1. Introduction
Atherosclerosis is the main underlying cause of heart disease, which is the mutual cause of death in developed countries [
1,
2]. The atherosclerosis process initiates as a protective response to injuries to the endothelium and smooth muscle cells of the arterial wall and leads to atherosclerotic plaque formation [
2]. This in turn narrows, and may block, the lumen of the affected artery. Atherosclerosis is responsible for myocardial ischemia/infarction, stroke, aortic aneurysms and peripheral vascular diseases [
2].
The relationship among diet, plasma lipid concentrations and atherosclerosis has been well documented and reviewed [
3]. Atherosclerosis lesions in animals and humans seem to be associated with elevated plasma total cholesterol (TC), decreased high-density lipoprotein cholesterol (HDL–C), increased low-density lipoprotein cholesterol (LDL–C) and over-consumption of fat [
4]. Dietary fats can be important determinants that regulate blood cholesterol levels. Generally, saturated fats are thought to increase blood cholesterol whereas unsaturated fats decrease blood cholesterol. Chiu et al. [
5] reported that unsaturated fats were less atherogenic than saturated fats and this outcome was further reduced by a cholesterol-poor diet.
Palm oil having 50% of its fatty acid composition as saturated fatty acids has been postulated to increase coronary heart disease risk by elevating blood cholesterol levels [
6]. However, several human clinical trials, including those from our laboratories, have discounted this postulation [
4,
7]. Instead it has been shown that palm olein could be used to replace canola, olive and rapeseed oils without affecting lipoprotein cholesterol and serum lipids levels [
8,
9]. In human studies designed to evaluate the atherosclerotic effects of palm-based diets, it was noted that the effects of palm oil were unlike saturated fats but resembled those of unsaturated fats [
10]. The existence of numerous fat-soluble micronutrients in palm oil, namely vitamin E, carotenoids and phytosterols, contributes to its pleiotropic nutritious profile. Oil palm vitamin E has been reported to act as a potent biological antioxidant which was suggested to reduce cardiovascular disease by modulating human lipids and lipoproteins [
6].
The American Heart Association (AHA) Nutrition Committee (2006) [
11] recommended that the fats people eat should have equal parts of saturated, monounsaturated and polyunsaturated fatty acids (Step I diet). This should have a distribution ratio 1:1:1 of all the fatty acids but in nature, no oils or fats that is available for consumption have this recommended ratio.
Previously, Sundram et al. [
7,
12] designed an AHA oil blend which give rise to a 1:1:1 ratio of monounsaturates, polyunsaturates and saturates. The oil later was given to 23 healthy normocholesterolaemic male volunteers who were fed a specially designed whole food diets and compared to palm olein (16:0-rich), canola oil (18:1-rich) or the AHA Step-1 diet, altogether contributing about <200 mg dietary cholesterol/day and 31% energy. Plasma TC and LDL-C were not significantly affected by the three diets regardless of modifications of the primary dietary fatty acids. The high 16:0 palm olein and 18:1 canola brought about nearly identical lipoprotein and plasma cholesterol levels. HDL–C was significantly increased after the AHA diet in comparison to palm olein and canola diets. The results of their dietary fats showed that a balanced fatty acid ratio incorporating palm oil may improve the LDL/HDL-cholesterol ratio and thus this could be cardio-protective.
The Malaysian Palm Oil Board (MPOB) has produced several innovative oil compositions to penetrate niche markets where consumers are concerned with diet and health. Two unique oil compositions with cold stability have been developed from laboratory to pilot plant scale, and are available for commercialization. These oils have palm oil products as base materials and enhanced fatty acid compositions by blending with other vegetable oils. A study of these oils compared with saturated oil or olive oil from the nutrition point of view could help in the promotion of such oils for commercialization and market studies. Therefore, the effects of these novel oils on plasma lipid profiles and their relation to the development of atherosclerotic plaques in a cholesterol-fed rabbit model was explored in the present study.
4. Discussion
When a new edible oil is introduced, the safety and nutritional qualities of the oil has to be acceptable or above average. The two oils (NoveLin I, NoveLin II) are intended for producing products which are not only marketable in temperate countries, but could be also promoted as healthy oils.
The purpose of the current study was to assess whether the two palm-based innovative oils namely the NoveLin I (which has an approximately equal distribution of saturated, monounsaturated and polyunsaturated fatty acids) and NoveLin II (which has moderately high levels of oleic acids with low content of saturated and polyunsaturated fatty acids), have potent hypocholesterolaemic and anti-atherogenic effects in comparison with olive oil, and whether these properties are connected with changes in plasma lipid compositions in rabbits.
The rabbit is a commonly used experimental model for assessing the effects of dietary fat and development of atherosclerosis. In the current study, a high-fat atherogenic diet formulation containing fat at approximately 35% energy with addition of 0.15% dietary cholesterol was fed to the rabbits. Idris et al. [
14] reported that the lesion morphology in rabbits was altered by the percentage of cholesterol added to the diet and the feeding period. Short feeding time with a high percentage of cholesterol (exceeding 2%) may lead to hypercholesterolaemia and atherosclerotic lesions rich in foam cells originated from macrophages. On the other hand, diet supplemented with high-fat, low-dietary cholesterol content, and long feeding duration, may cause atherosclerotic lesions that are rich in smooth muscle cells and contain cholesterol deposits leading to atherosclerotic lesions similar to those of humans [
16,
17,
18].
In the present study, animals were fed a high-fat atherogenic diet daily for 100 days duration. Changes in blood lipids resulting from these fatty acid manipulations were obvious. The test oils used (NoveLin I, NoveLin II, OLV and CNO) have different fatty acid compositions in terms of their saturated:monounsaturated:polyunsaturated ratios. Significant effects were evident in the rabbits fed the CNO diet, which resulted in changes of lipid profiles (TC, LDL–C and TG) that were clearly atherogenic. As a result, the CNO-fed group showed a significantly lower percentage of lesion-free area, with substantial development of fibrous plaques, compared to the other three groups. Similar observations were also reported by Wilson et al. [
19] and Kritchevsky et al. [
20].
Monounsaturated oils rich in oleic acid are believed to be the healthiest of edible oils in the human diet [
11]. Whereas rapeseed, olive and canola oils have in excess of 60% of their fatty acid composition as oleic acid [
11], both NoveLin I and NoveLin II, contain about 38% and 53% of oleic acid, respectively. In the present study, both NoveLin oils and olive oil led to significantly lower plasma TC and LDL–C in rabbits compared to the control CNO diet. NoveLin oils are based on palm oil, which has been identified as an exceptional source of two major phytochemicals, including vitamin E (tocopherols and tocotrienols) and carotenoids, both of which are fat-soluble [
17]. Vitamin E from palm oil has been reported to be a potent biological antioxidant, protecting against oxidative stress and the atherosclerotic process by modulating pathways of lipid metabolism and fatty acid biosynthesis [
21]. It could also decrease the expression of transcription factors regulating adipogenesis and increase apoptosis of adipocytes [
21]. A similar finding was reported by Wilson et al. [
19] on the effects of emu oil, which is mostly monounsaturated, though less so than olive oil. The same author [
19] stated that olive oil, crude emu oil, and refined emu oil diet-fed groups had significantly lower plasma TC and LDL–C concentrations compared to the coconut oil diet-fed group, without differences in plasma HDL–C or TG concentrations. Kritchevsky et al. [
20] evaluated the atherogenicity of avocado oil compared to coconut oil, olive oil and corn oil. Their findings showed that serum cholesterol levels were significantly higher in the rabbits fed coconut oil than the other groups, which is commensurate with our findings. Triglyceride levels were similar in all four experimental groups. In addition, Ng et al. [
9], who studied dietary effects of palm olein and olive oil in comparison to coconut oil on blood lipids of human subjects, reported that coconut oil demonstrated significantly higher blood lipid parameters i.e., TC, LDL–C and HDL–C; whereas palm olein and olive-fed groups resulted in similar blood lipid parameters.
The literature has highlighted the beneficial effects of polyunsaturated fats and the deleterious impact of saturated fats [
22,
23]. Currently, monounsaturated fats are regarded as neutral. The atherogenic effects of various oils, such as corn, coconut, olive and avocado, were also evaluated by Kritchevsky et al. [
20]. Their findings showed no significant difference in atherogenic effect between avocado oil, a monounsaturated oil, with olive oil, since both contain high levels of oleic acid; however, severe development of aortic lesions was apparent in the coconut oil-fed group, and similar observations were also made in the present study, as rabbits fed coconut oil displayed severe atherosclerosis, most likely due to the high ratio of saturated to monounsaturated and polyunsaturated (S:M:P) fatty acids in their diet [
24]. Meanwhile, the severity of lesions in both NoveLin oil groups were very similar to those seen in the olive-fed group, indicating that these newly developed oils have hypocholesterolaemic and anti-atherogenic effects similar to olive oil.