Purpose: Individuals with type 1 diabetes mellitus (T1DM) are encouraged to consume CHO to prevent hypoglycemia during or after exercise. However, the research comparing specific types of CHO is limited. This study compared the alterations in metabolism and fuel oxidation in response to running after preexercise ingestion of isomaltulose or dextrose in T1DM. Methods: After preliminary testing, on two occasions, eight T1DM individuals consumed 75 g of either dextrose (DEX; GI = 96) or isomaltulose (ISO; GI = 32), 2 h before performing 45 min of treadmill running at 80% perpendicular to 1% (V) over dotO(2peak). Blood glucose (BG) was measured for 2 h before and 3 h after exercise. Cardiorespiratory parameters were collected at rest and during exercise. Data (mean +/- SEM) were analyzed using repeated-measures ANOVA. Results: There was a smaller increase in BG in the preexercise period under ISO with peak BG occurring at 120 min after ingestion compared with 90 min under DEX (Delta+4.5 +/- 0.4 vs Delta+9.1 +/- 0.6 mmol.L(-1), P <0.01). During the final 10 min of exercise, there were lower CHO (ISO 2.85 perpendicular to 0.07 vs DEX 3.18 perpendicular to 0.08 g.min(-1), P <0.05) and greater lipid oxidation rates (ISO 0.33 perpendicular to 0.03 vs DEX 0.20 +/- 0.03 g.min(-1), P <0.05) under ISO. After exercise, ISO BG was lower than DEX for the entire 180-min period, with BG area under the curve and mean BG concentrations being 21% +/- 3% and 3.0 +/- 0.4 mmol.L(-1) lower, respectively (P <0.05). Conclusions: Consumption of ISO improves BG responses during and after exercise through reduced CHO and improved lipid oxidation during the later stages of exercise.