Impact of Highly Saturated versus Unsaturated Fat Intake on Carbohydrate Metabolism and Vascular Reactivity in Rat

Abstract

Palm olein (PO) and lard are considered harmful to health because of their highly saturated fatty acid content. On the contrary, olive oil (OO) with its high level of polyunsaturated fatty acids is considered healthier. This study aims to evaluate the effects of high consumption of these oils on carbohydrate metabolism and vascular function. Male Wistar rats were fed ad libitum for 12 weeks with different high fat diets (HFD) containing 30% of each oil. Systemic glycemia, insulinemia, and lipidemia were assessed by routine methods or by ELISA. GLUT4 muscular expression and hepatic and muscular Akt phosphorylation were analyzed by western blot. Vascular function was evaluated, ex vivo, on aortic rings and on the variations of isometric tensions. The results show that fasting blood glucose was increased with PO and OO diets and decreased with lard. Compared to control diet, this increase was significant only with PO diet. The area under the curve of IPGTT was increased in all HFD groups. Compared to control diet, this increase was significant only with PO. In contrast, stimulation of the pathway with insulin showed a significant decrease in Akt phosphorylation in all HFD compared to control diet. KCl and phenylephrine induced strong, dose-dependent vasoconstriction of rat aortas in all groups, but KCl EC₅₀ values were increased with lard and OO diets. The inhibitory effect of tempol was absent in PO and lard and attenuated in OO. Vascular insulin sensitivity was decreased in all HFD groups. This decreased sensitivity of insulin was more important with PO and lard when compared to OO diet. In conclusion, the results of this study clearly show that high consumption of palm olein, olive oil, and lard can compromise glucose tolerance and thus insulin sensitivity. Furthermore, palm olein and lard have a more deleterious effect than olive oil on the contractile function of the aorta. Excessive consumption of saturated or unsaturated fatty acids is harmful to health, regardless of their vegetable or animal origin.

Figure 1

Blood glucose level evolution during intraperitoneal glucose tolerance test performed on 16 h-fasted animals (a) and calculated area under curve from IPTG (b). Results were expressed as mean values ± SD, n = 7-8 animals per group. The limit of statistical significance was set at p < 0.05. The group mean values with different letters (a, b, c) are significantly different. AUC: area under the curve; IPGTT: intraperitoneal glucose tolerance test.

Figure 2

Effects of diets on phospho-Akt protein expression in muscle (soleus) in the absence or presence of insulin. Representative western blot of either total Akt or phospho-Akt with tubulin as a normalizing gene (a). The histograms of blot quantification in the absence of insulin (b). The histograms of blot quantification in the presence of insulin (c). Results were expressed as mean values ± SD, n = 7-8 animals per group. The limit of statistical significance was set at p < 0.05. The group mean values with different letters (a, b, c) are significantly different. Akt or PKB: protein kinase B; p-Akt: phosphorylated Akt; NS: not significant.

Figure 3

Contractile responses of rat aorta. Graphs represent cumulative response curves (a) to KCl (1–80 mM) and (b) to phenylephrine (0.01–100 μM). Histograms represent the maximal contraction (E_max) induced by a maximally active concentration of KCl (a) and phenylephrine (b). Results were expressed as mean values ± SD, n = 7-8 animals per group. The limit of statistical significance was set at p < 0.05. The group mean values with different letters (a, b, c) are significantly different. KCl: potassium chloride.

Figure 4

Effect of L-NAME on the contractile response to Phe. (a) Concentration-response curves to phenylephrine (0.01–100 µM) in aortic rings from control, olive oil, palm olein, and lard diets in absence and in presence of L-NAME (10 µmol/L). (b) Histogram represents the difference in area under curve between contractions with and without L-NAME for each group expressed as percentage of Phe effect. Results were expressed as mean values ± SD, n = 7-8 animals per group. The limit of statistical significance was set at p < 0.05. Histogram bars with different letters (a, b, c) are significantly different. AUC: area under the curve; L-NAME: nitro-L-arginine methyl ester.

Figure 5

Effect of tempol on the contractile response to Phe. (a) Concentration–response curves to phenylephrine (0.01–100 µM) in aortic rings from control, olive oil, palm olein and lard diets in absence and in presence of tempol (100 µmol/L). (b) Histogram represents the difference in area under curve between contractions with and without tempol for each group expressed as percentage of Phe effect. Results were expressed as mean values ± SD, n = 7-8 animals per group. The limit of statistical significance was set at p < 0.05. Histogram bars with different letters (a, b, c) are significantly different. AUC: area under the curve.

Figure 6

Effects of diets on relaxation of phenylephrine-precontracted aortic rings. Endothelium-dependent relaxation induced by acetylcholine (a) and endothelium-independent relaxation induced by SNP (b) and relaxant effect of insulin (c). Graphs represent cumulative response curves to acetylcholine (0.01–10 µM) in the absence and in the presence of L-NAME (10 µM), SNP (0.1 nM–1 µM) and Insulin (0.1–50 µM) on aortic rings previously contracted with 10 µM Phe. Histograms represent the maximal relaxation obtained for Ach and SNP and the relaxation induced by 20 µM insulin. Results were expressed as mean values ± SD, n = 7-8 animals per group. The limit of statistical significance was set at p < 0.05. The group mean values with different letters (a, b, c) are significantly different. Ach: acetylcholine; L-NAME: nitro-L-arginine methyl ester; SNP: sodium nitroprusside.