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Purpose: To study the effect of isoenergetic administration to adult rats of high-fat or high-fat-high-fructose diet for 2 weeks on skeletal muscle mitochondrial energetic. Methods: Body and skeletal muscle composition, energy balance, plasma lipid profile and glucose tolerance were measured, together with mitochondrial functionality, oxidative stress and antioxidant defense. Results: Rats fed high-fat-high-fructose diet exhibited significantly higher plasma triglycerides and non-esterified fatty acids, together with significantly higher plasma glucose and insulin response to glucose load. Skeletal muscle triglycerides and ceramide were significantly higher in rats fed high-fat-high-fructose diet. Skeletal muscle mitochondrial energetic efficiency and uncoupling protein 3 content were significantly higher, while adenine nucleotide translocase content was significantly lower, in rats fed high-fat or high-fat-high-fructose diet. Conclusions: The results suggest that a high-fat-high-fructose diet even without hyperphagia is able to increase lipid flow to skeletal muscle and mitochondrial energetic efficiency, with two detrimental effects: (a) energy sparing that contributes to the early onset of obesity and (b) reduced oxidation of fatty acids and lipid accumulation in skeletal muscle, which could generate insulin resistance. [ABSTRACT FROM AUTHOR]