Fasting and Women's Hormones: What the Evidence Says, Where to Be Careful, and Who Should Seek Professional Advice

Intermittent fasting has become a popular approach among people who care about their health, whether that means limiting eating to a window each day (time-restricted eating) or alternating normal days with low-intake days. When energy availability is insufficient, especially when the exposure is severe or prolonged, reproductive function may be disrupted. Responses vary between individuals, and REDs can occur in people of any sex. This article looks at what the research actually says about fasting and women’s sex hormones, menstrual cycles, and fertility, and where extra caution is warranted.

One thing to say up front: the evidence here is limited and preliminary. Most studies are small and short, around 6 to 8 weeks, and were done in women with obesity, so the findings cannot yet be generalized to women at a normal weight or over the long term. Everything here is population level understanding for prevention and health care, not a diagnosis or a personal prescription. If you have menstrual or hormone concerns, or you are planning a pregnancy, talk to a doctor first.

What the Research Says About Fasting and Women’s Hormones, and What It Doesn’t

In premenopausal women with obesity, some small studies report lower androgen markers such as testosterone and the free androgen index and higher SHBG during intermittent fasting or time-restricted eating, with a clearer signal in some early-day eating protocols. Estrogen, the gonadotropins that stimulate the ovaries, and prolactin have not changed consistently. The findings therefore do not all point in one direction across every hormone outcome.

To be plain, the evidence comes from several small intervention studies; some lack a control group, and some hormone outcomes are secondary analyses in reduced samples. These designs cannot clearly separate meal-timing effects from weight loss, total energy change, and other co-interventions, so causal conclusions remain premature. In premenopausal women, many studies did not measure estrogen or progesterone rigorously because they did not control which day of the cycle blood was drawn, so interpretation is limited.

One more finding is DHEA, a precursor of the sex hormones, which dropped in both premenopausal and postmenopausal women after about 8 weeks of time-restricted eating. What that drop means for fertility over the long term is not known. This part comes from a follow-up analysis of a small study, about 23 participants in total, so it should not be called firm evidence, and it deserves extra caution because the study that measured DHEA and the review that summarized the hormone findings come from the same research group, so they are not yet two truly independent sources. A review from a separate group that assessed the same body of evidence confirmed the same direction and stressed that the human evidence is still limited.

The Most Important Risk: Low Energy Availability and Your Cycle

The point that matters most here is a state called low energy availability (LEA): the dietary energy remaining for normal physiological functions after subtracting exercise energy expenditure, commonly expressed relative to fat-free mass. It is not the same as a simple negative daily calorie balance or weight loss. In sports medicine, prolonged or severe problematic LEA is linked with multisystem effects, including impaired reproductive function, but menstrual disturbance has multiple possible causes and requires medical evaluation.

The International Olympic Committee consensus defines REDs as a multisystem syndrome in athletes caused by exposure to problematic LEA that is prolonged and/or severe and may include impaired reproductive function. The Endocrine Society guideline describes functional hypothalamic amenorrhea (FHA) as a diagnosis of exclusion that may involve low energy availability, weight loss, psychological stress, or high exercise load.

Diagnosis and management of FHA belong to a licensed clinical team. A missed period has other important causes, including pregnancy, thyroid problems, high prolactin, PCOS, and primary ovarian insufficiency. Readers should not diagnose or treat FHA from this article; a gynecologist or reproductive endocrinologist should evaluate the cause.

PCOS: An Early Uncontrolled Signal With Very Limited Evidence

In women with the anovulatory form of polycystic ovary syndrome (PCOS), a pilot study reported within-person changes after an 8 hour eating window for about 5 to 6 weeks: weight, insulin and androgen measures, and menstrual regularity moved in a favorable direction. This is an early signal for further study.

But it needs to be read carefully, because it was a very small study with only 15 participants, no control group, and no randomization, so it cannot establish that fasting is what caused the improvement. A review that assessed the evidence for fasting in PCOS reached a similar conclusion: there are positive signals, but they are limited by study size and design. Larger, controlled studies are needed before anyone can be confident. And even where these measures improve, that does not make fasting a treatment for infertility or a way to boost the chance of pregnancy.

Fertility and Pregnancy: The Evidence Is Still Missing

The effect of fasting on fertility and pregnancy outcomes in women is a clear research gap. There is not yet high quality data to recommend that people trying to conceive use fasting to improve their chances. Both the independent review and the review of human trials stress the same thing: the number of studies in women is small, and data on reproductive outcomes is lacking.

Some of the data that does exist comes from Ramadan fasting during pregnancy, a context of temporarily going without food and water during daylight, which is different from fasting to lose weight. A meta-analysis found no clear effect on birth weight overall, and an umbrella review found weak evidence concerning low birth weight or preterm birth, while other outcomes remain too sparse for conclusions. These data should not be used as evidence that fasting or weight loss during pregnancy is safe. Pregnant people should discuss fasting, nutrition, and weight goals with their obstetric clinician and a dietitian.

After Menopause: What Changes and What Doesn’t

In postmenopausal women with obesity, participants in a small short-term study lost around 4 percent of body weight during time-restricted eating, while estrogen, progesterone, androgens, and SHBG did not change clearly. DHEA dropped, as it did in the premenopausal group. Without a direct non-TRE comparator for this hormone analysis, the study cannot establish that TRE caused these changes.

As with the other sections, this is short-term data from small samples; the study cited included about 11 postmenopausal women. Weight change is the main visible effect in this group, while the overall picture for sex hormones still shows small effect sizes and needs larger studies to confirm.

One group deserves a separate mention: women in perimenopause, the transition before menopause. During this stage cycles are naturally variable and irregular, which makes an irregular cycle harder to read as a fasting warning sign than it would be for someone with regular cycles. Women in perimenopause should discuss fasting and any changes to their cycle with a doctor rather than relying on self-monitoring alone.

The Risk of Disordered Eating, and Who Should Seek Professional Advice

One side of this that needs to be said clearly is that fasting, especially when done to control weight, is associated with disordered eating behaviors such as binge eating and compulsive exercise, and the signal is clearer in women and in adolescents and young adults. This comes from a cross-sectional survey, so it shows association rather than cause. Anyone with a history of, or currently being treated for, an eating disorder should not start restrictive fasting independently and should discuss it with their specialist care team.

Groups with individualized energy, nutrient, monitoring, or medication needs who should consult a clinician or dietitian before considering fasting include:

• People who are pregnant, breastfeeding, trying to conceive, or dealing with infertility, because there is no high-quality evidence that fasting improves conception and their energy and nutrient needs require individual assessment
• People with a history of, or currently living with or being treated for, an eating disorder, or a fragile relationship with food
• People whose periods are already absent or irregular, or who have low energy availability, such as athletes who meet the criteria for RED-S or the female athlete triad
• People who are underweight, malnourished, or ill or recovering
• Children and adolescents
• People with a chronic condition linked to meals or medication timing, such as diabetes treated with insulin or glucose-lowering drugs, who should discuss fasting with their clinician or pharmacist and should not adjust medication or insulin from this article

Warning signs to stop and see a doctor:

• Periods becoming absent or irregular after starting fasting, a signal that energy may be insufficient; stop and consult a doctor
• Unusually rapid weight loss, severe fatigue, dizziness, feeling cold easily, or hair loss
• Preoccupation with food or weight, binge eating, or compensatory compulsive exercise
• Mood swings, insomnia, or a clear drop in concentration
• In pregnancy, reduced fetal movement, dizziness to the point of fainting, or dehydration, which need immediate medical attention

The most important caution: LEA is the dietary energy remaining after exercise energy expenditure relative to fat-free mass, not a negative daily calorie balance. Severe or prolonged problematic LEA may impair reproductive function.

If a cycle becomes irregular or stops, pause the experiment and seek medical evaluation rather than self-diagnosing FHA or LEA. Diagnosis and management of FHA belong to a licensed clinical team. Sources: IOC REDs consensus (PMID 37752011) and Endocrine Society FHA guideline (PMID 28368518).

Questions to Discuss Before Trying It

If you are considering fasting, use this material as a basis for discussion with a clinician or dietitian rather than as a personal eating plan, especially if you have a chronic condition, take medication, are pregnant or planning pregnancy, have cycle changes, or have a history of disordered eating.

1. Review suitability with the relevant professional, including total energy and nutrient needs, medication, chronic conditions, and pregnancy goals; individualized plans belong to licensed professionals.
2. Use self-observation to support that discussion, including cycle pattern, energy, mood, and relationship with food, but not to diagnose a condition.
3. Pause and seek assessment for cycle changes, rapid weight loss, dizziness, marked fatigue, or compulsive eating or exercise behaviors.

Remember that most of the evidence in women comes from small, short studies in people with obesity, so the results may not be the same for someone at a normal weight. Extreme weight-control fasting is associated with disordered eating, especially in women, although cross-sectional evidence cannot establish that fasting causes those behaviors.

This article was prepared by a certified Health Coach from research and academic literature for general, population-level education. It is not a diagnosis, treatment, diet or exercise plan, or individualized advice, and it must not be used to start, stop, or adjust medication, supplements, diet, or exercise. Consult your own licensed physician or the relevant qualified specialist before acting when medical risks apply, especially if you have a chronic condition, take medication, are pregnant or planning pregnancy, are considering a prolonged fast, or have abnormal symptoms. Contact emergency services for an emergency.