Does Intermittent Fasting Affect Women's Hormones? What the Evidence Says

The short answer: there is no strong evidence that moderate fasting windows of 12 to 16 hours disturb reproductive hormones such as estrogen and LH in healthy women. But "no evidence" is not the same as "no risk." The studies are few, small and short. What matters most is often not the number of fasting hours but how much energy you take in over the whole day.

What Does the Science Say?

A 2022 review that pulled together human trials on intermittent fasting and reproductive hormones (Cienfuegos et al., Nutrients) found only 7 studies: 5 on time-restricted eating, one on the 5:2 diet and one on meal timing. Sample sizes ranged from 16 to 107, and reproductive hormones were mostly a secondary outcome rather than the main one.

The authors concluded that fasting did not affect estrogen, gonadotropin or prolactin levels in women. That conclusion rested on very few measurements, though: estradiol was measured in only one study, and LH and FSH in a single study as well.

Some things did change. In premenopausal women with obesity, SHBG (a protein that carries sex hormones) rose by 2% to 14% across several trials. In women with PCOS and obesity, androgens such as testosterone fell, especially when the eating window was early (before 4 p.m.). One study saw a 9% drop in total testosterone.

The authors are open about the limitations: most trials were short, many had no control group, and it is unclear which day of the menstrual cycle measurements were taken. Since hormones fluctuate substantially across the cycle anyway, that last point matters.

PCOS Is a Slightly Different Picture

In a randomized study published in Nature Medicine, 76 premenopausal women with PCOS either ate only between 1 p.m. and 7 p.m. (time-restricted eating) or counted calories for six months. Testosterone fell in both groups and both lost about 10 pounds (roughly 4.5 kg) on average. But only the time-restricted eating group saw a drop in the free androgen index (a marker of how much active testosterone reaches the tissues) and an improvement in HbA1c.

Menstrual irregularity did not improve in either group. The researchers think that may change with a longer time frame and more weight loss. About 80% of participants said they wanted to keep going with the diet. Because weight loss and insulin sensitivity already affect hormone balance in PCOS, these results shouldn't be read as an effect caused by fasting alone.

Why Is There More Caution for Women?

The reproductive axis is a system that monitors the body's energy status. When too little energy comes in, the brain can turn down the signals that support ovulation. The loss of periods seen in some athletes and in people on very low calories is a known example.

Whether intermittent fasting causes this is a separate question. With moderate protocols and total calories preserved, you don't seem to get near that threshold. But when long fasts, low calories and hard training stack up, the picture can change. On this point, the evidence we have is the small studies described above.

Fasting by Cycle Phase: What Does the Evidence Say?

Charts like "16:8 in the follicular phase, 12:12 in the luteal phase" circulate widely online. To be honest, we couldn't find a clinical study that tests fasting protocols by cycle phase or supports those charts. The review above also notes that cycle day wasn't standardized in most trials. What follows is therefore not a rule but a cautious approach:

• Start moderate: Begin with a 12-14 hour window (for example, close the kitchen at 8 p.m. and have your first meal between 8 and 10 a.m.). Move to 16:8 only if those first few weeks go smoothly.
• Don't cut total calories: Change the eating window first. If you need to reduce amounts, do that as a separate step, later.
• Track your cycle for 2-3 months: Note cycle length, spotting, late periods, sleep and energy. If you see changes, widen the window.
• Adjust to your own observations: If sleep disruption, increased appetite or fatigue gets worse in the days before your period, widening the window on those days is a reasonable experiment. This isn't an evidence-based rule, just an adjustment based on your own experience.

When You Combine It With Keto

Because keto can suppress appetite, it's easy to end up eating far too little without noticing when you add fasting. For a while, actually log what you eat and check that your total calories and protein are reasonable. In the first weeks, water and electrolyte loss can also make fatigue, dizziness and palpitations more likely; see our electrolyte balance article for that.

When Should You See a Doctor?

Don't start intermittent fasting or keto on your own if you are pregnant, planning a pregnancy, breastfeeding, have a history of an eating disorder, already have irregular periods or are underweight. If you take diabetes medication or insulin, don't fast without adjusting the dose with your doctor. If, after starting, you notice a late period, noticeable hair loss, constant feeling of cold or fatigue, stop and get checked.

Sources: Cienfuegos et al., Effect of Intermittent Fasting on Reproductive Hormone Levels in Females and Males: A Review of Human Trials (Nutrients, 2022) · UIC, time-restricted eating study in PCOS (Nature Medicine)