Effect of dietary elaidic versus vaccenic acid on blood and liver lipids in the hamster
Introduction
Epidemiological studies in the United States of America have suggested that dietary trans-fatty acids from partially hydrogenated vegetable oils, as found in margarine, but not those from ruminant fats, present in butter, are associated with an increased risk of coronary heart disease in women [1], [2]. Moreover, in the Scottish Heart Health Study dietary intake of total or partially hydrogenated vegetable oils-derived trans-fatty acids, but not of naturally occurring trans-fatty acids (mainly ruminant), tended to be positively associated with risk of coronary heart disease in women [3]. The observed difference in effect of the two types of trans-fatty acids on risk of coronary heart disease is attributed to the distinct structure of the primary trans-isomer in ruminant fat, i.e. vaccenic acid, which has the double bond at the 11 as opposed to the 9 position in elaidic acid, the predominant but not exclusive trans-isomer in partially hydrogenated vegetable oils. However, there is no clear biological explanation for such a differential effect. In human skin fibroblasts, elaidic acid has been shown to be a more potent inhibitor of Δ6-desaturase in vitro than vaccenic acid [4], whereas the opposite was observed in rat liver microsomes [5]. These findings, however, can not directly be related to coronary heart disease.
No metabolic studies have been published in which elaidic acid is directly compared with vaccenic acid in dietary triglycerides. In the present study both trans-fatty acids were therefore compared in hamsters with their cis-counterpart oleic acid; medium-chain fatty acids, which have been reported to have neutral effects on blood lipids [6]; and palmitic acid, which is hypercholesterolemic in the hamster [6]. The average human trans-fatty acids intake (as mixture of isomers) is around 2% of energy (en%) [7]. The intake of the two trans-fatty acids isomers in this experiment was set at the high level of 10 en% to enable detection of differential effects, if any. The hamster was used as an animal model because lipid metabolism in this rodent species resembles human lipid metabolism more than that in rats or mice [8], [9]. Elaidic acid and vaccenic acid were each randomly incorporated in dietary triglycerides.
Theoretically, the two trans-fatty acids could have a differential impact on lipid and eicosanoid metabolism in vivo, which in turn could explain a differential effect on coronary heart disease risk, if any. In the current study those aspects of lipid and eicosanoid metabolism were investigated that either have been reported, or were considered most likely to be effected by trans-fatty acid intake. The parameters measured were plasma lipids and lipoproteins, lipid transfer and transferase protein activities. In man dietary trans-fatty acids have been reported to simultaneously increase LDL-cholesterol and decrease HDL-cholesterol, or at least to raise the ratio LDL/HDL-cholesterol, when compared with oleic acid [10]. Lecithin:cholesterol acyltransferase (LCAT) and phospholipid transfer protein (PLTP) are HDL-bound enzymes. Previous studies have shown that cholesteryl ester transfer protein (CETP) activity, measured either with endogenous [11] or exogenous [12] substrates, may be affected by dietary trans-fatty acids in humans. To predict whether the two trans-fatty acids would have the potential to differentially affect eicosanoid metabolism [1], the fatty acid distribution in phospholipids was also determined.
Section snippets
Animals and design
The experimental protocol was approved by the Animal Experiments Committee of the Unilever Research Laboratory, Vlaardingen, The Netherlands.
A total of 150 male F1B hybrid Syrian (golden) hamsters, aged 10–11 weeks at arrival, were obtained from Biobreeder Inc., Fitchburg, US. The study was designed to have a large statistical power to be able to detect small differences between dietary treatments.
The hamsters were housed individually in Makrolon cages type II, with a layer of sawdust as
Diets
As intended, detailed analysis of the fatty acid pattern in the dietary fat blends revealed that the MCFA, SAFA, MUFA, Elaidic, and Vaccenic fats were increased with 28–34% of caprylic (octanoic) and capric (decanoic), palmitic (hexadecanoic), oleic (9c-octadecenoic), elaidic, and vaccenic acid, respectively (Table 2). The fatty acid patterns include fatty acids from mono- and di-acylglycerols and unesterified fatty acids, which were typically present at levels of <0.1, <5, and <0.1%,
Discussion
The purpose of this study was to investigate the effect of the exchange of a high level (10 en%) dietary elaidic acid as opposed to vaccenic acid on parameters of lipid metabolism in vivo. The effects of the two trans-fatty acids were also compared with those of their cis-counterpart oleic acid, medium-chain fatty acids, and palmitic acid in the diet. All diets contained a low background level of cholesterol (0.01%) to enhance the effects of the dietary fatty acids on blood (LDL-)cholesterol
Acknowledgements
We thank A Visser for preparation of the triglycerides containing the trans-fatty acids isomers, and P van den Berg, T van Gent, WJ Kloots and LM Scheek for their expert technical assistance.
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