Effects of the Menstrual Cycle on Fat Utilization During 1 Hour of Cycling at Fatmax.

TL;DR

Fat oxidation during moderate-intensity cycling remains stable across menstrual cycle phases in healthy, active women , but perceived exertion and heart rate responses vary, suggesting the menstrual cycle may influence how exercise *feels* rather than metabolic fuel utilization.

What the Study Found

Researchers tracked 15 physically active women through three distinct menstrual cycle phases: early follicular (low hormone levels), peri-ovulatory (estrogen surge), and midluteal (progesterone dominant). Each woman completed a one-hour cycling session at "Fatmax" intensity, the workload where the body oxidizes the maximum amount of fat per minute. This intensity was individually determined for each participant using metabolic testing, which is the gold standard approach for this type of research.

The headline finding was clear: the amount of fat burned during exercise did not differ meaningfully across the three cycle phases. Total carbohydrate oxidation and total energy expenditure also showed no significant variation. This challenges a longstanding assumption in exercise physiology that the menstrual cycle substantially reshapes how the body uses fuel during moderate-intensity work. The measurement methods were rigorous, using breath-by-breath gas analysis to directly measure fat and carbohydrate oxidation in real time, rather than relying on estimates.

However, the study revealed important differences in how the exercise was experienced. During the peri-ovulatory phase (when estrogen peaks around ovulation), women showed a higher average heart rate compared to the early follicular phase. More notably, perceived exertion, measured on the 6-20 Borg scale, was significantly higher in the midluteal phase compared to both the early follicular and peri-ovulatory phases. This suggests that the same absolute workload feels harder during the luteal phase, even though the metabolic demand and fuel utilization remain constant.

Menstrual symptoms themselves followed an expected pattern: symptom scores were substantially higher during the early follicular phase (the transition into menstruation) compared to the follicular and ovulatory phases. This aligns with clinical observations and supports the study's phase identification methodology.

What This Means for You

If you menstruate and train regularly, this research suggests you don't need to fundamentally alter your fuel strategy or expect your fat-burning capacity to change across your cycle. For sessions targeting fat adaptation or aerobic base building, consistent training at your Fatmax intensity should yield similar metabolic outcomes regardless of cycle phase.

That said, the shift in perceived exertion and heart rate response is worth noting practically. If you track training load, expect your heart rate to run slightly higher during ovulation and your perception of effort to climb during the luteal phase, even at the same power output. This doesn't mean the training is less effective, but it's useful information for pacing subjective difficulty and managing fatigue. You might benefit from adjusting recovery strategies during the midluteal phase if perceived exertion is higher.

The study was limited to a small sample of young, physically active women with high aerobic fitness (VO2max around 38 mL/kg/min). Results may not generalize to sedentary populations, older women, those with irregular cycles, or women using hormonal contraceptives, which were not included. A larger sample would strengthen confidence in these findings.

Study Details

Sources

Menstrual cycle phase effects on fat oxidation during cycling. International Journal of Sports Physiology and Performance. PubMed: 41838469