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Asia Pacific J Clin Nutr (1997) 6(2)116-118

Palm olein oil produces less lipid peroxidation products than
soya bean oil

Z Zaiton¹ MBBS, PhD, Z Merican² PhD, BAK Khalid³ MBBS(Homs), FRCP, PhD, JB Mohamed⁴ PhD and S Baharom²

Departments of Physiology¹ and Medicine³,Faculty of Medicine and Departments of Pharmacy² and Biomedical Sciences⁴, Faculty of Allied Health Sciences, Universiti Kebangsaan Malaysia, Kuala Lumpur, Malaysia

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The soleus muscles of hyperthyroid rats were used to investigate the effect of palm olein oil and soya bean oil on the production of lipid peroxidation products. It was found that palm olein oil but not soya bean oil significantly decreased malonaldehyde and conjugated diene levels of the soleus muscles of hyperthyroid rats. These findings suggest that palm olein per se produces less lipid peroxidation products than soya bean oil. Such an assay method gives a composite net picture of the propensity of an oil to produce lipid peroxidation products.

Key words: Conjugated diene, malonaldehyde, lipid peroxidation, palm olein, soya bean oil, hyperthyroidism, rats, soleus muscle

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Introduction

Refined deodorised and bleached palm olein oil (palm olein) and soya bean oil are used as cooking oils. Palm olein consists of 45.8% saturated fatty acids, 42.5% monounsaturated and 11.6% polyunsaturated fatty acids while soya bean oil consists of 15.1% saturated fatty acids, 23.4% monounsaturated and 61% polyunsaturated fatty acids^1,2. It has been reported that unsaturated fatty acids increase lipid peroxidation products more than saturated fatty acids^3,4. Besides the degree of saturation of an oil, other components of the oil can influence the formation of lipid peroxidation products. Tocopherols found in palm oil and soya bean oil⁵ and tocotrienols found in palm olein⁵ are antioxidants and decrease the formation of lipid peroxidation products by acting as free radical scavengers. Tocopherols and tocotrienols are believed to be the major lipid-soluble chain-breaking antioxidants found in blood plasma and membranes. Increases in lipid peroxidation products are thought to be associated with various diseases such as atherosclerosis, cancer^6,7 and myopathy⁸. In ischaemic heart disease, it is thought that lipid peroxidation may be an important event in the process of atherosclerosis⁹. Studies have shown that these free radical mediated diseases were significantly suppressed with administration of antioxidants in 1000 the form of palm oil and vitamin E ^10,11. Thus, besides knowledge of the lipid profile of an oil, knowledge of its antioxidant potential is equally important in these diseases.

In this study, the effects of oils on the production of lipid peroxidation products in vivo were measured by using the soleus muscle of the hyperthyroid rat. It has been reported ¹² and confirmed by the authors¹³ that lipid peroxidation products malonaldehyde (MDA) and conjugated diene (CD) increase in the soleus muscle of the hyperthyroid animal. Thus, the ability of an oil to decrease the level of lipid peroxidation products in the hyperthyroid soleus would indicate a desirable property in this respect.

Materials and Methods

40 male Wistar rats of similar age (8 weeks old) were divided into 4 treatment groups of 10 rats each. Each rat weighed between 250-300 grams and the groups were weight-matched. The rats were treated for a period of 8 weeks. Group 1 rats (control group) were injected intraperitoneally with alkalinised saline (pH 9-9.5) thrice weekly and were fed ground rat chow. Group 2 rats were injected intraperitoneally thrice weekly with 500 m g/kg L-thyroxine dissolved in 0.9% sodium chloride made alkaline with sodium hydroxide and adjusted to pH 9-9.5 with hydrochloric acid and were fed ground rat chow (hyperthyroid group). Group 3 rats were treated similarly to group 2 rats but, in addition, received 20% w/w of palm olein mixed with ground rat chow (hyperthyroid + palm olein group). Group 4 rats were treated similarly to group 2 rats but, in addition, received 20% w/w of soya bean oil mixed with ground rat chow (hyperthyroid + soya bean oil group).

Twenty-four hours after the final injection of the 8 week period, the rats were anaesthetized with sodium pentobarbitone injected intraperitoneally.

Preparation of homogenate and analysis of MDA and CD

The soleus muscles were removed, weighed and homogenised in phosphate buffer saline. The homogenate was centrifuged at 3000 rpm for 15 mins and the supernatant was kept at -70° C. Assays for MDA^13,14 and CD^13,15 were done within one week of killing. The MDA content of the sample was calculated as follows:

MDA =

0.5 x f sample x V final
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F std x V sample

f sample: absorbance of sample
F std: absorbance of standard solution
V sample : volume of sample (ml)
V final : volume of final mixture (ml)

Statistical analysis

The data are presented as mean ± SD. Statistical significance was by unpaired t-test after a one-way analysis of variance.

Results

MDA level in the soleus muscle of the hyperthyroid group was significantly increased compared to the control group (Table 1). The MDA level in the soleus muscles of the hyperthyroid and palm olein group was significantly different from the hyperthyroid group but not significantly different from the control group showing that palm olein successfully brought down the MDA level to the control value. The MDA level in the soleus muscles of hyperthyroid and soya bean oil group was not significantly different from the hyperthyroid group (Table 1). This shows that consumption of palm olein produces less 1000 MDA than consumption of soya bean oil.

Table 1. Malonaldehyde levels in the soleus muscles of the four groups of rats.

Group	Malonaldehyde (nmol/mg protein)
Control (8)	3.17 ± 0.60
Hyperthyroid (8)	6.20 ± 2.60a
Hyperthyroid + palm olein (8)	3.06 ± 1.10c
Hyperthyroid + soya bean oil (8)	5.98 ± 0.94bd

Values indicate mean ± SD. Number of observations are given in parenthesis. Superscripts indicate significant difference from: Control ^ap<0.01, ^bp<0.0001; Hyperthyroid ^cp<0.05; Hyperthyroid + palm olein ^dp<0.0001.

The CD level in the soleus muscle of the hyperthyroid group was also significantly increased compared to the control group (Table 2). The CD level in the soleus muscles of the hyperthyroid and palm olein group was not significantly different from either the control group or the hyperthyroid group, showing that palm olein has partially decreased the CD level in the soleus muscle of the hyperthyroid group. The CD level in the soleus muscle of the hyperthyroid and soya bean group was still significantly different from the control group though at a lower level of significance. This shows that consumption of palm olein produces less CD than consumption of soya bean oil.

Table 2. Conjugated diene levels in the soleus muscles of the four groups of rats.

Group	Conjugated Diene (OD/g protein)
Control (8)	119 ± 25
Hyperthyroid (8)	228 ± 47a
Hyperthyroid + palm olein (8)	203 ± 71
Hyperthyroid + soya bean oil (8)	251 ± 137b

Values indicate mean ± SD. Number of observations are given in parenthesis. Superscripts indicate significant difference from: Control ^ap<0.0001, ^bp<0.05;

Discussion

Two products of lipid peroxidation, that is MDA and CD, were measured since a single product may not be reflective of the lipid peroxidation process. In this study, the MDA and CD contents of the soleus muscles of hyperthyroid rats were shown to be about twice their respective control values. Yet, when palm olein was added to the rat chow of hyperthyroid rats (hyperthyroid and palm olein group) the MDA and CD levels of their soleus muscles were reduced to values not significantly different from their respective control values. This shows that palm olein has antioxidant properties. When soya bean oil was added to the rat chow of hyperthyroid rats (hyperthyroid and soya bean oil group) the MDA and CD level of the soleus muscle were not raised further, but remained not significantly different from those of hyperthyroid rats. This shows that soya bean oil too has antioxidant properties; if this were not so the level of MDA and CD in the soleus would be higher compared to the level in the soleus of the hyperthyroid animal (since soya bean oil contains a high percentage of unsaturated fatty acids). Collectively, the result also shows that consumption of palm olein produces fewer lipid peroxidation products than consumption of soya bean oil. It is suggested that two factors may be responsible for the observed effect of the oils on MDA and CD. First, palm olein contains both tocopherol and tocotrienol¹⁶ but soya bean oil contains only tocopherol¹⁶ in quantities less than that of palm olein. Also tocotrienol has been reported to have a higher antioxidant activity than tocopherol¹⁷. These facts could account for the lower production of MDA and CD with palm olein consumption. However, the suggestion is made based on the assumption that the levels of tocopherol and tocotrienol in the soleus of palm olein-fed and soya bean oil-fed rats are reflective of the levels found in the oils. In future studies, the levels of tocopherol and tocotrienol in the soleus of palm olein and soya bean oil-fed rats will be measured. Another factor is that palm olein is a more saturated oil than soya bean oil and MDA and CD production would be lower with increasing saturation of an oil. Thus, the higher level of MDA and CD in the soleus of soya bean oil-fed rats compared to palm olein-fed rats could be due to its lower antioxidant activity and to its high content of unsaturated fatty acids. These are two of the factors that could contribute to the fact that rats fed palm olein produce less MDA and CD than those fed soya bean oil.

The soleus muscles from the hyperthyroid rats rather than the soleus from normal rats were used because the authors are of the opinion that it is easier to detect antioxidant activity (of the oil or any other compound) in a situation where lipid peroxidation products are raised rather than when they are at their basal level. This in vivo method gives a net picture of the propensity of an oil to produce lipid peroxidation products. Furthermore, it takes into account all factors inherent in the oil, known and still unknown, which may influence the production of lipid peroxidation prod 1000 ucts.

In conclusion, the study suggests that palm olein oil produces less MDA and CD than soya bean oil. As such, palm olein oil may be a better oil to use for normal cooking and also in other pathological conditions where lipid peroxidation levels are raised. In addition since repeated use of an oil exposes it to peroxidation, the less unsaturated palm olein oil would have an added advantage over soya bean oil.

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Palm olein oil produces less lipid peroxidation products than soya bean oil
Z Zaiton, Z Merican, BAK Khalid, JB Mohamed and S Baharom
Asia Pacific Journal of Clinical Nutrition (1997) Volume 6, Number 2: 116-118

Copyright © 1997 [Asia Pacific Journal of Clinical Nutrition]. All rights reserved.

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