The effect of an oxidized dietary oil on plasma cholesterol and thyroid hormone concentrations in miniature pigs fed on a hyperlipidaemic diet
Authors: Eder K. 
Published: March 2003

Introduction  Polyunsaturated fatty acids (PUFA) are labile compounds which under the promoting effect of oxygen, high temperature or metallic catalysts undergo peroxidative damage. In the course of lipid peroxidation, several primary and secondary products such as peroxides, aldehydes, dimers or polymers are formed. Human western diets are not only generally rich in fats, but also rich in fats that have been subjected to various degrees of processing and heat treatment, particular deep fat frying. The absolute intake of lipid peroxidation products in western countries varies over a wide range, depending on the individual eating habits; however, it can be assumed that on average of the population at least 50% of the fat consumed has been heated during industrial processing of foods or preparing of meals. Therefore, the potential toxic effects of products formed from polyunsaturated fatty acids during heating or frying of fats are of particular interest for human health, particularly in the development of coronary heart disease. In literature, there is one study which reports that the development of atherosclerosis is accelerated by dietary oxidized lipids in cholesterol‐fed rabbits (S
trapans et al. 1996). The reason for this observation, however, is not clear. The pathogenesis of atherosclerosis is very complex. Undoubtedly, the concentration of cholesterol in plasma and in lipoproteins as well as the susceptibility of low‐density lipoproteins to lipid peroxidation are major factors influencing the risk of coronary heart disease (S
teinberg et al. 1989; M
archioli et al. 1996). In order to clarify a potential relationship between the intake of dietary oxidized lipids and an increased risk of coronary heart disease, this study investigated the effects of feeding oxidized oils on the concentrations of lipids in plasma and lipoproteins and the susceptibility of plasma lipids to peroxidation (S
taels et al. 1990; T
anis et al. 1996). The concentrations of thyroid hormones in plasma were measured because they are in close relationship with the plasma cholesterol concentration. Miniature pigs that were fed a cholesterol‐enriched diet were used in this study because they have been described to be an adequate model for the investigation of the lipid metabolism in humans (B
arth et al. 1990). A general problem in the investigation of the effects of dietary oxidized lipids is that the administration of a diet containing highly oxidized fats reduces food intake, food efficiency and the digestibility of fatty acids (C
orcos benedetti et al. 1987; Y
oshida and K
ajimoto 1989; B
lanc et al. 1992; H
ayam et al. 1993; B
orsting et al. 1994; L
iu and H
uang 1996; H
ochgraf et al. 1997). Moreover, because of strong oxidation, highly oxidized oils usually contain significantly less PUFA and tocopherols than the equivalent fresh oils (D&
rsquo;
aquino et al. 1985; C
orcos benedetti et al. 1987; B
lanc et al. 1992; B
orsting et al. 1994; H
ayam et al. 1995; L
iu and H
uang 1995; H
ochgraf et al. 1997). In order to avoid these problems a controlled feeding system was used in this study and dietary fats were equalized for their fatty acid composition and tocopherol concentrations. Studies from literature reported that dietary oxidized oils reduce tocopherol concentrations and thereby alter the fatty acid composition of tissues and increase the fragility of erythrocyte membranes in animals (C
orcos B
enedetti et al. 1987; Y
oshida and K
ajimoto 1989; B
lanc et al. 1992; H
ayam et al. 1993). However, as mentioned above, in most of the studies reported in literature, the concentration of vitamin E was significantly lower in diets containing oxidized oils than in control diets. In order to find out whether those effects are also observed if diets are adjusted for identical vitamin E activities, this study also measured the tocopherol concentration in plasma, the fatty acid composition of erythrocyte lipids and the osmotic fragility of erythrocytes.