Metabolically healthy skeletal muscle possesses the ability to switch easily between glucose and fat oxidation in response to homeostatic signals. In type 2 diabetes mellitus and obesity, the skeletal muscle shows a great reduction in this metabolic flexibility. A substrate like dodecanedioic acid (C12), able to increase skeletal muscle glycogen stores via succinyl-CoA formation, might both postpone the fatigue and increase fatty acid utilization, since it does not affect insulin secretion. In healthy volunteers and in type 2 diabetic subjects, the effect of an oral C12 load was compared to a glucose or water load during prolonged, moderate-intensity, physical exercise. C12 metabolism was analysed by a mathematical model. After C12, diabetics were able to complete the 2 hrs exercise. Non-esterified fatty-acids increased both during and after the exercise in the C12 session. C12 oxidation provided 14% of total energy expenditure and the sum of C12 plus lipids oxidized after the C12 meal was significantly greater than lipids oxidized after the glucose meal (P<0.025). The fraction of C12 which entered the central compartment was 47% of that ingested. During the first phase of the exercise (~60 min), the mean C12 clearance from the central compartment towards tissues was 2.57 l/min and 1.30 l/min during the second phase of the exercise. In conclusion, C12 seems to be a suitable energy substrate during exercise, since it reduces muscle fatigue, is rapidly oxidized and does not stimulate insulin secretion, which implies that lipolysis is not inhibited as reported after glucose ingestion.