What the research found

A study of 209 naturally cycling women wearing continuous glucose monitors for up to two weeks found that glucose variability—not average glucose levels—fluctuates predictably across the menstrual cycle. Six measures of glucose fluctuation showed a consistent pattern: they were lowest around day 27–29 of the cycle (late follicular phase, just before ovulation) and peaked around day 23–25 (late luteal phase, just before menstruation). The swings in variability reached 6–10% above and below the cycle mean.

Notably, average glucose and overall glucose control (measured by the Glucose Management Indicator) remained stable throughout the cycle. However, how quickly blood glucose returned to baseline after meals shifted with cycle phase—meals eaten during the luteal phase showed faster glucose descent, particularly at breakfast. When researchers controlled for what women actually ate, these patterns persisted, suggesting hormonal rather than behavioural drivers. A matched male control group showed no cyclic pattern, confirming the effect was sex-specific.

Why it matters for you

If you're a premenopausal woman tracking glucose via CGM, your variability metrics will naturally oscillate across your cycle independent of your diet or training. This means a high standard deviation or coefficient of variation reading in your late luteal phase isn't necessarily a red flag—it reflects normal endocrine physiology. You might see tighter glucose control in the week after ovulation and looser control in the days before your period, even on identical meal compositions and activity levels.

For breakfast specifically, luteal-phase glucose peaks may be slightly higher and descend faster than follicular-phase responses to the same meal. This could influence how you interpret postprandial glucose targets or decide whether a particular meal timing or composition "works" for you. Tracking these patterns yourself over several cycles—noting both CGM metrics and where you are in your cycle—will show you whether this population trend matches your individual response.

The stability of average glucose is reassuring: cycle phase doesn't appear to degrade overall metabolic control in healthy women, so optimising other variables (sleep, training intensity, stress) likely remains the priority.

Caveats

• Cross-sectional observational design: Real-world eating and activity patterns weren't controlled, only measured and adjusted for statistically
• Moderate effect sizes: Peak-to-nadir amplitudes were 6–10% of mean glucose, clinically small but statistically reproducible
• Limited mechanistic insight: The study documents when variability changes, not why—hormonal mediators (oestrogen, progesterone, insulin sensitivity) were not directly measured
• Age and BMI specificity: Mean age ~43 years with low-normal BMI; generalisability to younger or higher-BMI populations unclear
• Single population: All participants from one research cohort; findings need independent replication