis of these results, the use of the isomeric mixture of CLA as an aid to weight loss cannot be recommended.

Beef tallow increases the potency of conjugated linoleic acid in the reduction of mouse mammary tumor metastasis

Animal studies consistently show that dietary conjugated linoleic acid (CLA) reduces mammary tumorigenesis including metastasis. Relatively low concentrations of CLA are required for those effects, and a threshold level exists above which there is no added reduction. We reasoned that the concentration of CLA required to effectively alter mammary tumor metastasis may be dependent on the type of dietary fat because select fatty acids can enhance or suppress normal or malignant cell growth and metastasis. For this study, the diets (a total of 12 different groups) differed in fatty acid composition but not in energy from fat (40%). In experiments involving spontaneous metastasis, mice were fed for 11 wk; in experiments in which mice were injected i.v. with tumor cells, they were fed for 7 wk. Mice were then assessed for the effect of CLA concentration on mammary tumorigenesis. Mammary tumor growth was not altered, but metastasis was significantly decreased when beef tallow (BT) replaced half of a defined vegetable fat blend (VFB). That blend reflects the typical fat content of a Western diet. In addition, that same VFB:BT diet lowered the concentration of CLA required to significantly decrease mammary tumor metastasis from 0.1% of the diet to 0.05%. A diet in which corn oil replaced half of the VFB did not lower the threshold from 0.1 to 0.05%. In vitro, the main fatty acid in vegetable oil, linoleic acid, reduced the efficacy of CLA toxicity on mammary tumor cells in culture. Alternatively, fatty acids normally found in BT, such as oleic, stearic, and palmitic acids, either did not change or enhanced the cytolytic effects of CLA isomers on mouse mammary tumor cells in culture. These data provide evidence that dietary BT, itself with negligible levels of CLA, may increase the efficacy of dietary CLA in reducing mammary tumorigenesis.