This study corresponds to Task 8.1 of the project "Patient-tailored solutions for blood glucose control in type 1 diabetes- TAILOR" (PID2019-107722RB-C21).

It has been observed in different studies in healthy people that the glucose rate of appearance or disappearance during exercise is attenuated with the coincident rise in estrogen and progesterone during the mid-luteal phase of the menstrual cycle versus the early luteal phase.

The investigators hypothesize that in women with type 1 diabetes, glucose behavior when performing aerobic exercise may be different depending on the phase of the menstruation cycle. This analysis is necessary to improve physical exercise recommendations, both educational and technological, in women with type 1 diabetes, as well as improve the performance of artificial pancreas systems for automatic control of glucose levels under exercise in women.

This study will consist of two aerobic exercise sessions, one conducted at the beginning of the late follicular phase, where maximal estrogen concentration coincides with low progesterone levels, and the other session in the middle of the luteal phase, where peak values of estrogen and progesterone are generally observed.

Prior to the exercise sessions, an incremental exercise test will be performed to determine the working power in the cycle ergometer. At the beginning of the test, subjects will be sit quietly on the cycle ergometer for 3 min (0 W) before they will start the warm-up period of 3 min with cycling at a workload of 40 W. Then, the workload will increase by 20 W every minute until volitional exhaustion. Finally, 3 min active recovery at 40 W followed by 3 min passive recovery (0 W) will be conducted on the cycle ergometer. Lactate turn point 1 (LTP1) and maximum power (Pmax) will be determined in order to prescribe the exercise intensity.

The aerobic exercise session will begin with a 3 min resting period (sitting quietly on the cycle ergometer (0 W), followed by a 3 min warm-up at 40 W and a stepwise intensity increase by 20 W / min (to control for day-to-day variations in exercise response) until the 5% of Pmax from IET above PLTP1 is achieved (i.e. moderate steady state walking or low-intensity running or cycling). This target workload will be maintained for 30 min. Active and passive recovery periods of 3 min each will be the same as the incremental test.

To monitor the impact on blood glucose, blood draws will be performed at initial and every 10 minutes for aerobic sessions. A sample size of n=30 will be chosen.

Inclusion criteria:

• age between 18-45 years
• T1D with a diabetes duration for more than 2 years
• glycated hemoglobin (HbA1c) < 8.5% (<69 mmol mol-1)
• stable insulin regimen in the past 6 months with less than 20% change in total insulin daily dose
• multiple daily injections
• weekly physical activity of 90 min or more, but no practicing any sport as amateur or professional.

Exclusion Criteria:

-clinical conditions or use of medications (other than insulin) known to affect glycemic control (e.g., oral/parenteral steroids or metformin, among others).