Prolonged fasting identifies skeletal muscle mitochondrial dysfunction as consequence rather than cause of human insulin resistance.

AbstractObjective: Type 2 diabetes and insulin resistance have been associated with mitochondrial dysfunction, but it is debated whether this is a primary factor in the pathogenesis of the disease. To test the concept that mitochondrial dysfunction is secondary to the development of insulin resistance we employed the unique model of prolonged fasting in humans. Prolonged fasting is a physiological condition in which muscular insulin resistance develops in the presence of increased free fatty acid (FFA) levels, increased fat oxidation and low glucose and insulin levels. It is therefore anticipated that skeletal muscle mitochondrial function is maintained to accommodate the increased fat oxidation unless factors secondary to insulin resistance exert negative effects on mitochondrial function. Research Design and Methods: Twelve healthy males fasted for 60h or received a control diet, while in a respiration chamber. Afterwards, insulin-sensitivity was assessed using a hyperinsulinemic-euglycemic clamp and mitochondrial function was quantified ex vivo in permeabilized muscle fibers using high-resolution respirometry. Results: Indeed, FFA levels were increased approximately 9-fold after sixty hours of fasting in healthy male subjects leading to elevated intramuscular lipid levels and a decreased muscular insulin sensitivity. Despite an increase in whole-body fat oxidation, we observed an overall reduction in both coupled state 3 respiration and maximally uncoupled respiration in permeabilized skeletal muscle fibers, which could not be explained by changes in mitochondrial density. Conclusions: These findings confirm that the insulin resistant state has secondary negative effects on mitochondrial function. Given the low insulin- and glucose levels after prolonged fasting, hyperglycemia and insulin action per se can be excluded as underlying mechanisms, pointing towards elevated plasma FFA and/or intramuscular fat accumulation as possible causes for the observed reduction in mitochondrial capacity.