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Does Intermittent Fasting Affect Your Period?

Does intermittent fasting disrupt your period or hormones? What small human studies found, why PCOS results conflict, and when a missed period needs a doctor's visit.

By Published 9 min read

You started a 16:8 routine a month ago and your period is late. Is the fasting window to blame?

Online, one camp says fasting wrecks female hormones and the other says it fixes them. The human research is more limited than either suggests. Most studies involved women with obesity or PCOS, and measuring a few hormones doesn’t tell you whether a period will arrive on time.

The short version

A fasting routine can contribute to missed periods if it leaves you eating too little for your needs. Small studies in women with obesity, including a 12-month trial analysis, found no clear changes in several measured hormones, but they do not establish menstrual-cycle or fertility safety. PCOS studies report some encouraging changes, with uncertain benefits beyond weight loss. Evidence for lean, highly active women remains especially limited. If pregnancy is possible, take a test when a period is late, and seek medical advice for persistent changes.

Where the warning came from

A 2022 review by Sofia Cienfuegos, Krista Varady and colleagues at the University of Illinois Chicago (UIC) traced the fear largely to one rat experiment. Young female rats fasted for 24 hours every other day for 12 weeks; their estradiol rose, luteinizing hormone fell and their cycles were disrupted. That animal protocol cannot tell us what happens to an adult’s menstrual cycle with a daily eight-hour eating window.

What happened in adult women

The same group later measured hormones in 23 women with obesity who had eaten within a 4- or 6-hour window for 8 weeks. Faiza Kalam and colleagues published the analysis in Obesity in 2023, using stored blood from the trial behind our post on whether a shorter fasting window works better. It isn’t an independent second trial.

Twelve women were premenopausal and eleven postmenopausal. Both groups lost 3% to 4% of their weight. The researchers found no significant changes in testosterone, androstenedione or sex hormone binding globulin (SHBG, which carries sex hormones in the blood). Postmenopausal estrogen and progesterone measurements also showed no significant changes.

DHEA, an adrenal precursor of other sex hormones, fell by about 13% to 14%, with wide individual variation. Levels stayed within normal ranges; the clinical meaning was unclear.

The gaps matter more than the results. Premenopausal estrogen and progesterone weren’t measured, because cycle day wasn’t recorded when blood was drawn. Cycle length and ovulation weren’t tracked, perimenopausal women were excluded, and there was no non-fasting comparison group.

There is also longer research. Shuhao Lin and colleagues’ 2024 randomized-trial analysis compared time-restricted eating, daily calorie restriction and a control group over 12 months in adults with obesity. It found no significant changes in testosterone, DHEA or SHBG over time or between groups in the female subgroups. Estrogen and progesterone were measured only after menopause. The published abstract supports these results; it does not establish that fasting leaves menstrual cycles unaffected.

A July 2026 systematic review by Aimee Jirapak and colleagues brought together six studies and still judged the evidence inconclusive. It highlighted inconsistent hormone-sampling times and weight loss as an alternative explanation for some PCOS findings.

PCOS: promising, but the reviews disagree

Polycystic ovary syndrome often involves insulin resistance, higher androgens and irregular ovulation. Results in that population need their own interpretation; they don’t predict what fasting does to someone with regular cycles.

The study most often quoted is Chunzhu Li and colleagues’ 2021 trial in the Journal of Translational Medicine. Eighteen women in China with PCOS and absent ovulation ate only between 8 a.m. and 4 p.m. for five weeks, and 15 finished. They lost about 1.3 kg on average. Testosterone and the free androgen index fell, SHBG rose and insulin resistance improved. Eleven of the fifteen showed an improvement in cycle irregularity.

That’s a striking number from a tiny study. There was no control group, and the paper neither defines “improvement” nor confirms ovulation with testing.

Two 2025 meta-analyses then tried to pool the PCOS evidence:

  • Ranneh and colleagues in Nutrients reviewed five papers and reported lower weight, insulin resistance, free androgen index and DHEA-S, plus higher SHBG. Total testosterone and AMH didn’t change significantly. They called the evidence preliminary.
  • Aolymat and colleagues in Metabolites reviewed four studies; its pooled analyses found no statistically significant change in BMI, blood sugar, insulin resistance or blood fats. They couldn’t pool sex hormones at all, because the data were too sparse, and rated Li’s trial at serious risk of bias.

The reviews included different studies and fasting protocols, including Ramadan fasting in Aolymat’s review. Both drew on before-and-after comparisons, which cannot cleanly separate fasting from weight loss or other changes. Neither pooled menstrual regularity. Their disagreement leaves the benefit uncertain; a statistically non-significant result also doesn’t prove there is no effect.

How much to trust this

The evidence is limited even with the 12-month analysis. Female subgroups were small, several studies had no comparison group, and hormone outcomes were often secondary. Most participants had overweight, obesity or PCOS. These results don’t establish safety for every body size, training load or fasting schedule.

These studies don’t adequately answer what happens to cycle regularity in lean women combining fasting with hard training. What is known is that eating too little for the energy you use can stop periods. NICHD lists strenuous exercise that burns more calories than you eat, eating disorders and extreme stress among the causes of missed periods. MedlinePlus adds sudden weight loss from strict or extreme diets and very low body fat. Do not assume fasting caused a missed period, but do not dismiss a persistent energy shortfall because a small hormone study looked reassuring.

What to do with this

  • Treat a late period as information, not a verdict. If pregnancy is possible, take a test; pregnancy is a common cause of missed periods.
  • Eat enough inside the window. If fasting has shrunk your meals or your cycle changes while training hard, pause the restriction and seek advice about fueling. The Endocrine Society’s guidance for energy-related amenorrhea focuses on correcting the energy shortfall, which can mean more food and less exercise. Choosing a window you can sustain matters more than the hours.
  • Keep a simple record. Note cycle dates next to your fasting routine so a clinician can see what changed and when. Atomic Fast doesn’t track menstrual cycles; its free timer records how long each fast ran, so keep cycle dates in whatever you already use.
  • With PCOS, treat fasting as one option, not a treatment. Discuss it with whoever manages your PCOS, especially if you take metformin or are trying to conceive.

When to see a doctor

MedlinePlus advises seeing a healthcare provider if you’ve missed more than one period. Mention any other changes too, such as rapid weight loss, breast discharge, new acne or extra hair growth, because they help narrow down the cause.

Fasting isn’t a good fit if you’re pregnant, breastfeeding, trying to conceive with medical help, or have a history of disordered eating; our guide to who shouldn’t do intermittent fasting covers the full list. If fasting has become a way to eat as little as possible, that matters more than any cycle change; talk to someone about it.

Frequently asked questions

Can intermittent fasting make your period late? It can contribute if the routine leaves you eating too little for your activity level. But a late period has other possible causes, including pregnancy, stress, medication and thyroid problems. The hormone studies described here cannot tell you the cause of your own cycle change.

Does 16:8 lower estrogen? There isn’t a reliable yes-or-no answer. The 2026 review found one uncontrolled PCOS study reporting lower estradiol, while other studies found no significant change or measured it only after menopause. Those findings don’t establish what 16:8 does to estrogen across a regular menstrual cycle.

Is intermittent fasting good for PCOS? Some studies report improved hormone and metabolic markers, but they cannot reliably separate meal timing from weight loss. The studies are small and short, often without control groups, and two 2025 meta-analyses disagreed, so it’s an option to discuss rather than an established treatment.

Should I fast differently during my period? The studies reviewed here do not establish an effective cycle-synced fasting schedule. If you feel worse on certain days, shortening or skipping the fast on those days is reasonable.

Does fasting affect fertility? The hormone and cycle findings reviewed here do not establish whether fasting improves or harms the chance of pregnancy or a live birth. If you’re trying to conceive, talk to your clinician before starting a fasting routine.

Sources

Sources checked: October 13, 2026.

Keep track of your fasting hours with Atomic Fast, free on the App Store and Google Play.

This article is for general education and isn’t medical advice. If you’re pregnant, managing a health condition, taking medication, have a history of disordered eating, or are considering fasts longer than 24 hours, talk to a healthcare professional first.

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