Rationale: In healthy individuals ingestion of high molecular mass carbohydrates (HMM-CHO) empty from the stomach sooner, cause faster and greater increases in blood glucose and post-exercise muscle glycogen resynthesis and improve cycling performance compared with low molecular mass carbohydrates (LMM-CHO) (Leiper et al., 2000; Piehl Aulin et al., 2000, Stephens et al., 2008). Pre-exercise carbohydrate ingestion is an effective strategy for reducing the occurrence of hypoglycaemia during or after exercise in individuals with type 1 diabetes (T1DM) but the metabolic effects of ingestion of different carbohydrates in this group for glycaemic or performance gain has been under-researched. This study examined the metabolic and performance responses of pre-exercise ingestion of a low- and high-molecular mass CHO during running in T1DM. Methods: With ethical approval, seven participants (34.3±5.5 years, 69.9±2 kg, HbA1c 76.6±6.5 mmol/mol) attended the laboratory on two occasions after preliminary testing. On each visit participants consumed 0.6 g.kg-1 BM of either LMM (Dextrose, DEX, MyProtein®, Cend ltd, UK) or HMM (Waxy Barley Starch; WBS, Vitargo®, Swecarb, Sweden) as a 9.2±0.1% solution, with 50% reduced rapid-acting exogenous insulin dose 2-h before exercise. Blood samples were taken over 2-h rest. Participants then completed a discontinuous incremental treadmill run which consisted of five 4 min stages at 31±2%, 41±2%, 53±3%, 69±4%, 80±4% VO2peak interspersed with 90s recovery. After 5 minute recovery, participants performed a 10 min performance run on a non-motorised treadmill (Curve, Woodway, Germany). Capillary blood samples were analysed for glucose (BG). Rates of CHO and lipid oxidation were determined using principles of indirect calorimetry. Data were expressed as mean±SEM and analysed using repeated measures ANOVA (P<0.05). Results: Fasted BG reached similar peak values one hour after CHO ingestion. Immediate pre-exercise BG were comparable (WBS 15.9±1.7 vs. DEX 15.0±1.9 mM, P=0.47). Resting CHO oxidation was elevated under WBS (WBS 0.31±0.03 vs. DEX 0.19±0.04 g.min-1, P=0.024) and lipid oxidation lower in WBS (WBS 0.03±0.01 vs. DEX 0.06±0.01 g.min-1, P=0.017). BG increased similarly following performance running (WBS 0.9±0.2 vs. DEX 0.7±0.3 mM, P=0.45). There was a greater distance completed in the final quarter of the run in WBS (WBS 323±21 vs. DEX 301±20 m, P=0.02) equating to 0.5±0.2 km.h-1 greater speed during the last quarter of the test. Conclusions: The results demonstrate maintained glycaemia and improved run performance following ingestion of a high molecular mass carbohydrate in individuals with type 1 diabetes.