Fasting in women: why the cycle decides which form of fasting fits

In many cases yes, but the blanket answer falls short. A pooled analysis of three studies with 75 adults found no meaningful differences between premenopausal women, postmenopausal women and men when fasting every other day over twelve weeks. Weight, fat mass, insulin and blood pressure fell similarly in all three groups.

These participants, however, had obesity and therefore ample energy reserves. That is exactly the point. The question is not whether women are allowed to fast, but how much energy is actually left over for the body at the end of the day.

In lean women, in women who train a lot or who carry a heavy load, the same form of fasting can produce a very different calculation. For this group, robust data are almost entirely missing.

Because a possible pregnancy is biologically the most expensive project the body has. The hypothalamus therefore checks continuously whether there is enough energy before it lets the cycle continue.

This check runs via signalling molecules from fat tissue and the gut, above all leptin, and also via insulin, blood glucose and ketone bodies. If these signals fall below a certain threshold, the body throttles reproduction first, because it is dispensable in survival mode.

Men have a comparable safeguard, but it engages considerably later and with less visible consequences. This is why women often notice an energy gap first in their cycle, while men may not feel it for much longer.

Energy availability is what remains after exercise: energy intake minus the energy burned through movement, related to fat-free body mass. Exactly this remainder is what your metabolism has left for the heart, brain, immune system, bones and hormones.

In a randomised study with 29 regularly menstruating women, the release of luteinising hormone remained unchanged as long as energy availability was at 30 kilocalories per kilogram of fat-free mass per day or above. Below this threshold, pulse frequency fell and pulse amplitude rose.

Two women with identical calorie intake can therefore be in completely different positions if one of them trains every day. That is why, on the topic of fasting, I always ask about movement and not only about the plate.

To my knowledge there is no direct proof in the form of a study linking intermittent fasting to loss of the cycle. That honesty belongs at the start of the answer.

What is well documented: a clearly too low energy availability can disturb the control of the cycle, and the Endocrine Society guideline describes functional hypothalamic amenorrhea as a consequence of stress, weight loss, high training load or a combination of these. If an eating window leads someone to eat considerably less while training a lot at the same time, exactly this constellation can arise.

In my consultations I hear this temporal connection regularly. I cannot derive causality from it, but I take it seriously enough to ask.

Usually quite a lot, and long beforehand. A study with daily hormone measurements in 67 physically active women with seemingly normal cycle lengths found that only half of the observed cycles actually involved ovulation.

29.2 percent showed a shortened or weak luteal phase, 20.8 percent were anovulatory. Among the less active comparison women, the share of abnormal cycles was 4.2 percent.

The decisive part: these disturbances were not visible on the calendar. Bleeding in the usual rhythm is therefore not reliable proof that everything is running smoothly hormonally.

Leptin is a messenger from fat tissue and works like a fuel gauge. Kisspeptin is a small group of nerve cells in the hypothalamus that reads this gauge and gives the start signal for the reproductive axis. A review in Nature Reviews Endocrinology describes these cells as the central hub between energy balance and reproduction.

How closely the two are linked is shown by a controlled study in eight women with hypothalamic amenorrhea: under leptin administration, levels of luteinising hormone and pulse frequency rose after just two weeks, and three women had a cycle with ovulation over the course of the study.

That is a very small study and explicitly not a treatment recommendation, but a strong argument that the body really is reading an energy report here and not simply a number on the scale.

Because your basal metabolic rate rises slightly in this phase and your metabolism works differently. In a study with 38 naturally cycling women, resting energy expenditure in the luteal phase was on average around 40 kilocalories per day higher than in the follicular phase, and around 44 kilocalories after excluding individual outliers, with a confidence interval of 12 to 76.

At the same time, leptin, the thyroid hormone T3 and fat intake were higher in the luteal phase. In women using a combined pill this difference did not appear.

The amount sounds small. It adds up, though, on top of an eating window that already saves energy, and it meets a phase in which many women describe more hunger and more appetite for carbohydrates.

It appears so, and the most interesting finding on this comes from Tübingen. In a study with eleven women and four elaborate clamp measurements per person, insulin delivered through the nose directly into the brain improved whole-body insulin sensitivity only in the follicular phase. In the luteal phase this effect was absent.

A second group of 15 women showed the same pattern on MRI: the hypothalamus responded to insulin follicularly, but not luteally. The authors conclude that insulin resistance in the brain may contribute to the less favourable metabolic situation in the second half of the cycle.

For practice this means: the same meal does not land in the same body in every cycle phase. The study is small, so it remains a direction and not a rule.

Less than the hype promises. The TREAT study randomised 116 people with overweight over twelve weeks to 16:8 or to three fixed meals. The fasting group lost 0.94 kilograms, and the difference to the control group of 0.26 kilograms was not statistically meaningful. In the in-person subgroup, a loss of muscle mass in the arms and legs was also noticeable.

Narrower windows can do more: with 4 and 6 hour eating windows, weight fell by around 3 percent over eight weeks, because energy intake dropped by about 550 kilocalories per day without any counting.

The mechanism is therefore simply eating less, not the clock as such. That is an important reality check before asking a lot of your body.

This is the most interesting special case. In polycystic ovary syndrome there is often insulin resistance and elevated androgens, and fasting could act favourably on exactly these levers.

In a small study with 15 women with anovulatory PCOS over five weeks with an eating window from 8 am to 4 pm, body fat, testosterone, the free androgen index, fasting insulin, HOMA-IR and the inflammatory marker hsCRP fell, while SHBG rose. In 11 of 15 women the cycle became more regular. A review of the available human trials describes the same trend, especially when food is eaten early in the day.

For context: this study was small, uncontrolled and short. As a pointer to a direction it is useful, as proof it is not.

In many cases yes, because the cyclical control falls away that previously created a different starting position every week. A pooled analysis of 75 adults found similar results with every-other-day fasting in premenopausal women, postmenopausal women and men, with LDL cholesterol falling more strongly in the postmenopausal women.

After menopause, fat distribution also shifts more towards the abdomen, which can be one reason to address metabolism deliberately.

What still matters is protein and nutrient supply, because in this stage of life preserving muscle and bone gains importance. A short eating window makes both harder if nobody pays attention to it.

In my view always when you are pregnant or would like to become pregnant, when you are breastfeeding, when there is an eating disorder in your history, when you are underweight, when your period has become irregular or has stopped, when you train a great deal, when you have a thyroid condition or diabetes, or when you take medication that affects blood glucose.

In these situations fasting is not a neutral experiment, and a missing cycle is never merely a cosmetic topic. It concerns bone density, the cardiovascular profile, mood and fertility.

A sensible workup rules out pregnancy and includes thyroid, prolactin, iron status, blood count and the sex hormones before any nutritional strategy is even up for discussion.