Is Intermittent Fasting Bad for Women? (What the Research Says)

If you’re a woman with even a passing interest in this whole health and fitness thing, you’ve probably heard some version of the claim that fasting is uniquely bad for women.

While the specific claims vary, they generally revolve around the same idea that fasting can mess with women’s hormones, which can then disrupt their menstrual cycles, and even harm their reproductive health.

From what I can tell, this claim originates from ​a 2013 study​ that investigated the effects of alternate-day fasting on reproductive health in young male and female rats. For 12 weeks, the rats alternated between 24 hours without food and 24 hours with unrestricted access to food.

Within about two weeks, the female rats’ reproductive cycles stopped functioning normally, and they stopped ovulating for most of the experiment. Their oestradiol (a form of oestrogen) increased, while luteinising hormone (LH), which helps trigger ovulation, decreased.

Cool. Very interesting.

But I’m going to go out on a limb here and assume you’re not a rat.

So, what does the human research show?

A recent ​systematic review​ investigated whether time-restricted eating (TRE) affects reproductive hormones in women. The review included six studies involving 239 women, most of whom had overweight, obesity, or PCOS.

Most studies used a 16:8 protocol (participants fasted for 16 hours and ate their food within an eight-hour window). The studies included in the review generally lasted 5-8 weeks, although one lasted 12 months.

The researchers found no consistent evidence that TRE negatively affected reproductive hormones, including oestrogen, progesterone, and testosterone, in women without PCOS.

In women with PCOS, some studies reported improvements in hormonal profiles, although this might have been due to the weight loss rather than fasting itself.

However, the evidence is limited. Most studies were small, and few accounted for menstrual-cycle fluctuations when measuring hormones, and participants were predominantly women with overweight or obesity (as opposed to leaner, physically active women). 1 While research on 16:8 fasting in leaner, physically active women, including resistance-trained women and female dancers, exists, those studies have largely focused on body composition and exercise performance rather than reproductive hormones or menstrual health.

That last bit is important, and I suspect it’s where a lot of the “fasting is bad for women” stuff comes from.

See, there’s a legitimate reason why restricting food intake can negatively affect women’s reproductive health.

But it has less to do with fasting itself and more to do with something called energy availability.

Don’t worry, I’ll try to keep this short.

​Energy availability​ is simply the amount of energy your body has left over to carry out its normal physiological functions after accounting for the energy you expend through exercise. 2 Technically, energy availability is calculated by subtracting the calories expended through exercise from daily calorie intake, then dividing the result by lean body mass in kilograms. For example, someone eating 2000 calories while expending 400 through exercise and carrying 50 kg of lean body mass would have an energy availability of (2,000 − 400) ÷ 50 = 32 kcal/kg of lean body mass/day.

For example, let’s say you eat 2000 calories per day and burn 400 calories through exercise. That leaves 1600 calories to support everything else your body needs to do, like regulating your metabolism, producing hormones, and supporting reproductive function. And, if you’re my four-year-old, producing a ridiculous amount of sass.

Now, imagine you reduce your intake to 1400 calories but continue burning 400 calories through exercise. Now, you’ve only got 1000 calories left to support those same functions.

Also, to be clear because I don’t want anyone misconstruing what I’m saying: the numbers above are for illustration only. Whether someone has low energy availability depends on their individual energy needs, including their body composition and activity levels, not on whether they started eating 1400 calories. Cool? Cool — let’s keep it moving then.

In short, the less energy you consume relative to what you expend through exercise, the less energy remains available to support your body’s other physiological functions.

When energy availability stays too low for too long, your body starts making trade-offs, and one of these trade-offs is dialling down reproductive function.

After all, if your body senses there isn’t enough energy to go around, reproduction (an energetically costly process) tends to drop down its priority list.

This isn’t just theoretical either.

In ​a 2003 study​, researchers took 29 healthy, normal-weight women with regular menstrual cycles and manipulated their energy availability by changing their calorie intake relative to their exercise expenditure. They found that when energy availability dropped below roughly 30 kcal/kg of lean body mass/day, it disrupted the release of luteinising hormone (LH), which plays an important role in regulating ovulation and the menstrual cycle.

Data highlighted in a ​2024 review​ found that even short periods (4-5 days) of low energy availability could affect the release of hormones involved in regulating ovulation and the menstrual cycle. Longer studies in their review found insufficient energy availability could lead to menstrual disturbances, including irregular cycles and problems with ovulation.

However, that 30 kcal/kg of lean body mass figure I mentioned earlier isn’t a hard-and-fast rule. Rather, it’s a threshold below which the risk of menstrual disturbances could increase.

The same 2024 review noted that some women experienced disturbances above that threshold, while others maintained normal menstrual function despite being below it.

Now, this is the part where the C-word comes in (what? Not that C-word): Context.

First off, most of the energy availability studies involved young, normal-weight women – not women with overweight or obesity trying to lose excess body fat. And you can lose weight without messing up your reproductive health. And for women with overweight or obesity, losing excess body fat can improve several aspects of health, including reproductive function. For example, in women with PCOS, weight loss can improve hormonal profiles and, in some cases, menstrual regularity and ovulation.

Second, being in a calorie deficit isn’t the same as having problematically low energy availability.

In fact, the ​International Olympic Committee’s 2023 consensus statement​ distinguishes between adaptable and problematic low energy availability.

• Adaptable low energy availability is a temporary period of low energy availability with generally mild effects that are quickly reversible. For example, dieting to lose body fat might cause some short-term hormonal changes that return to normal once you end the diet and return to maintenance.

• Problematic low energy availability, on the other hand, occurs during prolonged or severe underfuelling and can cause more serious disruptions to physiological functions, like menstrual irregularities, hormonal disturbances, worse recovery, or a decline in training performance. Unlike the temporary changes associated with adaptable low energy availability, these effects can take much longer to reverse.

And while we’re on the topic of exercise, we should quickly address the claim that women shouldn’t exercise fasted because, yep! You guessed it, it messes with their hormones.

A ​2026 review​ investigated this claim by examining studies comparing exercise performed before and after eating. They found no consistent evidence that fasted training negatively affected muscle growth, strength, fat loss, or other measured training outcomes in women. They also found no convincing evidence that fasted exercise causes uniquely harmful hormonal responses in women, including the commonly cited ​concern about cortisol.​

To quote the researchers:

Fasted training is safe for most healthy women; choose fed or fasted sessions based on comfort, performance goals and overall daily energy needs.

Of course, this doesn’t mean you have to train fasted. But if you prefer to train fasted, then you can keep doing it without worrying about crashing your hormones or impacting your progress. Likewise, eating before or around training may make more sense for more demanding sessions, especially if you’re training more than once a day or already struggling to eat enough.

Now, let’s bring this back to women and fasting

Consider a hypothetical scenario.

A normal-weight woman wants to lose body fat, so she decides to try intermittent fasting. She skips breakfast and eats just two meals per day. She isn’t tracking her calorie intake, so she doesn’t know how much she’s eating. On top of that, she wants to lose fat as quickly as possible, so she’s also exercising a lot.

After a while, she starts noticing that her periods are becoming irregular, her mood is shit, she’s constantly tired, and her training performance is getting worse. Naturally, she starts wondering whether fasting is messing with her hormones.

From the outside, without understanding energy availability, it’s easy to point to the most obvious culprit: her fasting habit. But the problem could be that she’s eating too little while exercising too much. Sure, fasting could contribute by making it harder to eat enough, but she could run into the same issue even if she ate three or four meals and was still underfuelling. In other words, fasting might be how she ends up eating too little, but it isn’t necessarily the cause of the underfuelling.

All said, there’s no convincing evidence that intermittent fasting is inherently harmful to women’s hormones or reproductive health. What we have much stronger evidence for is that consistently underfuelling your body, especially when combined with high levels of exercise, can have negative consequences for hormonal and reproductive health.

So, if you’re a woman who finds intermittent fasting helpful for managing your calorie intake, or it just works better for your schedule, I don’t really see a reason to stop. Likewise, if you don’t enjoy fasting, don’t do it.

Concerns about hormones, disrupted cycles, and reproductive health have less to do with when you eat, and more to do with whether you’re fuelling appropriately for your activity levels.

As long as you eat enough to support your health and activity levels and don’t combine aggressive calorie restriction with excessive exercise, how you distribute your caloric intake throughout the day is entirely up to you, man or woman.

TL;DR

There’s no convincing evidence that intermittent fasting inherently harms women’s hormones or reproductive health. The bigger concern is consistently eating too little relative to how much you’re exercising, which can disrupt hormonal and menstrual function regardless of when you eat. As such, you can choose an eating pattern that helps you fuel adequately and works for your health, training, and schedule.