Medium-chain fatty acids ameliorate insulin resistance caused by high-fat diets in rats.

BACKGROUND: High dietary intake of saturated fat impairs insulin sensitivity and lipid metabolism. The influence of fatty acid chain length, however, is not yet fully understood, but evidence exists for different effects of saturated long-chain (LC) versus saturated medium-chain (MC) fatty acids (FA).
METHODS: To investigate the effects of the FA chain length, male Wistar rats were fed high-fat diets containing triacylglycerols composed of either MC- or LCFA for 4 weeks; rats fed maintenance diet served as a control. The animals underwent euglycemic hyperinsulinemic clamping or oral metabolic tolerance testing respectively; enzyme activities of mitochondrial (EC2.3.1.21 carnitine palmitoyl transferase) and peroxisomal (EC1.3.3.6 acyl-CoA oxidase) FA oxidation were measured in liver and muscle.
RESULTS: LCFA consumption resulted in higher fasted serum insulin and glucose concentrations compared to controls, while MCFA-fed animals did not differ from controls. Insulin sensitivity was reduced by 30% in the LCFA group while the MCFA group did not differ from controls. Feeding MCFA resulted in the controls' lowered fasted and post-prandial triacylglycerol concentration compared to LCFA, while triacylglycerol concentrations in muscle were higher in both high-fat groups compared to controls. No diet-induced changes were found in acyl-CoA oxidase (ACO) activity (liver and muscle), while LCFA feeding significantly raised carnitine palmitoyltransferase activity.
CONCLUSIONS: The chain length of saturated fatty acids in isocaloric diets affects insulin sensitivity, lipid metabolism and mitochondrial fatty acid oxidation without influencing body weight. While dietary LCFA clearly impair insulin sensitivity and lipid metabolism, MCFA seem to protect from lipotoxicity and subsequent insulin resistance without caloric restriction. Copyright (c) 2009 John Wiley & Sons, Ltd.