In this study, 14 subjects with type 1 diabetes are studied in a randomized crossover study in which the subjects cycle at a fixed intensity at 60% of their maximum oxygen capacity (VO2 max) for 1 hour. Thirty minutes before each cycling test, participants consume a 200 ml beverage, consisting of either: 1) dextrose (20 g), 2) galactose (20 g), 3) lactose (20 g) or 4) water (sweetened). If blood sugar drops below 3.9 mmol/l, glucose infusion is given and blood sugar is kept just above 5 mmol/L.

The trial days take place at least 4 days apart.

On study days, a catheter is placed in two antecubital veins for regular blood sampling and for glucose infusion (if needed). The participant will rest in a bed, when not performing the 1-hour bicycle ergometer test.

Thirty minutes before the leg-cycle ergometer test, the participants consume a 300 ml beverage containing either: 1) dextrose (20 g), 2) galactose (20 g) 3) lactose (20 g) or 4) water (sweetened). At t=0, the ergometer test is started and continued for 1 hour (fixed intensity cycling at 60% of VO2-max). If blood glucose drops below 3.9 mmol/l, glucose infusion (5 % glucose) will be administered, and blood glucose is kept just above 5 mmol/l and infused volume is registered.

Blood samples are collected at t= -40, 0, 30, 60, 90, 120 for measurement of plasma lactate, non-esterified fatty acids (NEFA), plasma triglyceride and hormonal concentrations (epinephrine, norepinephrine, cortisol, and glucagon). Plasma blood glucose measurements are performed at t = -40 and every 10-15 minutes until the end of the study day.

Respiratory exchange ratio is measured by indirect calorimetry (collection of respiratory gasses VO2 and VCO2) pre-exercise, at the end of the exercise bout, and 30 minutes post-exercise. Protein oxidation rate is estimated from urinary urea excretion, and net lipid and glucose oxidation rates are calculated from indirect calorimetry measurements with correction for protein oxidation. Therefor a urine sample will also be collected at the end of the study day.

The bicycle ergometer test is discontinued after 1 hour of bicycling and the participant is observed until 2 hour post exercise where the study day is ended.

Inclusion Criteria:

• type 1 diabetes for at least 2 years
• age between 18 and 65 years
• HbA1c less than 70 mmol/mol
• Physically active (at least 150 minutes of moderate-to-vigorous intensity activity weekly)
• Insulin pen and pump treatment
• Men and women are eligible, but women will have to perform study days in the mid-cycle phase of the menstrual cycle.

Exclusion Criteria:

• - Diabetic nephropathy, neuropathy, and proliferative retinopathy
• Hypoglycemia unawareness
• Lactose intolerance
• Heart disease and other conditions which may be negatively affected by the VO2 max test and 1-hour cycling on study days

The participants will receive either dextrose, water, lactose, galactose in a randomized order (computer generated block randomization) prior to a one-hour exercise bout. Participants, but not investigators, are masked to nutrition allocation.