Regular endurance training enhances fat oxidation during exercise at a given exercise load and decreases muscle glycogen utilisation. In addition to training status the daily dietary macronutrient intake as well as the consumption of carbohydrate containing beverages and/or food during exercise will influence fuel utilisation during exercise. After ultra endurance exercise where different exercise modalities are performed more or less continuously for several days fat oxidation during exercise is increased both at rest and during submaximal exercise (1). However, the increased fat oxidation may well be due to decreased glycogen stores and inability to maintain energy balance, both of which will increase fat utilization during exercise. To further elucidate the effect of excessive prolonged exercise on fat oxidation during exercise 14 days of very prolonged submaximal exercise was studied in 6 elderly male subjects. During a 14 day period the subjects performed approximately 10 hours of cycle exercise per day. During the 14 days the subjects consumed an ad lib high carbohydrate diet and were able to maintain body weight despite of the massive exercise load. Before and approx. 30-34 hours after completion of the last of the 14 days maximal fat oxidation was measured during a graded cycle exercise protocol in the overnight fasted condition. Interestingly a marked decrease in maximal fat oxidation and a decreased maximal oxygen uptake was observed after regular excessive prolonged exercise (Authors unpublished findings). The first part of this talk will focus on the result of this study and further discuss the possible mechanisms explaining this observation. In diabetes and obesity a common trait is the occurrence of excess fat accumulation in skeletal muscle and insulin resistance. Although muscle triacylglycerol is often correlated to insulin resistance, a causal link remains to be demonstrated. However, increased intracellular concentrations of long chain Acyl CoA are probably present when muscle triacylglycerol content is high, and this may well lead to increased concentrations of skeletal muscle bioactive lipid intermediates, such as ceramide and diacylglycerol (2). Ceramide and/or diacylglycerol can attenuate insulin signalling in muscle and may be the link between muscle triacylglycerol and skeletal muscle insulin resistance. In skeletal muscle ceramides are generated primarily through de novo synthesis from palmitate and serine or through breakdown of sphingomyelin in the cell membranes (2). In the second part of this talk focus will be on the muscle ceramide content in man and the available evidence for a role in human skeletal muscle insulin resistance. In particular focus will be on the role of manipulation of plasma fatty acid content achieved either by lipid heparin infusion during a euglycemic hyperinsulinemic clamp or by prolonged intake of fat rich diet. Furthermore the focus will be on the effect of training and/or inactivity on muscle ceramide content studied with a longitudinal approach.