REF-1743 · García-Ortiz et al., 2026 · International Journal of Sports Physiology and Performance, · Paper · PubMed

In 15 physically active, naturally menstruating women riding 1 h at their maximal-fat-oxidation intensity, cycle phase did not change total fat or carbohydrate oxidised or energy spent, but heart rate was higher around ovulation than in the early follicular phase and RPE was higher in the mid-luteal phase than in the other two (one small crossover, read from its abstract; substrate use is a marker, and no magnitudes are given)

Main caveat: One crossover of 15 active women; the abstract gives P values only, so the size of any phase difference, and the uncertainty around the nulls, are unknown.
limited, mixed evidenceMeasured on recreational peopleRead from the abstract only

García-Ortiz and colleagues (Del Coso group) asked whether the menstrual cycle changes fat use during an hour of cycling at the intensity of maximal fat oxidation, and, second, how menstrual symptoms vary across the cycle.

Fifteen physically active, naturally menstruating women (age 25 ± 4 years, VO2max 38.15 ± 7.02 mL/kg/min) rode 1 h at Fatmax in the early follicular, peri-ovulatory and mid-luteal phases, in random order, with each phase identified individually by an app, urinary luteinising-hormone tests and daily basal temperature and body mass.

Phase had no main effect on total fat oxidised (P = .388), total carbohydrate oxidised (P = .265) or total energy spent (P = .839). Mean heart rate was higher in the peri-ovulatory than the early follicular phase (P = .004), and mid-luteal RPE exceeded both early follicular (P = .003) and peri-ovulatory RPE (P = .024).

Symptom scores were highest in the early follicular phase (P < .001). The authors conclude that fat oxidation at Fatmax did not depend on phase, while heart rate and perceived exertion may.

Participants

n = 15 physically active, naturally menstruating women; age 25 (4) years; VO2max 38.15 (7.02) mL/kg/min.
Training volume, sport, cycle length and diet: not in the abstract.

Protocol

Phases: early follicular, peri-ovulatory, mid-luteal, in randomised order; identified individually with a mobile app, urinary luteinising-hormone tests, and daily basal body temperature and body mass. No blood hormone measurement is described.
Exercise: 1 h of cycling at the intensity of maximal fat oxidation (Fatmax).
Measures: breath-by-breath gas exchange to estimate fat and carbohydrate oxidation; heart rate; RPE on the Borg 6-20 scale; menstrual symptom scores.
Analysis: linear mixed-effects models.
Not in the abstract: how Fatmax was determined, pre-test diet and fasting, time of day, the symptom questionnaire, and the size of any difference.

Key Results

OutcomeResult across phasesP
Total fat oxidisedno main effect of phase.388
Total carbohydrate oxidisedno main effect of phase.265
Total energy expenditureno main effect of phase.839
Mean heart ratehigher peri-ovulatory than early follicular.004
RPE (Borg 6-20)higher mid-luteal than early follicular; higher mid-luteal than peri-ovulatory.003; .024
Menstrual symptom scorehigher early follicular than peri-ovulatory and mid-luteal< .001
No means, differences or intervals are given in the abstract for any outcome.

Practical Application

Cycle phase did not change how much fat or carbohydrate active women burned in an hour at Fatmax, so there is no reason from this study to time easy "fat-burning" rides by phase; the same ride did feel harder in the mid-luteal phase and ran a higher heart rate around ovulation. One small crossover read from its abstract; the sizes of the RPE and heart-rate differences are not given.
For [SYNTH-39]: the mid-luteal phase is one more ordinary reason for a session's RPE to sit above its power-based score, and the peri-ovulatory phase a reason for heart rate to run higher at the same power. The guide's list of causes names cycle phase as a candidate beside sleep and stress, citing it; this study measured in-session Borg RPE, not session RPE against a training-stress score.
Against [SYNTH-37]: consistent with "the phase is not the lever" for substrate use and for planning. The guide notes that exercise heart rate did not differ by phase in pooled heat studies (REF-709); here (laboratory temperature not reported in the abstract) mean heart rate was higher peri-ovulatory than early follicular. One small study, with no magnitude given, against a pooled heat analysis.
Inference, not tested here: when a naturally cycling athlete reports a steady ride as harder than usual in the second half of her cycle, log it with the phase rather than reading it as lost fitness; her own record over several cycles (SYNTH-37) is what shows whether the pattern is hers.

Limitations

One crossover of 15 active women; the abstract gives P values only, so the size of any phase difference, and the uncertainty around the nulls, are unknown.
Substrate oxidation estimated from gas exchange is a marker of fuel use, not a performance or body-composition outcome.
Phases were identified with app tracking, urinary luteinising-hormone tests and basal temperature; blood oestrogen and progesterone are not described in the abstract, so whether every mid-luteal test had a raised progesterone is not known.
Fatmax determination, diet and fasting before each test are not in the abstract.
Written from the abstract and the PubMed record; full text not read.
How this entry was checked and changed
Why this level

a crossover of 15 (the rubric's B+ band) taken one step down because its main outcome, substrate oxidation, is a marker, and held at B- because it is read from the abstract only, which gives P values but no magnitudes for any outcome