Should I time my fasting to the phase of my menstrual cycle?

Four gaps matter here, and missing any of them misrepresents someone. First, the subject: Sims withdrew a prescription about training by cycle phase, not about fasting by cycle phase. She has a separate and firm position against fasting for active women, but her dated withdrawal is not about fasting, and presenting it as though it were would misquote her.

Second, audience: Sims writes for women who train deliberately, and scopes her fasting objection to the intersection of training stress and low energy availability. Pelz writes for a general female reader, weighted toward midlife and perimenopause, with no training load assumed.

Third, cycle status: Pelz's framework depends on a predictable cycle, and her own answer for women without one is unsettled, having been an internal-signal rule in Nov 2025 and a lunar-calendar rule in May 2026. Fourth, the counterweight is narrower than it looks: the peer-reviewed case against prescribing by cycle phase (Colenso-Semple, D'Souza, Elliott-Sale and Phillips, Frontiers in Sports and Active Living, 2023, concluding that building resistance-training prescriptions on cyclical hormonal changes is not an evidence-based approach) is about training, not fasting, and no published statement on cycle-phase fasting from anyone else was found.

More than the headline suggests, and it should be shown. Both hold that fasting research built on men does not transfer to women unexamined. Both hold that a woman whose period has stopped from under-fuelling or stress should eat rather than fast (Sims on low energy availability and Pelz on amenorrhea agree almost word for word).

Both have publicly moved from a calendar rule toward a readiness rule, Sims on 14 Oct 2025 and Pelz on 21 May 2026, and neither cites the other. And both currently practise and endorse roughly the same window: Sims accepts 12 to 13 hours and calls it time-restricted eating rather than fasting, and Pelz reports her own settled window at 12 to 13 hours. The live disagreement is not about that window. It is about everything above it, and about whether the calendar or the body sets it.

A trial of cycle-phase-timed fasting against a fixed window in regularly cycling women, with cycle phase verified by hormone assay rather than by counting days, and endpoints on the things both sides claim: reproductive hormones, sleep, cycle regularity, adherence and body composition. It does not exist.

Pelz's own citation list is the evidence that it does not, and she has separately said in print that the trials exclude perimenopausal women outright. Absent that study, this dispute cannot resolve on mechanism, because both sides have one.