New research to be presented at the Annual Meeting of the European Association for the Study of Diabetes (EASD) in Milan, Italy (Sept 28th – Oct 2nd) shows, with the use of continuous glucose monitoring (CGM) devices people with type 1 diabetes can complete a marathon safely and achieve finishing times similar to runners without diabetes. The study is conducted by Michał Kulecki, Dr Andrzej Gawrecki, and colleagues at Poznan University of Medical Sciences and Raszeja City Hospital, Poznan, Poland.
Completing a marathon with type 1 diabetes (T1D) requires balancing energy expenditure, carbohydrate intake, insulin adjustment, and glucose monitoring. Evidence on blood sugar patterns and CGM accuracy during prolonged endurance exercise under real-life race conditions is limited. The researchers therefore assessed glucose levels, carbohydrate intake, and CGM accuracy during the 2025 Poznań Marathon, a standard 42 km race.
The observational study included 20 amateur runners: 10 with T1D of at least 1 year's duration and 10 controls without diabetes. For participants with T1D, the target pre-race glucose range was 140-200 mg/dL. Basal insulin dose was reduced by 25% in multiple daily injections and by 50% in non-hybrid insulin pumps, while participants using hybrid closed-loop systems set a target glucose of 150 mg/dL. Five participants used multiple daily injections (MDI), three used continuous subcutaneous insulin infusion (CSII), and two used automated insulin delivery (AID) systems. At five checkpoints (start, 10 km, 19 km, 30 km, and finish), glucose was measured with a glucometer and compared with two CGM systems, one intermittently scanned and one in real-time. The researchers administered carbohydrates or insulin as required. They evaluated CGM accuracy using a standard method for these devices called mean absolute relative difference (MARD), which represents the average absolute percentage difference between sensor glucose readings and reference glucose values.
Participants with and without T1D showed comparable baseline characteristics. They did not differ significantly in age (35.4 vs 39.7 years), and each group included 8 men and 2 women. In the T1D group, participants had a median diabetes duration of 16.5 years and a glycated haemoglobin (HbA1c) of 6.4%. Marathon performance did not differ significantly between groups, with completion times of 228 minutes in T1D and 248 minutes in controls.
During the race, participants with T1D consumed a median of 53.5 g of carbohydrate per hour, equivalent to 2.61 g/kg body weight over the marathon. Capillary glucose measured with the glucometer was 183.5 mg/dL at the start, 119.5 mg/dL at 10 km, 142.5 mg/dL at 19 km, 121.5 mg/dL at 30 km, and 108.5 mg/dL at the finish (median values).
Two hypoglycaemic measurements occurred in the same participant, who nevertheless completed the race. This participant consumed 49.5 g of carbohydrate per hour.
CGM accuracy was reduced during the marathon. CGM readings differed from glucometer measurements by an average of approximately 43% for the intermittently scanned system and 37% for the real-time system and overestimated capillary glucose by +32.2 and +50.4 mg/dL, respectively.
The authors say: "In this small observational study, all runners with type 1 diabetes completed the marathon, with performance comparable to controls. The runner who experienced low blood sugar had started the race with a glucose level below 140 mg/dL. During the marathon, CGM readings differed from glucometer measurements. For longer endurance events, runners should therefore consider checking their glucose with a glucometer, especially when the sensor reading does not match how they feel."
They add: "Fear of hypoglycaemia is the main barrier to physical activity and affects up to 45% of people with T1D despite the major health benefits of regular exercise. Managing glucose is challenging when glucose levels change rapidly and responses vary between individuals. Our study provides another example that, with appropriate education and careful blood sugar management, people with T1D can successfully take part in even very demanding endurance exercise. "
But they also explain: "People with T1D preparing for a marathon should discuss an individual glucose, carbohydrate, and hydration plan with their doctors before the event. The most important aspect is an appropriate insulin management strategy, including reductions in basal and/or prandial insulin. Of course, much depends on baseline blood sugar control, exercise experience, diabetes duration, the presence of diabetes-related complications, and many other individual factors. For example, some people should consult a cardiologist before starting endurance training. Our team supports many athletes with type 1 diabetes who have achieved remarkable goals, including competing at the Olympic Games, completing an Ironman triathlon or running 10 marathons in 10 consecutive days.
