A randomised, crossover, counterbalanced repeated measures study will be conducted to examine the effect of acute heat exposure on rate of change in blood glucose concentration during 40 minutes of moderate-intensity exercise and for 30 minutes after exercise. Participants will complete two experimental conditions during two separate laboratory visits, with the order of conditions randomised. One condition will be a temperate condition of 20°C with 50% relative humidity (Temperate), the other will be 40°C with 50% relative humidity (Heat). Visits will be identical, including time of day, except for the condition. Visits will be separated by 24h. Testing will take place in the laboratories of the Research Institute for Sport and Exercise Sciences at Liverpool John Moores University.

Inclusion Criteria:

• T1D diagnosis more than 1 years ago (to ensure participants are out of the honeymoon period)

• Using insulin therapy with multiple daily injections (MDI), insulin pump (pump), or hybrid closed-loop (HCL)

  o If using a HCL system have experience of using a pre-programmed basal rate in manual mode during exercise

• Aged 18-65 years

• Regularly engaged in endurance exercise (training at least twice per week for ≥30 minutes per session)

Exclusion Criteria:

• Pregnancy
• <6 months postpartum or stopped breastfeeding <1 month before recruitment
• Existing cerebrovascular or cardiovascular disease
• Significant history of hyperglycaemia (HbA1c >85 mmol/mol)
• History of severe hypoglycaemia requiring third party assistance within the last 3 months
• Body weight <36.5kg
• Difficulty swallowing tablets
• Appointment for an MRI test that cannot be rescheduled
• Obstructive gastrointestinal disease
• History of gastrointestinal surgery
• Have an implanted medical device

On request to PI