REF-1685 · Britton & Duncan, 2026 · Reproduction & Fertility, · Paper · PubMed · Free full text

VO2max did not differ significantly between the early follicular and mid-luteal phases (Cohen's d −0.07, 95% CI −0.45 to 0.30; I² 0%) in a meta-analysis of five small studies of trained, naturally cycling women whose phase was confirmed by blood hormones (52 women by our sum); the review assessed no risk of bias, and the title's "elite" is not shown by competition data. Read in full

Main caveat: The authors': very few studies met the hormone-confirmation rule, so power was low and small effects may have been missed.
limited evidenceMeasured on trained to highly trained athletesRead in full

Britton and Duncan (University of Edinburgh) asked whether the phase of the menstrual cycle changes VO2max in trained female athletes, and kept only studies that confirmed the phase with blood hormone measurements, which they call the gold standard.

Searching PubMed and Scopus to February 2025, they found five eligible studies (1990 to 2023) with 8, 16, 5, 13 and 10 women, all naturally cycling and off hormonal contraception for at least six months; four tested VO2max on a treadmill (three in runners, one in a mixed-sport sample) and one estimated it with a shuttle-run test.

Pooled, VO2max did not differ between the early follicular phase (menstruation, low oestrogen and progesterone) and the mid-luteal phase (Cohen's d −0.07, 95% CI −0.45 to 0.30), with no heterogeneity (I² 0%, heterogeneity P = 0.7536); the negative sign is a small, non-significant lower value in the early follicular phase.

The authors argue that both phases may be poor for performance for different reasons, that VO2max barely moves week to week and may be the wrong outcome, and that athletes' own reports of cycle effects may reflect symptoms and perceived effort that a VO2max test does not capture.

They propose a standardised research protocol covering more sub-phases, hormone confirmation and sport-specific outcomes.

Participants

Five studies, 52 women by our sum: De Souza 1990 (8), Lebrun 1995 (16), Thiros and Van Guilder 2022 (5), Recacha-Ponce 2023 (13), Goldsmith and Glaister 2020 (10).
Training status (from each study's inclusion rule): VO2max of at least 50 mL/kg/min (Lebrun); at least 35 or 30 miles a week of running for at least a year (De Souza; Thiros and Van Guilder); McKay levels II and III (Recacha-Ponce); training at least three times a week for over a year or a 10-km best under 50 min (Goldsmith and Glaister). The authors call these elite or well-trained; no study reports international competition.
Sports: runners in three studies (De Souza, Thiros and Van Guilder, Goldsmith and Glaister); Lebrun's sample mixed running, cycling, rowing and cross-country skiing; Recacha-Ponce's sport is not stated in the text.
Age: study mean ages 23.8 ± 6.6 to 32.0 ± 6.0 years.
Reproductive status: eumenorrhoeic and no hormonal contraception (pill, patch, hormonal IUD) for at least 6 months before the study.

Protocol

Search: PubMed and Scopus with MeSH terms for menstrual cycle and phases combined with performance terms (endurance, strength, VO2max, power output, time trial, laxity); filters English, adult, female, no reviews; published before the end of February 2025; reference lists hand-searched.
Phase confirmation: blood oestradiol and progesterone in all five, drawn before testing and analysed afterwards; cycle tracking, basal temperature or urinary LH tests set the timing in some, and mid-luteal confirmation used a progesterone threshold in some (for example above 7 ng/mL, or above 16 nmol/L). Four studies tested within one cycle, one across two.
VO2max tests: treadmill with rising speed and gradient (two studies), rising speed only (two), or an estimate from the Course Navette shuttle test (one); termination criteria differed between studies; three studies asked for no hard exercise for 24 h before testing, one monitored training load and one did not report controlling it.
Analysis: random-effects meta-analysis of Cohen's d between early follicular and mid-luteal VO2max, with the SD pooled from the two phases as if they were independent groups.
Not done: risk-of-bias assessment, certainty grading, publication-bias testing, sensitivity analysis, registration.
Funding and interests: no specific funding; the second author is an associate editor of the journal and was not involved in its review; the first author is an elite athlete.

Key Results

ComparisonEffect (95% CI)Heterogeneity
VO2max, early follicular vs mid-luteal (5 studies)Cohen's d −0.07 (−0.45 to 0.30), not significantI² 0.0%, P = 0.7536
Negative values mean lower VO2max in the early follicular phase; the per-study effects are in a forest plot not reproduced in the text.
Late follicular VO2max was measured in two studies (Recacha-Ponce, Goldsmith and Glaister) but not pooled.
The authors' wider reading: their result fits an earlier meta-analysis that found only trivial reductions in the early follicular phase compared with all other phases (REF-335).

Practical Application

Do not move a VO2max test or plan aerobic capacity around cycle phase: in trained, naturally cycling women whose phase was confirmed by blood hormones, VO2max did not differ between menstruation and the mid-luteal phase (d −0.07, CI −0.45 to 0.30). The interval is wide (five studies, 52 women), so a small effect is not excluded, and the review assessed no risk of bias.
This agrees with [SYNTH-37]: the phase is not the lever at group level. The guide's default, ask about symptoms and act on them, is what the authors point to as well: they suggest symptoms and perceived effort may explain why athletes report cycle effects that VO2max tests do not show (their reasoning, not tested here).
Inference, not tested here: for an athlete who repeats lab or field tests to track fitness, noting cycle day and symptoms beside each test helps read an odd result, but the phase alone is not a reason to discard or reschedule a VO2max test.
Not settled by this design: time-trial or race performance across the cycle, the late follicular or late luteal phases, individual responders, and women on hormonal contraception (REF-338 covers the pill).

Limitations

The authors': very few studies met the hormone-confirmation rule, so power was low and small effects may have been missed; VO2max protocols and stopping criteria differed and one study estimated VO2max from a shuttle test; VO2max is a capacity measure that changes little over weeks and is not sport performance; participants were young adults (means 23.8-32.0 years); training load before testing was not controlled in one study; only two phases were compared, and both may be unfavourable for different reasons.
Ours: no risk-of-bias or certainty assessment, no publication-bias test and no protocol registration; two databases and English only; the effect size treats the two phases as independent groups although the same women were tested in both, which, if the variances were computed the same way, would likely make the interval wider than a paired analysis (our reading of the formula given; the variance formula is not stated); the title and abstract say "elite", but the samples are trained to highly trained by their inclusion rules and no study reports international competition.
Not read: the PRISMA flow numbers and the forest plot's per-study values, which are given only as figures.
How this entry was checked and changed
Why this level

a systematic review that assessed no risk of bias cannot reach A- (rule 1), so it is capped at B+, and held at B because it pools five within-person studies of 5 to 16 women from two databases, one of which estimated VO2max from a shuttle-run test, with an effect size that ignores the pairing of phases in the same women (our reading)