Background

Increased plasma free fatty acids reduced insulin-stimulated glucose-uptake. The mechanism responsible for this inhibition, however, remains unclear. There is sufficient evidence suggesting that the defects in glucose uptake and storage in the presence of free fatty acids may be related to abnormal fat metabolism [1, 2]. Most patients with type 2 diabetes are obese and have elevated plasma free fatty acids concentrations [3], and these have been shown to inhibit insulin-stimulated glucose uptake [4, 5]. The Randle glucose-fatty acid cycle has been used to explain insulin resistance in skeletal muscle of patients with type 2 diabetes or obesity. An alternative hypothesis proposes that muscle insulin resistance results from decreased mitochondrial oxidation of fatty acids [6–9]. The unoxidized fatty acids are re-routed toward the synthesis of diacylglycerol and ceramide, which in turn stimulate stress-induced protein kinases that inhibit insulin signalling [8, 9]. This hypothesis is supported by measurement of the concentrations of lipids in mitochondrial oxidative capacity, and the phosphorylation state of several components of the insulin-signalling pathway in hearts perfused with palmitate in pathological samples from type 2 diabetic patients [10].

Increasing evidence implicates dietary fat composition in the development of insulin resistance, including impaired fasting glycaemia and Type 2 diabetes mellitus [11, 12]. Several cross-sectional studies have shown an association between dietary saturated fat content and hyperinsulinaemia [13] impaired glucose tolerance [14, 15] and overt diabetes [16]. Groundnut oil contains 46 and 32% of mono unsaturated fatty acids (MUFA) and polyunsaturated fatty acids (PUFA), respectively Palm oil contains approximately 50 % of saturated and 50 % unsaturated fatty acids [17], whilst coconut oil is, a saturated fat which contains predominantly medium-chain saturated fatty acids with high antioxidant properties [18–20].

Several management strategies of diabetes such as dietary modification, exercise, oral hypoglycaemic agents, and insulin have been in used for decades. However, these regimen have not adequately proven to be effective in curbing the complications associated with diabetes mellitus [21]. Vegetable oils have also been found to be useful in the management of diabetes [22]. These oils contains different fractions of fat and other component such as tocotrienols, tocopherols, oryzanol, phytosterols which have varying effect on insulin resistance and metabolic control [23]. Many vegetable oils including groundnut oil, red palm oil, coconut oil and palm kennel oil are widely used in many developing countries as part of their dietary preparations. A favourable glucose response was also induced by fish oil feeding and this was mediated through body weight loss [24]. Noninsulin diabetic model mice fed on palm oil had improved glycaemic control despite their marked obesity [25]. However, public perception that intake of these edible oils contributes to dyslipidaemia and increase cardiovascular risk has reduced their consumption. It is against this background that this study was aimed at determining the metabolic effect of some vegetable oil consumption in the management of diabetes in diabetic mice.

Methods

All procedures and techniques used in this study were in accordance with the National Institute of Health Guide lines for the care of Laboratory Animals (NIH). Department of Health Service Publication No 83-23, revised 1985). The protocol for the study was approved by the Health Service Ethics committee, School of Medical Sciences, Kwame Nkrumah University of Science and Technology Kumasi- Ghana.

Forty eight (48) male diabetic mice (BKS.Cg-m+/+Lepr db/BomTac) strain each weighing between 25 and 40 g and a mean weight of 35 g and approximately 8 weeks old were obtained from the Department of Animal Experimentation, Nuguchi Memorial Institute for Medical Research, University of Ghana. The mice were transported in specialized cages and brought to the Animal house of the pharmacology Department, Kwame Nkrumah University of Science and Technology (KNUST)-Ghana where the study was carried out. The mice were housed in groups of two at 25–30 °C with 12 h daylight. The mice were fed ad libitum with normal rodent chow (composed of 21 % protein, 5 % fat and 34 % carbohydrate, made from fish meal, maize, rice and wheat bran) was bought from the Nuguchi Memorial Institute for Medical Research laboratories (Department of Animal Experimentation) Accra – Ghana.

Diet preparation

The test diets were prepared by mixing 10 g vegetable oils with 90 g normal commercial rodent chow to obtain a 10 % by weight of the vegetable oils. Three different 10 % fat diets were prepared by adding red palm oil (extracted from the pulp of African oil palm (Elaeisguineensis), groundnut oil (extracted from the nuts of Arachishypogaea) and coconut oil (extracted from the kennel of Cocosnucifera) and pure chow as control diet and kept in a refrigerator until use. The supplementation in vegetable oils is in addition to the 5 % fats that are in the 90 % of the chow. The vegetable oils were bought from the Ghana Food Distribution Corporation. The red palm oil is composed of 51 % saturated fatty acid, 39 % monounsaturated fatty acids and 10 % polyunsaturated fatty acids. The groundnut oil is composed of 18 % saturated fatty acid, 48 % monounsaturated fatty acids and 34 % polyunsaturated fatty acids, whilst the coconut oil is composed of 92 % saturated fatty acid, 6 % monounsaturated fatty acids and 2 % polyunsaturated fatty acids.

Experimental design

After one week of acclimatization, during which period mice were fed on normal chow ad libi tum, overnight fasting glucose levels were determined. Only mice whose fasting glucose was ≥ 11.0 mmol/L were selected for the experiment.

The selected animals were randomly divided into eight groups of six animals each:

Groups I–IV were on diet treatment only and group V–VIII, were additionally treated glibenclamide (2 mg/kg body-wt, i.p) daily at 8.00 AM, for the 4 week period as follows)

  • Group I: chow (control)

  • Group II: groundnut oil diet

  • Group III: palm oil diet

  • Group IV: coconut oil diet

  • Group V: Chow + glibenclamide (2 mg/kg b-wt)

  • Group VI: groundnut oil diet + glibenclamide (2 mg/kg b-wt)

  • Group VII: palm oil diet + glibenclamide (2 mg/kg b-wt)

  • Group VIII: coconut oil diet + glibenclamides (2 mg/kg b-wt)

Biochemical assay

After 28 days of treatment, the 12 h-fasted animals were killed by cervical decapitation between 8 AM. and 9 AM. About 2.0 ml of blood sample was taken from each mouse, of which 1.5 ml was dispensed into a plain tube to clot whiles 0.5 ml of the blood was put into separate EDTA tubes. The tubes were then placed in a centrifuge and spun at 3000 x g for 10 min to obtain the plasma and sera. Plasma glucose was measured immediately and the serum for the measurement of other biochemical variables was stored at−80 °C until analysis.

Biochemical assays including glucose, Total cholesterol, LDL-cholesterol, HDL-cholesterol, triglycerides, were analysed using the automated analyser, Mindray Chemistry Analser (BS 120) (China) with reagent kits manufactured by the (ELITech Clinical System SAS 2.1 61500 SEES- FRANCE 2014-2010) compatible with the auto analyser, and according to the reagent kit manufacturer’s protocol.

Statistical analysis

The data analysis was done using Graph Pad Prism version 5.00 for windows (GraphPad Software, San Diego California, USA). Baseline characteristics were expressed as mean ± standard error of means (SEM). One-way analysis of variance (ANOVA) with Dunnett’s test was used for multiple comparisons between metabolic groups. P value <0.05 was considered significant.

Results and discussion

The management of type 2 diabetes mellitus includes the use of diet, exercise, oral hypoglycemic agents, and insulin. However, these remedies do not effectively prevent the complications of diabetes mellitus [21]. Extracts of many local medicinal plants have been successfully used in the management of diabetes [26]. Some dietary fats have also been reported to be effective in lowering blood glucose [16]. However, dietary fat may also have a negative effect on the lipid profile [27] which may negate the gains of glucose lowering.

Dietary fats differ in chain length and degree of unsaturation and this affects plasma lipoprotein constitution [28]. Serum cholesterol level increases when dietary carbohydrates are replaced by saturated fatty acids and decreases when carbohydrates are replaced by polyunsaturated fatty acids containing omega 3 [27]. Studies have suggested that the cholesterol-decreasing effect of polyunsaturated fatty acids affects both LDL and HDL cholesterol [28, 29]. However, the mechanism by which these fats affect serum lipoproteins concentration is unclear. Plasma total cholesterol, and LDL (Table 1) were significantly (p <0.05) reduced and HDL-cholesterol was increased in diabetic mice fed on the groundnut oil diet, whilst the palm oil and coconut oils diets did not have any significant effects. None of the diets significantly affected the plasma triglycerides level. This means that the consumption of these vegetable oils does not pose cardiovascular risk. This is supported by the low cardiovascular risk index (HDL-cholesterol/Total cholesterol) Tables 1 and 2.

Table 1 The effect of vegetable oil consumption on plasma lipid profile in diabetic Mice
Full size table
Table 2 Effect of vegetable oils consumption and Glibenclamide treatments on serum lipid profile of diabetic mice
Full size table

Humans and animal studies have shown that, saturated fat intake increases both LDL and high-density lipoprotein (HDL) cholesterol levels [30, 31]. The increased LDL is due to the inhibiting of LDL receptor activity and thus enhancing apolipoprotein (apo) B-containing lipoprotein production [32]. In contrast, intake of polyunsaturated fatty acids had a modest but significant LDL cholesterol-lowering effect [33]. The lowering of LDL cholesterol in mice fed with groundnut oil diet is consistent with other findings, particularly considering the fact that groundnut oil predominantly consists of polyunsaturated fatty acids [17]. In this study groundnut oil significantly (p <0.05) raised plasma HDL contrary to findings by Mattson and Grundy which suggest that polyunsaturated fatty acids lower HDL [34]. Similar results to our finding have been reported: streptozotocin-induced diabetes in rats lowered HDL-cholesterol whilst 8 % groundnut oil raised plasma HDL in streptozotocin-induced diabetic rats [35]. This may be because diabetes has a metabolic defect which is compensated for by an increase in HDL levels.

The exact mechanism by which groundnut oil raised HDL levels in the diabetic rats is not understood.

When the mice in addition to the oil feed diet were administered with (2 mg/kg. wt. glibenclamide i.p) daily for the 4 week period, plasma Total cholesterol was significantly (p <0.05) reduced in the control and mice fed on palm oil diet. Plasma LDL levels in diabetic mice were significantly (p <0.05) reduced in all the oil diets and the control plus glibenclamide treatment. HDL was significantly (p <0.05) increased only in groundnut oil and coconut oil formulated diets. Triglycerides were only significantly (p <0.05) increased in the groundnut oil diet (Table 2). It is unclear how glibenclamide treatment increases triglyceride lipid subfraction while the others are reduced. To consider the effect of the oils on treatment with glibenclamide, the lipid levels were compared to the control plus glibenclamide treatment. Total cholesterol was significantly increased by groundnut oil and coconut oil. LDL was significantly raised in the coconut oil. HDL was increased only in the groundnut oil and coconut oils, while triglycerides were also significantly increased only in the groundnut oil. Even though total cholesterol was increased in the groundnut and coconut oil there was a corresponding significant decrease in cardiovascular risk (Tables 1 and 2).

Mice fed on groundnut oil, palm oil and coconut oil diets had significant (p < 0.05) reduction in blood glucose. The reduction in blood glucose in groundnut oil diet-fed mice may be due to the presence of mono unsaturated fatty acids (MUFA) [36]. A reduction in plasma glucose concentration with the consumption of a MUFA-rich diet has been reported [37]. Indeed it has also been shown that when the groundnut oil was replaced with sesame oil (a much higher percentage polyunsaturated fatty acid than groundnuts) the reduction in glucose was even more significant [38].

Ikemoto et al., attributed reduction of blood glucose in mice fed on palm oil to palm oil-induced hypersinsulineamia [25]. The high antioxidant effect of both processed and unprocessed palm oil which could also lead to reduced blood glucose is due to the carotenoids, phosphatides, sterols, tocopherols and trace metals content, and these have been shown to be effective against oxidative stress in in vitro and in vivo studies [39]. The carotenoids, together with vitamin E, ascorbic acid, enzymes and proteins, are members of the biological antioxidant network converting highly reactive radicals and free fatty peroxyl radicals to less active species [40] thus, protecting against oxidative damage to cells. β-carotene is the most abundant carotenoids which can be converted to vitamin A; which is important in the visual process. In addition, it is an antioxidant that destroys singlet oxygen and free radicals [41]. Oral administration of antioxidants extracts from Aloe vera gel significantly decreased the levels of blood glucose, glycosylated hemoglobin and increased haemoglobin levels [42]. Several other studies have shown that antioxidants administration indeed reduces blood glucose, glycosylated haemoglobin and the upregulation of oxidative stress enzymes in diabetic animals [42, 43]. The administration of coconut oil diet, even though coconut oil is a saturated fat, significantly reduced blood glucose. The exact mechanism is not understood. This could also be mediated through its antioxidant effect [44]. Indeed Houssay and, Martínez showed that administration of coconut oil completely protected against diabetes [45].

In a Post Hoc Multiple Comparisons analysis (Fig. 1), plasma glucose in the glibenclamide treated diabetic mice was surprisingly significantly higher in the palm oil diet and groundnut oil diet fed mice than in the non glibenclimide treated mice on palm oil diet and groundnut oil diet (p < 0.05). The values were however significantly lower than in the control. On the other hand, plasma glucose in the glibenclamide treated mice on coconut oil diet was significantly decreased (p < 0.05) compared to the coconut oil diet only fed mice (Fig. 1).

Fig. 1
figure 1

The effect of vegetable oil consumption on glibenclamide activity in diabetic Mice. Mean effect of vegetable oils on glucose level on diabetic mice after oil diet feed. * indicate significant difference compared to controls, (p < 0.05). Mean value were significantly different from the control group. # indicate significant difference compared to other vegetable oils (p < 0.05). Mean value were significantly different from its comparative vegetable oil only (*P < 0.05, #P < 0.05, Post Hoc Multiple Comparisons)

Full size image

Sulfonylurea, such as glibenclamide has been widely used to treat type 2 diabetic patients over a long period. (Glibenclamide was chosen in this study because it is one of the antidiabetic drugs recommended by the Ministry of Health- Ghana for the management of diabetes). The mechanism of action of this drug has been so controversial. However; all do agree that the hypoglycaemic effect is mediated through increased insulin secretion [46]. Free fatty acids have been shown to impair insulin sensitivity [47]. The effect of fatty acids on sulfonylureas may be varied. Palm oil and groundnut oil had inhibitory effect on glibenclamide activity, hence plasma glucose was significantly (p < 0.05) higher in mice fed on these oils plus glibenclamide treatment than mice on the oils feeds alone (Fig. 1). Zambon et al., showed similar effect of fish oil (ω3) on glyburide. Fish oil alone significantly reduced blood glucose. However when the Fish oil was administered together with glyburide, the hypoglycaemic effect was reduced [48]. The mechanism by which these vegetable oils partially impair the activities of glibenclamide and fish oil impairs glyburide is unclear. It may be that these oils partially impair the activity of the glibenclamide-induced insulin secretion, as suggested by Boden [47]. However, the effect of coconut oil on glibenclamide activity was additive. The hypoglycaemic effect of glibenclamide plus coconut oil alone was significantly (p < 0.05) higher than the effect of coconut alone (Fig. 1). Coconut oil may therefore induce insulin secretion that may argument the insulin secretion induced by glibenclamide or at least improves insulin sensitivity, or the effect may be mediated through some other mechanism.

Food consumption rate was determined by the amount of food intake per kilogram body weight of the mice. Even though there was no significant change in food consumption per gram weight of mice compared to the control, there was a consistent tendency for decreased consumption of the oil formulated diets compared to the control chow (Fig. 2). Recent studies have shown that body weight loss can be achieved by increase in fat intake, which is mediated through early satiety or filling, resulting in lower total calorific intake compared to a carbohydrate diet that would require high filling and higher calories [49].

Fig. 2
figure 2

The effect of vegetable oil consumption on food intake in diabetic Mice. Mean effect of vegetable oils on feed consumption per body weight for diabetic mice. *Mean value were nonsignificantly different from the control group (*P < 0.05,Post Hoc Multiple Comparisons)

Full size image

This non-significant reduction in food consumption corresponded with a nonsignificant body weight loss. (Table 3). Since growth rate is time dependent, it is possible that the reduction in food consumption and body weight loss would have become significant if the period of study had been extended to six or more weeks as has been reported by Danqua.et al. [50].

Table 3 Effect of vegetable oil diet on weight of diabetic mice
Full size table

Conclusion

There was significant improvement of lipid profile in diabetic mice fed on 10 % vegetable oil fortified meal. Total cholesterol was significantly reduced in mice fed on groundnut oil only and palm oil diets plus glibenclamide treatment. Palm oil and groundnut oil diets and coconut oil plus glibenclamide treatment significantly reduced plasma LDL levels in mice fed on the diets. HDL cholesterol was significantly increased in mice on groundnut oil diets only. Plasma glucose in the diabetic mice was significantly reduced after consuming diets fortified with 10 % palm oil, groundnut oil and coconut oil. 10 % Groundnut oil, palm oil and coconut oil feeding had antidiabetic properties with no cardiovascular risk in diabetic mice.