Supplementation with n-3, n-6, n-9 fatty acids in an insulin-resistance animal model: does it improve VLDL quality?

Insulin-resistance (IR), of increased cardiovascular risk, is characterized by the production of altered VLDL with greater atherogenicity. Dietary fatty acids influence the type of circulating VLDL. But, it is not clear how dietary fatty acids impact VLDL characteristics in IR. AIM: to evaluate the effects of n-3, n-6 and n-9 fatty acid supplementation on preventing atherogenic alterations in VLDL, in a diet-induced IR rat model. Male Wistar rats (180-200 g) were fed: standard diet (control, n = 8) and a sucrose rich diet (30% sucrose in water/12 weeks, SRD; n = 24). Simultaneously, SRD was subdivided into SRD-C (standard diet), and three other groups supplemented (15% w/w) with: fish oil (SRD-n3), sunflower oil (SRD-n6) and high oleic sunflower oil (SRD-n9). Lipid profile, free fatty acids, glucose, and insulin were measured. Isolated VLDL (d < 1.006 g ml-1) was characterized by chemical composition and size (size exclusion-HPLC). In comparison with SRD-C: SRD-n3 showed an improved lipoprotein profile (p < 0.01), with lower levels of insulin and HOMA-IR (p < 0.05). SRD-n6 showed increased levels of HDL-cholesterol and lower insulin levels. SRD-n9 did not exhibit differences in lipid and IR profile, and even favored weight gain and visceral fat. Only SRD-n3 prevented the alterations in VLDL-TG% (54.2 ± 4.4% vs. 68.6 ± 8.2, p < 0.05) and showed lower large VLDL-% (22.5[19.7-35.6] vs. 49.1[15.5-82.0], p < 0.05), while SRD-n6 and SRD-n9 did not show effects. CONCLUSION: In IR, while n-3 PUFA showed expected favorable effects, supplementation with n-6 PUFA and n-9 MUFA did not prevent atherogenic alterations of VLDL. Thus, the recommendations of supplementation with these fatty acids in general diet should be revised.

Circulating very-low-density lipoprotein characteristics resulting from fatty liver in an insulin resistance rat model.

The close association between nonalcoholic fatty liver and insulin resistance is now widely recognized. While the former is characterized by excessive intrahepatic triglyceride accumulation, the latter induces overproduction of very-low-density lipoprotein (VLDL) particles. It has not been well elucidated whether these apparently opposite mechanisms impact on VLDL characteristics or not. The aim of the present study was to evaluate the VLDL secretion and features resulting from insulin resistance and fatty liver in rats fed a sucrose-rich diet (SRD, i.e. addition of sucrose to drinking water during 12 weeks). No differences in calorie intake were observed in comparison to controls. Both groups showed similar weight gains throughout the treatment period. However, SRD rats showed an increased proportion of body fat as assessed by X-ray absorptiometry, increased visceral obesity, liver weight and fat accumulation in the liver (p < 0.04). Histological study revealed moderate micro- and macrovesicular steatosis. Fasting insulin, triglyceride and free fatty acid (FFA) levels increased while VLDLs decreased in SRD rats (p < 0.05). The chemical composition of VLDLs of SRD rats showed a higher percentage of triglycerides, and the VLDL triglyceride/protein ratio, an estimator of lipoprotein size, suggests that VLDL particles of SRD rats are larger than those of controls (p < 0.0005). FFA levels correlated with VLDL triglycerides (r = 0.49, p = 0.03) and liver fat content correlated with plasma triglycerides (r = 0.65), VLDL triglycerides (r = 0.55) and triglyceride/protein ratio (r = 0.52, p < 0.02). The VLDL secretion rate assay showed an increase in SRD rats (p < 0.02), confirming an overproduction despite liver fat accumulation. Our findings are consistent with an insulin resistance development model in which hepatic lipid content would constitute an important determinant of a triglyceride-rich, large-particle VLDL secretion; both features would increase its atherogenic potential.