REF-730 · Lei, Cotter, Schlader, Stannard, Perry, Barnes & Mündel, 2019 · The Journal of Physiology, 2019;597 · Paper · PubMed
Chronic pill use impairs the sweating response in the heat
The sudomotor threshold and thermosensitivity are both blunted versus naturally cycling athletes, so an athlete on the pill is not simply "a cycle that got standardised"
Protocol
n = 10 well-trained women taking a combined monophasic oral contraceptive, compared with matched eumenorrhoeic (naturally cycling) athletes. Two environments matched at 27°C WBGT — dry versus humid; two hormonal states (quasi-follicular and quasi-luteal); rest, fixed-intensity cycling at 125 and 150 W, and a self-paced 30-min effort. The authors note the scale of the question: one in two female athletes chronically takes a combined monophasic pill.
Key Findings
Chronic OCP use impaired the sweating onset threshold and sweating thermosensitivity versus matched eumenorrhoeic athletes (both p < 0.01). The sudomotor response is genuinely altered.
Behavioural thermoregulation was NOT impaired — the athletes adjusted their own work rate to compensate. The authors describe a performance-thermoregulatory trade-off requiring behavioural adjustment.
Rectal temperature was 0.15°C higher in the quasi-luteal than quasi-follicular state (p = 0.05) — the same direction as the natural-cycle offset, much smaller than it.
Humidity impaired performance by 5 ± 7% despite matched WBGT — the same finding as REF-711 in naturally cycling women, and worth more attention than the hormonal variable.
Performance did not differ between the two hormonal states.
Practical Application
In 10 well-trained women (V̇O₂max 57 ± 7) on a combined monophasic pill, chronic OCP use impaired the sweating onset threshold and thermosensitivity versus matched naturally cycling athletes (both p < 0.01), while behavioural thermoregulation was maintained and humidity cost 5 ± 7% of 30-min work at matched 27°C WBGT. An athlete on the pill is a different thermal athlete, not a "standardised" one.The common framing that the pill removes cycle variability is true for the hormone swings and false for the sweating response — hers is measurably blunted relative to a naturally cycling athlete.
Expect her to self-pace down in the heat rather than overheat, because behaviour compensated for the impaired sweating. A slower hot session from an athlete on the pill may be correct regulation, not a bad day.
Do not extrapolate [REF-709] to [REF-712] to contraceptive users. Those studies excluded them by design; this entry is the reason that exclusion matters.
Humidity remains the bigger lever, exactly as in naturally cycling women (REF-711) — check dew point before hormones.
Limitations
n = 10, combined monophasic pills only, well-trained women. Small, and one class of drug among many (REF-574).
"Quasi-follicular" and "quasi-luteal" in pill users are pharmacological states, not menstrual phases — the comparison with naturally cycling athletes is between-group, with the confounding that implies.
The impaired sudomotor response is a physiological measurement, not a demonstrated performance cost — performance did not differ by hormonal state here, and behaviour compensated.
One WBGT (27°C) and one exercise model; hotter conditions, longer durations and unacclimated versus acclimated states are untested.
No injury, heat-illness or long-term outcome; nothing about whether heat acclimation closes the sweating gap.
How this entry was checked and changed
Why this level
regraded 2026-09-27 from B+: within-subject crossover of n=10 plus a between-group comparison with matched eumenorrhoeic athletes (rubric: crossover <15 = B)
Content correction 2026-09-27
Level B+ -> B: a crossover of n=10 (four trials per athlete) with a between-group comparison; the rubric puts a controlled crossover under 15 at B. All numbers re-checked against the abstract and unchanged.
"one combined monophasic formulation" -> the abstract says a combined monophasic pill taken for at least 12 months, not a single named formulation.
Sport line: "endurance" is not a discipline; the study was run on a cycle ergometer.
Lead bullet now states the finding with its numbers and population before the interpretation.