What is intermittent fasting?
Intermittent fasting is an eating pattern which cycles between eating and fasting periods. At least 12 hours of fasting is generally recommended, although some people fast for 14 or 16 hours. With this type of intermittent fasting, the fasting period usually occurs overnight.
Benefits of intermittent fasting
Intermittent fasting has been documented as leading to numerous health benefits:
- Improved weight and body composition — a number of research papers have found an association between intermittent fasting and weight loss. One trial found participants lost about 8% of body weight and around 16% of fat mass over six months, and a review of 27 trials similarly concluded that intermittent fasting shows promise for treating obesity.
- Improved metabolic health — intermittent fasting has been shown to improve blood sugar control, blood lipids, and reduce the risk of type 2 diabetes.
- These benefits may not only be due to calorie reduction, but also to something called metabolic switching — where fasting triggers the body to switch its source of energy from glucose stored in the liver to ketones made from stored fat. Although a 12-hour fast will only put you at the threshold of ketosis, it still helps train your cellular machinery to burn fat.
- Gut microbiome health — intermittent fasting can positively influence gut microbiome health, increasing diversity and beneficial bacteria such as Akkermansia muciniphila, which helps strengthen the gut barrier. Restricting food intake to daytime also synchronises microbial activity and gut motility with your body’s natural internal clock, optimising digestion and metabolic health.
This works alongside another mechanism: the migrating motor complex (MMC). This is a repeating, wave-like pattern of contractions that only occurs when you’re not eating — it’s often described as the gut’s “street sweeper,” clearing residual food, bacteria, and waste from the small intestine roughly every 90–120 minutes. The MMC needs a genuine gap between meals to activate properly, which is one more reason constant grazing throughout the day may work against gut health, even if total calories stay the same.
Why intermittent fasting may not always be beneficial for women
Although the benefits of intermittent fasting are well-documented, what’s important to note is that the majority of medical research has been carried out on young, healthy men — not women. Women are not hormonally static, and they are not simply “small men.” Results from research carried out on men may not be directly applicable to women.
The cortisol awakening response
One of the clearest examples of this gap is cortisol. Every morning, healthy people experience the cortisol awakening response (CAR) — a natural surge in cortisol in the first 30–45 minutes after waking, which primes the body for the day. This is a normal and useful process.
The issue is that fasting appears to interact with this rhythm differently in women. Research suggests fasting can not only elevate cortisol overall, but also disrupt the cortisol awakening response itself, leading to a dysregulated HPA (hypothalamic-pituitary-adrenal) axis that may affect long-term hormone health. In women specifically, short-term fasting has been shown to amplify cortisol secretion and disrupt the normal coupling between cortisol and other hormones like luteinizing hormone and leptin — a synchrony that matters for reproductive and metabolic signaling.
This is why the length of the fast matters so much. Shorter fasts, in the 12-hour range, tend to align with the body’s natural overnight rhythm and produce only a modest, expected cortisol response. Extended fasts — 24, 48 hours or longer — are where the more disruptive cortisol elevation tends to show up, and where the CAR itself starts to lose its normal shape.
Practical takeaway: a 12-hour overnight fast is a safer, better-supported starting point for most women than the more aggressive 16:8 or longer protocols popularised online. That simply means finishing dinner a little earlier and eating breakfast a little earlier too — for example, dinner by 7pm and breakfast around 7am — rather than pushing the fasting window later into the morning.
Don’t skip breakfast
This connects directly to the CAR. Eating your first meal of the day is part of what resets the cortisol curve back down after its natural morning peak. When that first meal is skipped or delayed too long, peak cortisol can persist for longer than it normally would. Habitually skipping breakfast — as opposed to a shorter, gentle overnight fast — is a different proposition for women’s hormone regulation, and isn’t the same thing as a well-timed 12-hour fasting window.
If you exercise first thing in the morning, this matters even more. Training in a fully fasted state stacks two stressors on the body at once — exercise and an empty stomach — right at the point when cortisol is already naturally peaking. A simple fix: have a small handful of nuts (or a small protein-containing snack) before an early morning workout, rather than training on nothing at all. This takes the edge off the combined cortisol load without meaningfully breaking the spirit of a 12-hour overnight fast.
Reduced nutrient intake
Compressing eating into a shorter window makes it easy to under-eat key nutrients even when total calories look fine — a particular concern for women, whose needs for iron, calcium, B vitamins, and protein are already significant and shift further with age.
Menstrual cycle differences
Fasting response isn’t fixed across the month. Insulin sensitivity is measurably higher in the follicular phase (the first half of the cycle) and drops in the luteal phase (the second half, before your period) — a difference confirmed in controlled studies using glucose tolerance testing. Practically, this means the body generally tolerates fasting more easily in the follicular phase, when rising oestrogen supports insulin sensitivity and metabolic flexibility. In the luteal phase, progesterone dominance increases caloric needs and reduces stress tolerance, so aggressive fasting here is more likely to elevate cortisol and suppress progesterone.
Early trial evidence also found a direct sex split: during alternate-day fasting, non-obese women experienced impaired glucose response after meals, while men’s glucose response didn’t change and their insulin sensitivity actually improved. Later, larger studies have pushed back on some of these findings, but it illustrates the point — women’s metabolic response to fasting is not simply a smaller version of men’s.
Why intermittent fasting may not always be beneficial for women, by hormonal life stage:
Reproductive years
For women with a regular cycle, the follicular/luteal distinction above is the main variable to work with — longer or firmer fasting windows tend to be better tolerated in the first half of the cycle, with a gentler approach (shorter fasts, more consistent meals, extra carbohydrate and calorie support) better suited to the luteal phase and the days just before a period.
Perimenopause
Perimenopause is arguably the hardest stage to apply a fixed fasting “rule” to, because nothing about the hormonal picture is stable. Estrogen isn’t declining smoothly here — it’s fluctuating erratically, which means the predictable monthly follicular/luteal pattern that younger cycling women can plan around becomes far less reliable.
Two things compound each other in this stage. First, insulin resistance tends to rise, because estrogen plays a direct role in maintaining insulin sensitivity — as it becomes erratic, the body needs to produce more insulin to keep blood sugar stable, often showing up as sugar cravings, post-meal fatigue, and stubborn weight gain. Second, cortisol regulation becomes less forgiving: declining estrogen during this transition further raises baseline cortisol, which is linked to the visceral abdominal fat that tends to increase cardiovascular risk.
This is exactly why fasting — a mild physiological stressor on its own — can misfire more easily in perimenopause than it did a decade earlier. High-quality, perimenopause-specific trials are still limited, but the consistent clinical advice is to start gently (around 12 hours overnight), keep any longer fasting windows to lower-stress points in the cycle, and back off in the week before a period.
Menopause and postmenopause
Once a woman is through menopause, there’s no monthly cycle left to plan fasting around — but the body hasn’t settled into something simple either. Visceral fat increases substantially through this transition (from roughly 5–8% to 15–20% of total body fat), and this tissue actively interferes with insulin function. Fasting research does show a genuine upside here — 8 weeks of time-restricted eating has been shown to improve fasting insulin and reduce insulin resistance in postmenopausal women.
But muscle loss is the part of the story that’s often left out. Muscle mass declines by roughly 3–8% per decade after age 30, and this accelerates further through the menopausal transition — less muscle means a slower resting metabolic rate, which makes fasting-driven weight loss harder to sustain and easier to regain. Fasting without adequate protein or resistance training can make this worse, not better; some research links prolonged fasting in postmenopausal women, without concurrent strength training, to reduced bone mineral density alongside accelerated muscle loss.
Bone density itself is a genuine question — short-term studies suggest fasting doesn’t harm bone density, but long-term data in postmenopausal women simply doesn’t exist yet in enough volume to be confident either way.
However, the research on postmenopausal fasting is also inconsistent. Some studies report a decrease in cortisol with fasting in this group, alongside an increase in DHEAS (a hormone that typically declines with menopause and supports bone health) — the opposite of what’s reported elsewhere. That contradiction is a useful reminder of how thin and unsettled the postmenopausal-specific fasting research base still is, compared to the confident tone most popular fasting content takes.
The bottom line
Intermittent fasting has real, well-documented benefits — but almost entirely in populations that don’t reflect the average woman’s shifting hormonal landscape. For most women, a gentle 12-hour overnight fast (dinner a little earlier, breakfast a little earlier, nothing more aggressive layered on top) captures much of the documented upside — supporting the migrating motor complex, gut health, and metabolic switching — without the cortisol and HPA-axis disruption that longer or more extreme protocols are more likely to trigger. Breakfast shouldn’t be skipped, morning workouts are better fuelled with a small snack first, and the right approach shifts again through perimenopause and menopause, when the body’s stress tolerance and metabolic flexibility are already in flux for reasons that have nothing to do with fasting at all.
