Fasting and women's hormones: a careful look at the evidence

The claim that intermittent fasting is uniquely harmful (or uniquely beneficial) for women is overstated in both directions online. The actual literature is small, mostly short-duration, and rarely powered to detect sex differences.

What's well-established#

Energy availability and the menstrual cycle. This is the most reliable underlying principle. Sustained low energy availability (insufficient energy after accounting for exercise) is associated with menstrual dysfunction, what's now formalised as Relative Energy Deficiency in Sport (RED-S). This effect doesn't require fasting; it's about total intake relative to demand. Any fasting protocol that produces sustained under-eating can plausibly contribute.

Lean and athletic women are more sensitive. Women with lower body fat or higher training loads show menstrual irregularities at smaller energy deficits than sedentary women with higher baseline body fat.

What the fasting-specific trials show#

Most published TRE trials have included women and either found similar effects to men or were underpowered to detect a difference:

• Sutton et al. 2018: small, men only.
• Cienfuegos et al. 2020: a time-restricted feeding trial in adults with obesity; the abstract does not report sex composition or menstrual outcomes.
• Liu et al. 2022 NEJM: included men and women; effects on weight and metabolic markers were similar.

There is no large, well-powered trial examining cycle effects of 16:8 in healthy lean women, and no convincing evidence that 16:8 specifically, as distinct from total under-eating, drives menstrual dysfunction.

What women's reports suggest#

In community surveys and observational data, a subset of women report:

• Cycle lengthening or amenorrhoea after several weeks of TRE, particularly with high training loads and lean body composition.
• Improvements in PCOS-associated markers in other women on similar protocols.

These responses are not uniform and likely depend on baseline body composition, training status, and total energy intake.

A pragmatic position#

For most healthy women in the UK:

• A moderate TRE window (10–12 hours of fasting) is unlikely to materially affect hormones if total energy intake remains adequate.
• A longer or stricter window (16:8 or beyond) combined with substantial calorie restriction or hard training is more likely to push some women into low energy availability.
• The menstrual cycle is one of the more useful biofeedback signals available: changes in cycle length, flow, or PMS pattern are worth noticing.
• Women trying to conceive, with a history of HPO-axis suppression, or with PCOS-related anovulation should discuss fasting protocols with a clinician, not rely on Instagram.

The accurate summary: fasting is not inherently dangerous for women, but the population most likely to be enthusiastic about fasting (lean, active women aiming for further leanness) is also the population most at risk of inadvertently combining it with low energy availability.