How Intermittent Fasting Affects Your Menstrual Cycle: What the Research Shows

How Intermittent Fasting Affects Your Menstrual Cycle: What the Research Shows

Researchers studied 18 women with PCOS who were not having regular periods. Fifteen finished a 6-week trial of eating only within an 8-hour window each day. After the trial, 11 of the 15 women (about 73%) had more regular periods, and several hormone and weight measures also improved.

Medical disclaimer: This article summarises published research for informational purposes only. It is not medical advice and is not a substitute for guidance from a qualified health professional. Always consult your doctor before starting any fasting protocol, especially if you have an existing health condition or take medication.

The Short Version

• What they tested: Eating only within an 8-hour window each day, to see if it helps irregular periods in women with PCOS.
• Who did it: C. Li and colleagues at Shengjing Hospital of China Medical University in China, published in Journal of Translational Medicine in 2021.
• How many people: 18 women joined, 15 finished. All were women, ages 18 to 31.
• How long: 6 weeks total — 1 week to settle at a stable weight, then 5 weeks of the eating window.
• What happened: Periods became more regular in 11 out of 15 women (about 73%).
• Read the study yourself: Journal of Translational Medicine 2021 →

What They Wanted to Find Out

Doctors wanted to know if an 8-hour eating window could help women with PCOS. PCOS is a hormone condition that can stop regular periods. Many women with PCOS do not release an egg on a normal schedule. This makes periods unpredictable and can affect having children.

This builds on other work linking fasting and PCOS to more regular periods, and on how fasting may affect PCOS hormones. Researchers wanted to see if this same eating pattern could also help periods become regular.

Who Was in the Study

Who they were: All 15 women who finished were between 18 and 31 years old. All had PCOS and were not releasing an egg on a regular schedule.

What the fasting actually looked like: Women ate only during an 8-hour window each day. The other 16 hours were a fasting period. Before this, everyone spent 1 week keeping their weight stable, so the study could measure a fair starting point.

What Happened

Most women's periods became more regular after 5 weeks of eating this way.

• Most women's periods became more regular after sticking to the eating window.
• Body weight and body fat both went down.
• Blood levels linked to male-type hormones also went down.
• The body handled insulin better by the end of the 5 weeks.
• Liver and inflammation blood tests moved in a healthier direction.

What Did Not Change

• Muscle mass stayed about the same — the plan did not appear to cost these women muscle.
• Plain blood sugar levels were not listed as changed, only insulin and its ratios.
• Reproductive hormones like LH and FSH were measured but not reported as changed.
• Cholesterol and uric acid levels were also not reported as changed.

What the Researchers Said

The researchers said this eating pattern may help women with this type of PCOS. They pointed to more regular periods, less body fat, and better hormone and sugar handling as the main gains.

What This Means for You

• If your periods are irregular because of PCOS: this small study suggests an 8-hour eating window may help, in about 5 weeks.
• Muscle was not lost: this may ease a common worry women have about fasting and muscle. See more on fasting and muscle.
• This is specific to PCOS: the study cannot tell you if fasting changes periods for women without PCOS, or for other causes of irregular cycles.
• Hormones shifted, not just weight: male-type hormone levels dropped too, not only body fat.
• Talk to a doctor first: insulin and hormone levels changed here, so medical guidance matters before you try this with PCOS.
• PCOS and periods are linked elsewhere too: see what a bigger fasting trial found for PCOS periods.

What This Study Could Not Tell Us

• Only 18 women joined, and just 15 finished — this is a small study.
• All participants were women with PCOS, so results may not apply to other women.
• It ran for only 6 weeks, so it cannot say what happens after a year.
• There was no separate comparison group, so other factors could have played a part.
• Exact numbers for weight loss and hormone changes were not given in the summary.
• The study does not say who paid for the research.

The Study Details

Li C, Xing C, Zhang J, Zhao H, Shi W, He B. (2021). Eight-hour time-restricted feeding improves endocrine and metabolic profiles in women with anovulatory polycystic ovary syndrome. Journal of Translational Medicine. PMID: 33849562

Common Questions

Does intermittent fasting change your menstrual cycle?

In this small study, 11 out of 15 women with PCOS had more regular periods after 5 weeks of an 8-hour eating window. This was in women who had not been releasing an egg regularly.

What is an 8-hour eating window?

It means eating all your meals within 8 hours of the day and not eating during the other 16 hours. In this study, that was the only fasting rule the women followed.

How many women's periods became regular in this study?

About 73% — 11 out of 15 women who finished the study. All of them had PCOS and irregular periods at the start.

Did the women in this study lose muscle?

No. Muscle mass was measured and was not listed among the things that changed. Body fat went down, but muscle stayed about the same.

Is this safe to try if I don't have PCOS?

This study only looked at women with PCOS, so it cannot answer that. Talk to a doctor before trying an 8-hour eating window for period changes.

More Research Like This

• 8-Hour Time-Restricted Eating and Hormones in Women With PCOS
• 5:2 Fasting and Menstrual Regularity in Women With PCOS
• Time-Restricted Eating and Weight Loss in PCOS
• Fasting, Insulin, and Hormones in PCOS
• Time-Restricted Eating Plus Calorie Cutting in PCOS
• Does Time-Restricted Eating Change Women's Hormones?
• Alternate-Day Fasting and PMS Mood in Women

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