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© 2005 The American Society for Nutritional Sciences J. Nutr. 135:1934-1939, August 2005

Nutrient Metabolism

Butter Naturally Enriched in Conjugated Linoleic Acid and Vaccenic Acid Alters Tissue Fatty Acids and Improves the Plasma Lipoprotein Profile in Cholesterol-Fed Hamsters1,2,3

Adam L. Lock, Claire A. M. Horne*, Dale E. Bauman and Andrew M. Salter*,4

Department of Animal Science, Cornell University, Ithaca, NY 14853 and * School of Biosciences, University of Nottingham, Loughborough, Leicestershire, LE12 5RD, UK

4To whom correspondence should be addressed. E-mail: Andrew.Salter{at}nottingham.ac.uk or debb{at}cornell.edu.


   ABSTRACT
TOP
 ABSTRACT
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 LITERATURE CITED
 
Butter, which is naturally enriched in cis-9, trans-11 conjugated linoleic acid (rumenic acid; RA) and vaccenic acid (VA), has been shown to be an effective anticarcinogen in studies with animal models; however, there has been no examination of the effects of a naturally derived source of VA and RA on atherosclerosis-related biomarkers. The current study was designed to determine the effect of a diet containing VA/RA-enriched butter on plasma lipoproteins and tissue fatty acid profiles in cholesterol-fed hamsters. Male Golden Syrian hamsters were fed diets containing 0.2% cholesterol and 20% added fat as: 1) Control, 20% standard butter (CT); 2) 5% standard butter + 15% VA/RA-enriched butter (EB); 3) 15% standard butter + 5% partially-hydrogenated vegetable oil (VO). After 4 wk, plasma lipoproteins were isolated, cholesterol quantified, and tissue fatty acid profiles determined. Tissue concentrations of VA and RA were increased by consumption of the EB diet compared with both the CT and VO diets, whereas the VO diet increased their concentration compared with the CT diet only. Total and LDL cholesterol concentrations were significantly reduced in hamsters fed EB and VO compared with CT, whereas VLDL cholesterol concentrations were reduced in hamsters fed EB compared with those fed CT and VO. HDL cholesterol concentrations did not differ among treatments. The ratio of potentially atherogenic lipoproteins [VLDL + intermediate density lipoproteins (IDL) + LDL] to antiatherogenic HDL was significantly lower in hamsters fed VA/RA-enriched butter (0.60) than in those fed either control diet (1.70) or the diet containing partially hydrogenated vegetable oil (1.04). Thus, increasing the VA/RA concentration of butter results in a plasma lipoprotein cholesterol profile that is associated with a reduced risk of atherosclerosis.

KEY WORDS: • conjugated linoleic acid • vaccenic acid • plasma cholesterol • functional food • human health

Conjugated linoleic acid (CLA)5 isomers have been shown to possess a number of properties that may be beneficial to human health (1,2). Ruminant products are the principal source of CLA in human diets with ~70 and 25% coming from dairy products and red meat, respectively (3); the cis-9, trans-11 isomer (rumenic acid; RA) represents 75–90% of total CLA in dairy foods (4). The predominant source of RA in dairy products is through endogenous synthesis in the mammary gland via {Delta}9-desaturase from vaccenic acid (trans-11 18:1; VA), the major biohydrogenation intermediate produced in the rumen (5). Due to the precursor:product relation between these 2 fatty acids, foods rich in RA are also a rich source of VA. Endogenous synthesis of RA from VA has also been reported in humans (6) and other species (7,8).

Most investigations concerning the health benefits of RA have examined its anticarcinogenic properties which are now well established for several types of cancer (1). We demonstrated that consumption of a VA/RA-enriched butter fat is effective in reducing the risk of mammary cancer in rats treated with a chemical carcinogen (8–10). By comparison, studies investigating the effect of CLA isomers on cholesterol and lipoprotein metabolism and atherosclerosis are limited. Synthetic CLA isomer mixes were shown to inhibit the development of cholesterol-induced atherosclerosis in hamsters and rabbits (11–15) and cause the regression of preestablished atherosclerotic lesions (11). Utilizing pure isomers it was shown that RA and trans-10, cis-12 CLA were equally effective in reducing cholesterol-induced atherogenesis in rabbits (16), and a dietary supplement of RA not only retarded further development of atherosclerotic lesions in the ApoE–/– mouse, but also induced regression of the lesions in the aorta (17). Reports on the effects of RA on cholesterol and lipoprotein metabolism have produced inconclusive results, with RA shown to have no effect in some studies (18,19), but having an effect in others (20).

To date, there have been no investigations into the effects of a naturally enriched RA food product on cholesterol and lipoprotein metabolism in a biomedical animal model of atherogenic risk. In addition to RA, dairy products contain VA, which also contributes to the available pool of RA through endogenous synthesis. The objective of the present study was to determine the effects of feeding a VA/RA-enriched butter on cholesterol and lipoprotein metabolism in male Golden Syrian hamsters. Because of concerns regarding trans fatty acids and their association with elevated plasma concentrations of LDL and lower concentrations of HDL cholesterol (21–23), we also compared the VA/RA-enriched butter to a partially hydrogenated vegetable oil (PHVO).


   MATERIALS AND METHODS
TOP
 ABSTRACT
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 LITERATURE CITED
 
    Production of experimental butter fats. The butter sources used in the current study were produced as described previously (24). The fatty acid composition of the 2 sources of butter and the PHVO are presented in Table 1; values for the butters are comparable to those used previously (8–10).


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  		TABLE 1 Fatty acid composition of standard butter, VA/RA-enriched butter, and PHVO

 
    Treatment protocol of hamster experiment. All procedures involving hamsters were subject to UK Home Office regulations. Male Golden Syrian Hamsters (n = 30; 16–24 wk old, weighing 134–175 g; Harlan UK) were housed individually and maintained in a controlled environment (21°C, 55% humidity) with a 12-h light:dark cycle. Upon arrival, hamsters were allowed a 2-wk acclimatization period during which they were fed a commercial rodent diet (Rat and Mouse Breeding Diet No. 3, Special Diet Services). This diet contained (figures as provided by Special Diet Services) 4.3% crude oil, 22.3% crude protein, 4.5% crude fiber, 7.7% ash, and 51% carbohydrate (35% starch and 7.7% sugar); total digestible energy was estimated as 12.8 MJ/kg. Detailed composition of the diet is available with the online posting (Supplemental Table 1). Throughout the study, all hamsters were checked daily for signs of ill-health; body weights were checked weekly. The experimental diets were fed for 28 d. Hamsters were anesthetized using sodium pentobarbitone (Sagatal, 1 mL/kg) and 3–4 mL blood was collected by centrifugation (1800 x g for 15 min, 4°C) by cardiac puncture and placed into EDTA-tubes. Plasma was isolated and stored at 4°C until lipoprotein separation, which was commenced within 48 h of collection. Livers and perirenal and epididymal fat pads were removed, weighed, and snap-frozen in liquid nitrogen.

    Diet formulation and feeding. The purpose of the overall dietary treatment scheme was to evaluate the modulation of cholesterol and lipoprotein metabolism by increasing the dietary supply of VA and RA and comparing this with a control diet and a diet containing trans fatty acids derived from PHVO. All 3 diets were supplemented with 0.2% crystalline cholesterol (Sigma) and contained 20% supplemental fat, which was derived from the 2 butter sources, and PHVO either alone or in combination (Table 2). All diets contained comparable amounts of SFA (12:0, 14:0, and 16:0) and linoleic acid. In addition, the proportion of total trans fat in the EB and VO treatments was approximately equal and represented ~5% of the energy (Table 3). Hamsters had free access to food and water, and food was replaced completely every 2 d.


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  		TABLE 2 Design of diets containing standard butter, VA/RA-enriched butter, and PHVO

 

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  		TABLE 3 Fatty acid composition of supplemental fats1

 
    Cholesterol and triacylglycerol (TAG) analysis. Plasma lipoproteins were separated from plasma by sequential ultracentrifugation; chylomicrons = 15,000 x g for 15 min; VLDL = 30,000 x g for 16 h; and IDL and LDL = 130,000 x g for 20 h. Chylomicrons, VLDL, intermediate density lipoprotein (IDL), LDL, and HDL were separated as described previously (25). Cholesterol and TAG concentrations in total plasma and individual lipoprotein fractions were determined using diagnostic kits from Thermo-Trace (Infinity Cholesterol and Infinity Triacylglycerol Enzymatic kits, Alpha Laboratories). Hepatic cholesterol and cholesterol esters were separated by TLC, extracted, and assayed enzymatically as previously described (26).

    Fatty acid analysis. The –80°C samples of epididymal and perirenal fat pads and liver were pulverized at liquid nitrogen temperature. Total lipids were extracted from pulverized tissues by the procedure of Hara and Radin (27) using a mixture of hexane and isopropyl alcohol. Fatty acids were methylated according to Christie (28) with modifications as described by Corl et al. (8). FAMEs were analyzed by GC (system 6890+ with flame ionization detector; Agilent) using a CP-Sil 88 capillary column (100 m x 0.25 mm i.d. with 0.2-µm film thickness; Varian) according to the methods described by Lock et al. (10). FAME standards were used to identify sample FAME (Nu-Chek-Prep).

    Statistical analyses. Data were analyzed by the general linear model procedure of SAS (SAS Institute). ANOVA was used to identify the effect of treatment, and differences between treatment means were identified using the probability of difference option of the LSMeans command. Results are expressed as means ± SE; treatment effects and differences between means were considered significant when P < 0.05.


   RESULTS
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 LITERATURE CITED
 
Body mass, liver and adipose tissue depot weights did not differ among dietary treatments and were in the normal range typically observed (data not presented).

    Cholesterol and TAG concentrations in plasma and liver. Dietary treatments had significant effects on total plasma cholesterol and cholesterol levels in individual lipoprotein fractions (Table 4). Total (P < 0.001), chylomicron (P < 0.01), LDL (P < 0.01), and IDL (P < 0.001) cholesterol concentrations were reduced in hamsters fed VA/RA-enriched butter (EB treatment) and PHVO (VO treatment) compared with hamsters fed the control diet (CT treatment). However, VLDL concentrations were significantly reduced only in hamsters administered the EB treatment compared with those administered the CT and VO treatments (P < 0.01). HDL cholesterol concentrations did not differ among treatment groups. Overall, the ratio of potentially atherogenic lipoproteins (VLDL + IDL + LDL) to antiatherogenic HDL was lower in hamsters fed VA/RA-enriched butter (0.60) than in those fed either the control (1.70) or PHVO diet (1.04).


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  		TABLE 4 Plasma cholesterol and TAG concentrations of hamsters fed a control diet, a VA/RA-enriched diet, or a PHVO diet1

 
The effects on plasma TAG concentrations were less pronounced (Table 4). VLDL-TAG did not differ among treatment groups, whereas chylomicron-TAG was reduced in both the EB and VO treatment groups compared with the CT group (P < 0.01). Total plasma TAG concentration differed between the CT and VO treatment groups (P < 0.05), with the EB treatment group intermediate and not different from either the CT or VO treatment group.

Hamsters administered the EB treatment had a greater concentration of cholesterol ester in their livers than those administered either the CT or VO treatment (P < 0.001), whereas liver free cholesterol concentration did not differ between treatment groups (Table 5).


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  		TABLE 5 Hepatic cholesterol and cholesterol ester concentrations of hamsters fed a control diet, a VA/RA-enriched diet, or a PHVO diet1

 
    Fatty acid analysis of tissue lipids. Tissue fatty acid compositions reflected dietary differences in the supplemental fat sources (Tables 6, , 7, , and 8). As expected, the greatest changes occurred with tissue and liver concentrations of trans fatty acids and RA. The lipid concentration of RA in liver, epididymal fat and perirenal fat was increased by 527, 473, and 478%, respectively, in hamsters administered the EB treatment compared with the CT treatment, whereas the VO treatment increased tissue RA concentrations by 103, 54, and 47%, respectively, compared with the CT treatment (P < 0.001). The proportion of fatty acids as VA was increased by 653, 608, and 591% (EB vs. CT treatment) and 144, 136, and 124% (VO vs. CT treatment) for liver, epididymal fat, and perirenal fat, respectively. The lipid concentration of total trans 18:1 fatty acids in liver, epididymal fat and perirenal fat were increased by 287, 336, and 335% (EB vs. CT treatment) and 299, 360, and 341% (VO vs. CT treatment), respectively (P < 0.001). Complete tissue fatty acid compositions are presented in Supplemental Tables 2, 3, 4.


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  		TABLE 6 Composition of fatty acids from liver lipids of hamsters fed a control diet, a VA/RA-enriched diet, or a PHVO diet1, 2

 

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  		TABLE 7 Composition of fatty acids from the epididymal fat pad of hamsters fed a control diet, a VA/RA-enriched diet or a PHVO diet1, 2

 

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  		TABLE 8 Composition of fatty acids from the perirenal fat pad of hamsters fed a control diet, a VA/RA-enriched diet or a PHVO diet1, 2

 

   DISCUSSION
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
LITERATURE CITED
 
Male Golden Syrian hamsters have been used extensively in studies of lipoprotein metabolism. We showed them to be a useful model in studying nutritional influences on cholesterol and lipoprotein metabolism (29–31). In the present study, we provide the first evidence that feeding butter, modified to increase RA and VA concentration, lowers total plasma and proatherogenic lipoprotein cholesterol concentrations. In addition to differences in RA and VA concentration of the diets, there were some other less-pronounced differences in fatty acid composition among diets. The total amount of SFA (12:0 + 14:0 + 16:0) differed slightly (6.45, 5.84, 5.36 g/kg for CB, EB, and VO, respectively). The EB diet also contained less stearic acid and oleic acid than either the CB or VO diet. Although the total trans 18:1 concentration of the EB and VO diets was similar (and considerably higher than CT), the isomer distribution differed. Thus, differences in lipoprotein cholesterol concentrations between hamsters fed these different diets may represent a combination of effects of these different fatty acids.

Each dietary treatment produced a unique pattern of fatty acid composition of liver and adipose tissue lipids. Compared with those fed the control diet, hamsters administered the EB treatment had higher tissue lipid concentrations of both RA and VA. This was predominantly at the expense of oleic acid and, to a lesser extent, palmitic acid. Increases in other trans 18:1 isomers were also seen, but these were much less pronounced and in general reflected the composition of the VA/RA-enriched butter. The tissues of hamsters administered the VO treatment also reflected dietary fatty acid composition with a much more uniform distribution of trans 18:1 isomers. It is of note, however, that even though RA was not present in the dietary fat, RA concentrations were higher in hamsters administered the VO treatment compared with those administered the CT treatment. We believe this is evidence of VA, present in the VO diet, being converted to RA through the action of {Delta}9-desaturase in these tissues of the hamsters.

In general, hydrogenated fat rich in trans 18:1 isomers has been considered to be associated with increased risk of developing cardiovascular disease (21–23). Human nutrition studies suggest that consuming partially hydrogenated vegetable oils increases plasma LDL cholesterol compared with monounsaturated fatty acids or PUFA, but they are less potent than SFA (32,33). However, compared with diets rich in saturated fat, trans fatty acid consumption may also be associated with reduced HDL cholesterol concentrations (32,33). Human nutrition studies have concentrated on the effect of trans fatty acids present in hydrogenated vegetable oils rather than those naturally occurring in ruminant milk and meat. However, epidemiologic evidence indicates that it is consumption of trans fatty acids derived from PHVO that is associated with increased risk of cardiovascular disease, whereas no clear relation exists for trans fatty acid intake from animal-derived foods (34–38). This suggests that the isomer profile of trans fatty acids may be an important consideration in evaluating the risk for coronary heart disease.

In hamsters, elaidic acid (trans-9 18:1) has been described as being biologically neutral in terms of its effect on plasma LDL concentrations and regulation of LDL receptor activity (39). This contrasted with an increase in LDL when myristic acid was fed and a decrease when oleic acid was fed (39). Nicolosi et al. (40) reported similar LDL cholesterol concentrations in hamsters fed oleic and elaidic acid, which were reduced compared with concentrations in those fed myristic acid. In a comparison of the effects of elaidic and VA, Meijer et al. (41) found no significant difference in VLDL, LDL, or HDL cholesterol concentrations in hamsters fed low-cholesterol (0.01%) diets.

The present experiment suggests that compared with CT, both EB and VO treatments reduce LDL cholesterol. This may be related to the slightly higher SFA concentration of the CT diet. In comparison with both CT and VO treatments, feeding EB specifically reduced VLDL cholesterol concentration. We demonstrated previously that development of atherosclerosis in hamsters was directly correlated with the concentration of VLDL, IDL, and LDL; hence each of these fractions can be considered to be "proatherogenic" (30). When considering the sum of these lipoproteins, or when expressing them as a ratio to the "antiatherogenic" HDL fraction, hamsters fed EB display the least atherogenic lipoprotein profile of the 3 treatments. Thus, some component(s) of EB appear to induce favorable changes in lipoprotein cholesterol concentrations.

The question remains concerning what fatty acid components of the EB diet are having these effects. Previous studies in hamsters suggested that purified mixtures of CLA isomers may have beneficial effects on plasma lipoprotein profiles (15,18,19). However, studies specifically investigating the effects of pure isomers yielded conflicting results, with one (20) showing that RA was effective in lowering plasma cholesterol levels, but others (18,19) indicating that this isomer had no effect. Differences in results may relate to the background diet of the animals, particularly cholesterol concentration, or the amount of CLA fed. Furthermore, most of these studies, with the exception of that of de Deckere et al. (18), have fed CLA isomers as FFA. Recent work from our laboratory (Salter et al., unpublished) suggests that RA is more readily absorbed when incorporated into dietary TAG. If RA is responsible for the beneficial effects of EB, then the conversion of dietary VA to RA through the action of {Delta}9-desaturase may further contribute to the effect. This may also explain some of the differential effects of the EB and VO diets, with the former containing considerably more VA.

Mechanisms for the effects of EB on cholesterol and lipoprotein metabolism remain to be established, but are presumably related to effects on hepatic cholesterol metabolism. Previous work showed that hamsters respond to diets supplemented with cholesterol by increasing hepatic stores of cholesterol ester in a dose-dependent manner (29). However, the storage of such cholesterol is modulated by the type of fatty acid in the diet (29,39,42). The current study suggests that consumption of the EB diet leads to a greater accumulation of cholesterol ester compared with either CT or VO. Whether this effect is related to the relative affinity of fatty acids as substrates (perhaps either RA or VA) for esterification or to some less direct effect remains to be established. Such sequestration of dietary cholesterol may explain, at least in part, why EB reduced plasma concentrations of VLDL, IDL, and LDL cholesterol. In addition, we showed previously that different dietary fatty acids modulated the fecal excretion of cholesterol (29,31). Other mechanisms, such as increased conversion to and/or excretion of bile acids or decreased cholesterol synthesis may also play a role.

To date, there have been 2 studies that examined the effects of RA on blood lipids in healthy humans. Differences in response were observed when the proportions of RA and trans-10, cis-12 CLA were altered in a CLA supplement; a 50:50 mixture of RA and trans-10, cis-12 CLA significantly improved plasma TAG and VLDL metabolism, whereas an 80:20 blend of the same 2 CLA isomers significantly reduced VLDL cholesterol concentrations (43). Utilizing relatively pure CLA isomers, it was recently reported that RA and trans-10, cis-12 CLA had opposing effects on blood lipids in healthy humans; plasma TAG, total plasma cholesterol, LDL cholesterol, and the LDL:HDL cholesterol ratio were all lower during supplementation with RA compared with trans-10, cis-12 CLA (44). Although data are limited at present, these 2 studies provide support that some of the cardioprotective effects of RA observed in the current study and previously may extend to humans. Additional studies are required to extend these results and examine the dissimilar effects of RA and trans-10, cis-12 CLA, and their possible interaction, on cholesterol and lipoprotein metabolism.

In conclusion, we produced butter with a modified fatty acid composition that yielded a more favorable lipoprotein cholesterol profile when fed to hamsters than standard butter or a diet containing partially hydrogenated vegetable oil. This is despite the fact that this butter was enriched in trans fatty acids. Current public health policy strongly recommends a reduction in the intake of trans fatty acids principally due to their putative association with elevated plasma concentrations of cholesterol and LDL along with lower concentrations of HDL (21–23). Although CLA are exempt from the imminent nutrition labeling of the trans fatty acid content of all food products in the United States, there will be no distinction between individual trans 18:1 fatty acids. Data presented here, and previously published studies on effects on cancer (8,10) suggest that perhaps VA should also be considered for an exemption. The VA/RA-enriched butter used in these studies is thus worthy of further consideration as a "Functional Food" that may offer benefits for human health.


   ACKNOWLEDGMENTS
 
The authors thank Matt Hurley and Richard Plant, University of Nottingham, for their technical assistance. The contributions of David Barbano, Jim Perfield, and Debbie Dwyer, Cornell University, are also gratefully appreciated.


   FOOTNOTES
 
1 Presented in abstract form at the 6th Congress of the International Society for the Study of Fatty Acids and Lipids July 2004, Brighton, UK [Horne, C.A.M., Lock, A. L., Hurley, M., Bauman, D. E. & Salter, A. M. (2004) Effect of a vaccenic acid (VA)/conjugated linoleic acid (CLA)-enriched butter on plasma lipoproteins in the cholesterol-fed hamster. Proc. 6th Congress of the International Society for the Study of Fatty Acids and Lipids, p. 96] and at the American Dairy Science Association Annual Meeting, July 2005, Cincinnati, OH [Lock, A. L., Horne, C.A.M., Bauman, D. E. & Salter, A. M. (2005) Effect of vaccenic acid/conjugated linoleic acid-enriched butter on plasma lipoproteins in the cholesterol-fed hamster. J. Dairy Sci. 88 (suppl. 1) (abs.)]. Back

2 Supported in part by a grant from the National Dairy Council, Rosemont, IL; support was also received from Northeast Dairy Foods Research Center and Cornell University Agricultural Experimental Station. Back

3 Supplemental tables with complete tissue fatty acid data and diet composition are available as Online Supporting Material with the online posting of this paper at www.nutrition.org. Back

5 Abbreviations used: CLA, conjugated linoleic acid; CT, control diet containing standard butter; EB, diet containing vaccenic acid/rumenic acid-enriched butter; IDL, intermediate density lipoprotein; PHVO, partially hydrogenated vegetable oil; RA, rumenic acid (cis-9, trans-11 conjugated linoleic acid); TAG, triacylglycerol; VA, vaccenic acid; VO, diet containing partially hydrogenated vegetable oil. Back

Manuscript received 14 March 2005. Initial review completed 15 April 2005. Revision accepted 26 April 2005.


   LITERATURE CITED
TOP
 ABSTRACT
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 LITERATURE CITED
 

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