title: Prolonged fasting identifies skeletal muscle mitochondrial dysfunction as consequence rather than cause of human insulin resistance.
abstract: OBJECTIVE
  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 physiologic 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 increased fat oxidation unless factors secondary to insulin resistance exert negative effects on mitochondrial function.
  
  
  METHODS
  While in a respiration chamber, twelve healthy males were subjected to a 60 h fast and a 60 h normal fed condition in a randomized crossover design. Afterward, 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 ninefold after 60 h of fasting in healthy male subjects, leading to elevated intramuscular lipid levels and 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 toward elevated plasma FFA and/or intramuscular fat accumulation as possible causes for the observed reduction in mitochondrial capacity.
authors.affiliation: Department of Human Biology, School for Nutrition, Toxicology and Metabolism, Maastricht University Medical Centre, Maastricht, the Netherlands. j.hoeks@hb.unimaas.nl
authors.forename: Joris
authors.initials: J
authors.lastname: Hoeks
authors.forename: Noud A
authors.initials: NA
authors.lastname: van Herpen
authors.forename: Marco
authors.initials: M
authors.lastname: Mensink
authors.forename: Esther
authors.initials: E
authors.lastname: Moonen-Kornips
authors.forename: Denis
authors.initials: D
authors.lastname: van Beurden
authors.forename: Matthijs K C
authors.initials: MK
authors.lastname: Hesselink
authors.forename: Patrick
authors.initials: P
authors.lastname: Schrauwen
mesh_terms: D000328:Adult; D001835:Body Weight; D002950:Citrate (si)-Synthase / Q000235:genetics; D004272:DNA, Mitochondrial / Q000235:genetics; D003924:Diabetes Mellitus, Type 2 / Q000503:physiopathology; D005215:Fasting; D005230:Fatty Acids, Nonesterified / Q000097:blood; D015309:Glucose Clamp Technique; D006801:Humans; D006946:Hyperinsulinism / Q000097:blood; D007328:Insulin / Q000097:blood / Q000502:physiology; D007333:Insulin Resistance / Q000502:physiology*; D008055:Lipids / Q000097:blood; D008297:Male; D008928:Mitochondria / Q000502:physiology*; D018482:Muscle, Skeletal / Q000502:physiology / Q000503:physiopathology*; D012016:Reference Values; D014280:Triglycerides / Q000378:metabolism; D055815:Young Adult
chemical_list: D004272:DNA, Mitochondrial; D005230:Fatty Acids, Nonesterified; D007328:Insulin; D008055:Lipids; D014280:Triglycerides; D002950:Citrate (si)-Synthase