Explainer · Heart & Metabolic
Insulin needs rose in the second half of the menstrual cycle, in 380 cycles from people with type 1 diabetes
Automated insulin systems adjust doses around the clock, but they do not know about the menstrual cycle. Data donated by 77 users showed insulin needs and blood sugar rising after ovulation.
- An observational study of 77 adults with type 1 diabetes who used automated insulin systems and had regular periods.
- They donated glucose, insulin, carbohydrate and cycle data covering 380 menstrual cycles.
- Insulin doses, carbohydrate intake and average glucose all peaked in the second half of the cycle.
- Modeling suggested the body responded to insulin about 5% less well after ovulation than just after a period.
- Most followed the pattern, but not all, and volunteers may have had better control than average.
People with type 1 diabetes spend their days balancing insulin against food, exercise, stress and sleep. Automated insulin systems have taken over much of that work, adjusting doses around the clock. But hormones rise and fall across the menstrual cycle, and the systems are not built to account for that.
A study in the journal Diabetes Care used data donated by 77 users of automated insulin systems to track what happens across the menstrual cycle. Insulin needs and blood sugar rose in the second half of the cycle, after ovulation, and the body seemed to respond to insulin a little less well.
What type 1 diabetes and insulin do
MedlinePlus explains that with type 1 diabetes, your body makes little or no insulin. Insulin, it notes, is a hormone from your pancreas that helps glucose move from your body into your cells, where it is used for energy. People with the condition need insulin every day, through injections or a pump.
How much insulin someone needs depends on insulin sensitivity, meaning how strongly the body responds to each unit. The authors note that hormonal fluctuations across the menstrual cycle may alter insulin sensitivity and glycemia, or blood sugar levels, even with modern automated systems.
How the menstrual cycle study worked
Participants signed up online and shared anonymized data from their devices. The researchers collected continuous glucose monitoring (CGM), insulin delivery, carbohydrate intake, and menstrual cycle data, all from adults who reported regular cycles and used an automated insulin system. A continuous glucose monitor is a small sensor worn on the skin that reads glucose every few minutes.
In total, seventy-seven individuals contributed 380 menstrual cycles. The authors say it is the largest number of menstrual cycles analyzed to date for this question. They compared the phases of the cycle: the follicular phase, from the start of a period to ovulation, and the luteal phase, from ovulation to the next period. A statistical model then estimated how this responsiveness changed from phase to phase.
What happened to insulin and blood sugar
The pattern was consistent. Total daily insulin dose and carbohydrate intake peaked during the luteal phase, with higher average glucose and less time in range, meaning less of the day spent in the target glucose band.
The model put numbers on it. Insulin sensitivity was estimated to be about 2.6% above average just after a period and about 2.6% below average in the middle of the luteal phase, a swing of roughly 5%. Most participants followed this pattern, about 85%, but the authors saw substantial differences from one person to another.
Why cycle-aware insulin systems may matter
The study measures a pattern that current tools do not plan for. The authors conclude that sex-specific physiology is an important factor in glycemic regulation that is not yet addressed by current guidelines or by automated insulin systems.
In practice, that could mean systems that learn a user’s cycle and adjust in advance, or clinical advice that helps people plan for predictable changes. Because the pattern differed between individuals, any such adjustment would likely need to be tailored to each user.
What donated insulin-pump data cannot show
The authors list several limits. Recruiting through online diabetes communities may have drawn highly engaged people with fairly good control, so the findings may not reflect everyone. Illness, sleep, stress, alcohol, medicines and exercise were not measured, and cycle dates were self-reported, with ovulation estimated rather than confirmed.
Some participants used hormonal contraception that changes ovulation, which may have blurred the phases. Insulin sensitivity came from a model rather than a direct measurement. Users who already adjust their settings around their cycle could also have hidden part of the effect, so the true size may be larger than reported. The results may not apply to people with irregular cycles or those not using these systems.
What this changes for people with type 1 diabetes
For people who menstruate and live with type 1 diabetes, the study confirms that rising insulin needs in the second half of the cycle are common. Tracking cycle dates alongside glucose data can help spot a personal pattern.
Any change to insulin settings should be made with a diabetes team. For device makers and guideline writers, the study is a case for building the menstrual cycle into how insulin is dosed.
People also ask
What did the study find?
Total daily insulin dose and carbohydrate intake peaked during the luteal phase, with higher mean glucose and reduced time in range. Model-derived estimates indicated insulin sensitivity was 2.6% higher than average in the early follicular phase and 2.6% lower in the midluteal phase. About 84.7% of participants showed patterns consistent with this trend.
What are the phases of the menstrual cycle?
The follicular phase runs from the start of a period to ovulation, and the luteal phase runs from ovulation to the next period. Hormone levels, including estrogen and progesterone, shift across these phases.
What is an automated insulin delivery system?
A setup that links a continuous glucose monitor to an insulin pump, with software that adjusts insulin doses automatically based on glucose readings. It is sometimes called a hybrid closed-loop system.
What is time in range?
The share of the day that blood glucose stays within a target range. More time in range generally means better day-to-day glucose control.
Why would insulin needs change across the cycle?
The authors point to hormonal fluctuations across the cycle, which may alter how sensitive the body is to insulin. The study did not measure hormones directly.
Should I change my insulin settings around my cycle?
Not without talking to your diabetes team. The pattern varied between people, and tracking your own cycle and glucose data with your clinician is the safest way to decide. This is general information rather than medical advice.