In clinical practice, women living with type 1 diabetes frequently report that insulin requirements change across the menstrual cycle. Consequently, glycemic fluctuations are observed. This phenomenon could be explained by a decrease in insulin sensitivity during the second half of the menstrual cycle (luteal phase).

Overall, despite an important proportion of women reporting glycemic and/or insulin variations across the menstrual cycle, studies to date have involved small sample sizes, and have had inconsistent results. The objective of this study will be to study glycemic fluctuations across the menstrual cycle using CGM data, alongside insulin data, in a large sample of women.

Inclusion Criteria:

1. Females aged 18 to 50 living in Canada.
2. Clinical diagnosis of type 1 diabetes or latent autoimmune diabetes in adults (LADA) for at least one year.
3. Using insulin pump therapy, multiple daily injections or automated insulin delivery systems for at least 3 months.
4. Using a continuous glucose monitoring (CGM) system.
5. Having at least one menses in the last 40 days.
6. Accepting to share CGM data with the research team and if applicable insulin pump data.This access will be limited to the study period.
7. Having a smartphone or tablet to follow menstrual cycles.
8. Stable weight (less than 5% variation in the last 3 months).

Exclusion Criteria:

1. Using a hormonal contraception method that eliminates menses (Depo Provera, progestin intrauterine device, extented-cycle regimen with birth control pill)
2. Using regular insulin (Entuzity U500, Novolin ge Toronto or Humulin R).
3. Clinically significant nephropathy (eGFR < 30 ml/min/1.73m2, planned or on dialysis) or neuropathy (e.g., known uncontrolled gastroparesis) as judged by the investigator.
4. Recent (< 6 months) acute macrovascular event (e.g., acute coronary syndrome or cardiac surgery).
5. Anticipated therapeutic change (including change of insulin type and/or type of CGM sensor, insulin pump or AID (automated insulin device) system) between admission and end of the study.
6. Anticipated change in contraception method or plan to begin or stop a contraceptive method.
7. Anticipated need to use acetaminophen during the study period at a dose above 1g every 6 hours.
8. Pregnancy (ongoing or current attempt to become pregnant)
9. Breastfeeding
10. Uncontrolled thyroid disease (TSH should be in target range and treatment stable for at least 6 weeks).
11. Severe hypoglycemic episode within two weeks of screening
12. Severe hyperglycemic episodes requiring hospitalization in the last 3 months.
13. Current use of glucocorticoid medication (except low stable dose and inhaled steroids and stable adrenal insufficiency treatment e.g., Cortef®)
14. Agents affecting gastric emptying (Motilium®, Victoza®, Ozempic®, Trulicity®, Byetta® and Symlin®) as well as oral anti-diabetic agents (Metformin, Prandase®, DPP-4 inhibitors) unless at a stable dose for 3 months and without anticipated change during the study.
15. Current use of SGLT-2 inhibitors unless at a stable dose for at least 3 months, without anticipated change during the study and appropriate ketone testing is performed.
16. Other serious medical illnesses which likely interfere with study participation or with the ability to complete the study by the judgment of the investigator.
17. Anticipation of a significant change in exercise or diet regimen between admission and end of the study (i.e., starting or stopping an organized sport; planned significant diet change).
18. Anticipated radiologic examination incompatible with CGM wear for more than 10 days between admission and end of the study (e.g., repeated MRI).
19. In the opinion of the investigator, a participant who is unable or unwilling to complete the study.
20. If taking medication for hypothyroidism, no change of dose (Levothyroxin) in the last 6 weeks.

Women aged 18 to 50 living with type 1 diabetes.