Caffeine helped women's performance in every cycle phase

Caffeine is one of the most widely used performance aids in sport, but the bulk of the evidence underpinning that use comes from male or mixed-sex cohorts. Because oestrogen and progesterone fluctuate across the menstrual cycle and are altered by hormonal contraceptives, there is a reasonable biological argument that caffeine's effects might not be constant in women. That has fuelled a growing market for cycle-phase-specific supplement timing. This review asked whether the actual randomised trial evidence supports tailoring caffeine to hormonal status at all.

Six databases were searched for randomised controlled trials of acute caffeine ingestion with objective exercise performance outcomes in women. Methodological quality and risk of bias were assessed using a modified PEDro scale and RoB 2. Because individual studies contributed several related outcomes, the authors used three-level meta-analyses to account for dependent effect sizes. The primary moderator was reproductive-hormonal status, classified as early follicular, late follicular or peri-ovulatory, luteal or mid-luteal, or hormonal and oral contraceptive use. Exploratory analyses examined caffeine dose, exercise type, method of phase verification, timing of ingestion and habitual caffeine intake. Twenty studies contributed 144 primary effect sizes.

Acute caffeine improved exercise performance overall in women, with a Hedges' g of 0.37 (95% CI 0.24 to 0.50) — a small effect — accompanied by moderate heterogeneity and, notably, a prediction interval crossing the null. Positive estimates appeared across all reproductive-hormonal strata, but there was no clear evidence of moderation between strata. The overall result held up when outliers were excluded and under alternative correlation assumptions. Exploratory work suggested exercise-task type and phase-verification method might partly explain variation in results, whereas dose, timing and habitual caffeine intake did not support phase-specific prescriptions. Risk-of-bias concerns, sparse subgroup data and low-to-very-low GRADE certainty limited any individualised inference.

This extends caffeine's ergogenic evidence base into female-specific analysis rather than assuming male data transfers. It pushes back against the increasingly commercialised idea that supplement timing should be keyed to cycle phase, since no between-stratum moderation was detected. Two cautions temper that: a prediction interval crossing the null means some individuals or contexts may see no benefit, and inconsistent phase verification across studies could be masking real biological patterns. The authors argue for future female-specific trials with rigorous hormonal verification, detailed contraceptive profiling, prespecified exercise phenotypes and mechanistic measurement.

The takeaway is that caffeine's average performance effect in women appears small but real, and current evidence does not justify treating menstrual phase or contraceptive use as a reason to change the approach. The prediction interval crossing the null is a reminder that averages hide individual variation, so not everyone responds. Because certainty was rated low to very low, all of this could be revised by better trials. If you encounter marketing that promises cycle-synced caffeine protocols, this review is a reason to think that claim is running ahead of the data.