To clarify the role of fast-acting insulin aspart (Fiasp) in insulin pump-treated type 1 diabetes more research is needed.

The aim of this study is twofold:

1. to compare the effects of Fiasp and Iasp in adults with type 1 diabetes who are using insulin pump and CGM and who are attending a diabetes out-patient clinic with extensive expertise in insulin pump and CGM therapy.
2. to determine differences in insulin pump settings when insulin pumps are optimally adjusted to each of the two insulin types.

Inclusion Criteria:

• Type 1 diabetes for ≥ 5 years
• HbA1c 53-75 mmol/mol (7.0-9.0%)
• Insulin pump treatment for ≥ 6 months (all insulin pump makes except hybrid closed-loop systems are eligible for inclusion)
• CGM use for ≥ 6 months (all CGM makes are eligible for inclusion)
• Carbohydrate counting for all snacks and meals
• Use of the insulin pump bolus calculator for all meals and snacks

Exclusion Criteria:

• Breast-feeding, pregnant, planning to become pregnant or of child-bearing potential and not using adequate contraceptive methods
• Gastroparesis (clinical assessment)
• Shift work
• Changing insulin needs throughout the menstrual cycle that requires different basal rate patterns
• Use of a hybrid closed-loop system
• Use of flash glucose monitoring
• Use of anti-diabetic medicine (other than insulin), corticosteroids or other drugs affecting glucose metabolism during the study period or within 30 days prior to study start
• Chronic paracetamol use
• Alcohol or drug abuse
• Severe cardiac disease or retinopathy contraindicating HbA1c below 53 mmol/mol
• Impaired renal function (eGFR< 60 ml/min/1.73 m2)
• History of local skin reactions to Fiasp and/or Iasp
• Other concomitant medical or psychological condition that according to the investigator's assessment makes the patient unsuitable for study participation
• Lack of compliance with key study procedures at the discretion of the investigator
• Unacceptable adverse events at the discretion of the investigator
• Less than 40 weeks guarantee remaining on insulin pump

Group allocation is concealed to participants as well as investigators until all participants have completed the study. The allocation for a participant may be revealed in a medical emergency if knowledge about the treatment allocation would influence the treatment of the person.