Elsevier

Nutrition Research

Volume 28, Issue 7, July 2008, Pages 480-486
Nutrition Research

Research Article
Diet supplementation with the cis-9,trans-11 conjugated linoleic acid isomer affects the size of adipocytes in Wistar rats

https://doi.org/10.1016/j.nutres.2008.04.002Get rights and content

Abstract

Previous reports have demonstrated that conjugated linoleic acid (CLA) acts on body fat accumulation in a variety of animal models. The aim of the present study was to investigate the effect of cis (c)-9,trans (t)-11 and t10,c12 CLA isomers on the number and size of adipocytes from the inguinal and retroperitoneal fats in Wistar male rats. A 5.1% palm oil–based diet was supplemented with CLA isomers as follows: 0.6% of c9,t11, 0.6% of t10,c12, 1.3% of c9,t11 and t10,c12 isomers in mixture, and a control nonsupplemented group for comparative purposes. Fat tissues were prepared on microscope slides for histologic examination using an image-analysis software to count the number of adipocytes and measure cell sizes. The results showed that CLA isomers did not affect (P > .05) either final body and fat depot weights or serum lipids (with the exception of triacylglycerols) and adipocytokines (leptin and adiponectin). Animals fed the c9,t11 CLA isomer diet showed larger adipocytes when compared to other groups. Independently of the CLA dietary treatment, retroperitoneal fat showed larger adipocytes (3319 μm2) and therefore a smaller number of adipocytes per unit of area, compared to inguinal fat (3055 μm2). Taken together, the data suggest that a palm oil–based diet supplemented with the c9,t11 CLA isomer in Wistar rats, in contrast to the t10,c12 isomer and the mixture of both isomers, increases adipocyte dimensions in inguinal and retroperitoneal fat depots, while having a minor effect in serum lipids and adipocytokines.

Introduction

Conjugated linoleic acid (CLA) is a group of naturally occurring fatty acids produced by fermentative bacteria in the rumen of polygastric animals, due to linoleic acid (18:2 cis-9,cis-12) isomerization. Another source of CLA is the endogenous Δ9-desaturation of trans-11 octadecenoic acid [1]. Twenty different CLA isomers have been identified as occurring naturally in ruminant fat [2]. The CLA isomeric profile of meat and dairy products presents at least 60% of the c9,t11 isomer [3], whereas the content in the t10,c12 isomer is only vestigial. In contrast, the commercial synthetic CLA mixtures contain equal amounts of c9,t11 and t10,c12 isomers [4]. These 2 CLA isomers have been associated with beneficial health properties, such as anticancerogenesis [5] and antiatherogenesis [6]. In addition, numerous reports have demonstrated that dietary CLA can interfere with body composition, reducing the accumulation of adipose tissue in several mammalian species [7].

The distribution of white adipose tissue in mammals depends on both genetic and environmental factors. Specifically, it depends on the number of adipocytes, as well as on the metabolic status of the animal and the degree of filling with depot fat [7]. The main function of adipocytes is to store triacylglycerols during periods of energy excess and to mobilize this reserve when expenditure exceeds intake. In addition to their energy-storage function, adipocytes are connected to the vascular network and have an important endocrine role.

Adipocytes secrete several adipocytokines, such as leptin and adiponectin, therefore modulating energy and lipid metabolisms [8], [9]. Adiponectin and leptin are believed to display antagonistic physiologic effects in animals. Adiponectin is considered a defensive cytokine presenting important protective effects, such as antiatherogenic, antidiabetic, and anti-inflammatory properties [9]. In contrast, leptin is an offensive cytokine that controls food intake and energy expenditure, while being present at elevated circulating levels in obese humans [10]. The capacity of different individual CLA isomers to influence body composition and adipocytokine metabolism in humans is still not clearly established. The conflicting results reported in the literature may result from the distinct isomer-specific mechanisms, with the t10,c12 CLA isomer being possibly responsible for most of the antiadipogenic effects [11]. This observation highlights the need to study the influence of CLA isomers separately, thereby disabling the possibility of interactions between isomers that contribute to masking individual biological effects.

As CLA isomers are commercially available for antiadipogenic purposes, further studies on these conjugated fatty acid properties are of extreme relevance for human nutrition research. However, the influence of CLA on body fat is still far from well characterized and some contradictory effects have been reported [8], [11]. In addition, although some studies have reported that CLA affects the histologic properties of fat tissue [12], [13], [14], [15], [16], there is no information regarding the interaction between individual isomers in rats. Thus, this article aims to describe and compare the histologic effects of different dietary CLA isomers, individually and combined, through a histometrical analysis to quantify the size and number of adipocytes per area, both on inguinal and on retroperitoneal fat depots. Diets were formulated as saturated based on mimic patterns of human nutrition from the modern societies. Moreover, CLA seems to be more beneficial in controlling adiposity when supplemented in a diet rich in saturated vs unsaturated fats [17]. Therefore, the experimental model selected for this study was the Wistar rat fed on a 5.1% palm oil–based diet supplemented with 1.3% of a CLA mixture (equal proportions of c9,t11 and t10,c12 isomers) and 0.6% of the individual c9,t11 and t10,c12 CLA isomers. To clarify the possible mechanisms involved in CLA metabolism of adipocytes, the serum concentrations of leptin and adiponectin were also determined. In summary, it was hypothesized that saturated-based diets supplemented with c9,t11 and t10,c12 CLA isomers would contribute to altering adipose mass and fat cell size and, therefore, change the level of circulating adipocytokines.

Section snippets

Diet oils

The CLA mixture oil with 80% purity and identical proportions of c9,t11 and t10,c12 isomers was a generous gift from PharmaNutrients, Inc (Gurnee, Ill). The c9,t11 and t10,c12 CLA oils with 90% purity were supplied by Natural ASA (Hovdebygda, Norway). Palm and sunflower oils were purchased from Sigma-Aldrich, Ltd (St Louis, Mo).

Wistar rats and diets

The experimental protocol was reviewed by the ethics committee of CIISA/FMV and approved by the Animal Care Committee of the National Veterinary Authority (Direcção

Results

Table 2 presents the overall characterization of the studied animal groups, in terms of growth performance, tissue weight, and serum lipid profile. There were no significant differences (P > .05) among the dietary groups in relation to feed intake, body weight, and weight gain during the experimental period. In addition, dietary treatments had no effect on feed conversion efficiency and tissue weights, including retroperitoneal and inguinal fat depots. However, the c9,t11 CLA isomer treatment

Discussion

Commercial CLA preparations containing t10,c12 and c9,t11 isomers in various proportions are attracting consumers' interest owing to the putative fat-lowering effects of these conjugated fatty acids on body composition [26]. However, in humans, the effect of dietary CLA on body fat is still far from well characterized, and some conflicting data have been reported [8], [11], [26].

The main goal of this study was to evaluate, histometrically, the isomer-specific influence of CLA on body fat of

Acknowledgment

The authors would like to acknowledge Clínica Médica e Diagnóstico, Dr. Joaquim Chaves (Algés, Portugal) for the technical assistance concerning the serum lipids determination. We would also like to thank Maria do Rosário Luís for her assistance in microscope slides preparation.

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    This study was supported by an FCT grant (POCTI/CVT/2002/44750) and individual FCT fellowships to Paula Alexandra Lopes (SFRH/BPD/2005/23931) and Susana Martins (SFRH/BD/2005/22566).

    1

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