Fasting for non-religious reasons – and whether it’s good or bad for you – has become a hot topic in the health and fitness space. Everyone has an opinion and they seem to be getting more and more passionate about it by the day.
Jump into the comments section of an Instagram post of fasting and you’ll see what I mean.
Personally, I wake up HUNGRY.
My sister can’t look at food before noon.
I feel like crap if I don’t eat before working out.
My friend Jules feels like crap if she does eat before working out.
As far back as 2019, “intermittent fasting” was one of the most searched diet-related terms on Google and the topic only seems to have heated up since then.
With all the hype around fasting, especially for women, it’s easy to get caught up in the trend. But here’s the thing: fasting for women isn’t as simple as it may seem.
After diving into the research, my biggest takeaway is this: No matter your opinion on fasting, you will likely find research to support your point of view. But when it comes to fasting for women, we need to consider some important factors, from the research gaps to how fasting impacts active women’s needs for energy, muscle recovery, and hormonal balance. Here’s what you should know.
What Is Fasting, Really?
Fasting simply means taking a break from eating (and sometimes drinking) for a set period of time.
Technically, we all fast overnight while we sleep—and that fasting window is actually important! It gives our bodies time to rest, repair, and tap into stored energy instead of constantly processing new food. And breakfast is literally the meal when we break the overnight fast.
This natural “pause” requires that our bodies use up glucose and fatty acids circulating in our blood and tap into energy stores. It trains our bodies to get better at processing energy.
But What About Longer Fasts?
Some people intentionally stretch that overnight fast into a longer break, a practice known as intermittent fasting (IF). One of the most popular versions is called time-restricted eating (TRE), where you limit your eating to a specific window – often around 8 hours (or less) – and fast the other 16 hours of the day.
For example, you might eat dinner at 6 p.m. and then not have any more calories until 10 a.m. the next day—so that you’re fasting during periods of low or reduced activity.
There is a Lack of Research on Fasting for Women
Most of the research on fasting has been done on men and many have only been done on animals. People who support fasting often make the leap that findings of those studies apply to women as well, but that’s a pretty big leap because women’s bodies respond differently than men’s (or mice)—especially when it comes to reduced calorie intake, our hormonal cycles, and exercise.
The vast majority of studies on fasting for women focus on sedentary women and those considered overweight or obese. But with small sample sizes it’s hard to generalize the findings to active women and those who are not considered overweight or obese.
We do see that for those who are inactive or sedentary, studies have shown fasting can be a helpful stressor on the system that helps improve insulin sensitivity, glucose metabolism, among other things.
But if you are an exerciser or an active women or in perimenopause, there is a major gap in the research that makes it even harder to apply the current fasting research to your life—especially when it comes to understanding how fasting affects exercise performance and recovery.
Fasting for Weight Loss
The research on fasting and weight loss is quite clear: at the end of the day, when it comes to weight loss, fasting is essentially just a method of calorie restriction. While proponents claim fasting has specific metabolic benefits, there’s no solid evidence showing that it’s any more effective than other forms of calorie control, like portion control, tracking your food intake, or balanced eating.
Fasting is just a method of reducing your calorie intake by using the clock to limit your intake. And when calorie intake is controlled for, fasting isn’t better than other “diets” for achieving long-term weight loss or health improvements.
Fasting and Exercise
Combining fasting and exercise is where it gets tricky—especially for women. Primarily, this is because fasting and exercise do similar things—they both decrease your blood glucose and other important molecules circulating in your blood.
So when you combine them and exercise while fasted, you might end up with some issues.
First, for both men and women, exercising fasted could make your blood sugar levels go too low and leave you trying to literally run on an empty tank. No gas makes it really hard to work out or work out hard enough to get the results you’re after. It could also make you dizzy and pass out.
Second, fasted exercise is associated with decreased performance, increased fatigue, reduced skill, impaired concentration, and an increased perception of effort (so exercise feels harder than it should). This makes it less likely that you’ll have good training sessions or even want to work out.
Additionally, muscle preservation is essential for overall health and metabolism, and fasting alone may not do the job as well as other eating strategies that prioritize protein and carbohydrates for muscle recovery and growth.
For women who do choose to fast or workout fasted, it’s important that they continue to focus on eating adequate food – especially protein and carbs – throughout the day during their feeding window to minimize the risk of low energy availability and prevent muscle breakdown and loss.
Self-Awareness: The Key to Making Smart Fueling Choices
Whether you’re fasting or following another eating plan, self-awareness is critical. Everyone’s body is different, and that means tuning into your personal needs and how your diet (and exercise) impacts your energy, mood, and performance.
And context matters—so remember that it’s not just about skipping that one meal, it’s about how you’re balancing your needs across the day as well. So what works for one person, may not work for you. And what used to work for you, may one day no longer be appropriate or effective.
Some women thrive on fasting, but many others find that under-eating or fasting interferes with their workouts and recovery.
Everyone’s body has unique needs, and there’s no one-size-fits-all approach.
When I brought up the topic of fasting to my colleague, Jules Mitchell, author of Yoga Biomechanics: Stretching Redefined, she shared how challenging it can be for people who weren’t athletes growing up to recognize their body’s signals.
“The older I get, the more privileged I feel for having played sports as a kid and spent my childhood running races. At eight years old, I learned that a half banana before a 10k would make me sick. I’ve been paying attention to my body’s signals my whole life. But if you’re just now in perimenopause and getting into exercise, all the mixed messaging must be so confusing.”
Her experience reminds us that self-awareness isn’t just about noticing what we feel today—it’s about understanding how our bodies respond over time, through different phases of life and activity.
If you’re constantly feeling tired, weak, or sluggish during your workouts, it might be a sign that you’re not fueling your body properly.
Self-awareness is about listening to your body and making adjustments based on how you feel, not just following the latest trend.
Many Active Women Are Often Under-fueled
One of the key points that often gets overlooked when it comes to fasting for women is the issue of under-fueling.
Many active women underestimate their total daily energy expenditure (thanks to all those silly 1200-1500 calorie diet recommendations that we grew up with) and end up eating too little to meet their energy demands. This is especially true for women who do regular strength training, high-intensity workouts, or endurance training.
A 2016 study conducted on 109 female recreational exercisers found that 45% were at risk for low energy availability – that’s nearly half of the women recruited from local gyms and fitness centers were at risk due to under-fueling.
So under-fueling and low energy availability aren’t just a concern for “high-level” athletes, any exercising woman can be at risk.
Under-fueling can lead to poor recovery, decreased performance, and even hormonal imbalances. Women who train hard need to ensure they’re consuming adequate carbohydrates and protein to fuel their workouts and recover effectively. Without the proper nutrients, your body can’t rebuild muscle, and you may experience fatigue, mood swings, and hormonal disruptions.
In my work as a personal trainer helping women with osteoporosis build and maintain bone mass, most of my discussions with clients surround eating more, not less.
Nutritional Therapist Michaela Smith agrees. She shared her thoughts with me:
“The vast majority of women I work with are under-fueling already, even those who are looking to lose weight! Under-eating is a significant stressor that can manifest into hormonal imbalance, weight changes, or even cognitive changes.”
In her practice, she generally does not recommend intermittent fasting for women. Here’s why:
- Many women are already under-eating, which can contribute to anxiety, brain fog, overwhelm, and hormonal imbalances.
- Fasting can negatively affect blood sugar, leading to:
- Drops in blood glucose
- Release of stress hormones
- A rebound spike in blood sugar
- A “blood sugar roller coaster” that leaves women feeling off balance in terms of energy and food cravings
- There is limited research on the effects of fasting on women, making long fasts a questionable strategy.
Instead, Michaela recommends the following approach to support steady energy and hormonal health:
- Eat a balanced breakfast within 30–60 minutes of waking.
- Fuel consistently throughout the day with regular meals and snacks.
- Finish dinner early enough to allow time to digest and benefit from a gentle overnight fast.
- Aim for a 12-hour window between dinner and breakfast—this supports energy metabolism and gut health without over stressing the body.
“The long-term results I see with the women I work with are more even energy, fewer jittery feelings, improved blood sugar response, and more stable weight. All wins in my book!”
When You Eat Matters, But So Does What You Eat
While timing your meals or only eating within a specific window might seem trendy, it’s more important to focus on what you eat and whether it meets your body’s needs. Whether you’re eating before or after your workout, make sure you’re getting enough carbohydrates to fuel your exercise and protein to recover.
Carbohydrates are your body’s main source of fuel for exercise, and you need to replenish glycogen stores after workouts. The amino acids in protein are the building blocks for all tissue and helps rebuild muscle, maintain bone, and support recovery.
For active women, this balance is key—no matter when you eat. It’s all about giving your body the nutrients it needs to recover and perform at its best.
If you’re not used to eating before you work out, but you want to try it, start small with just 100 calories of easily-digestible, low-fiber carbs or a sports drink to avoid GI distress. Then, over time gradually increase the calories to train your system to tolerate food at that time of day.
Fasting and Hormonal Health: A Complex Relationship
For women with polycystic ovary syndrome (PCOS), intermittent fasting has shown some promising benefits in improving insulin sensitivity, regulating blood sugar levels, and promoting hormonal balance. Several studies have suggested that fasting can reduce insulin spikes, a key issue in PCOS, and may help alleviate symptoms like irregular menstrual cycles, weight gain, and acne.
However, it’s important to note that most of the research on fasting for PCOS has been conducted on overweight or obese premenopausal women and the findings are still emerging. While the results are encouraging, more research is needed to understand how intermittent fasting impacts women with different body types and at various stages of life, including those who are active and in perimenopause or beyond.
For active women, especially those in perimenopause, fasting can impact hormonal balance in ways that may not be immediately apparent. Low energy intake, especially when combined with lots of exercise, can lead to low energy availability and/or ultimately Relative Energy Deficiency in Sport (RED-S), a condition that affects reproductive, metabolic, and bone health. Signs of RED-S can include menstrual irregularities, fatigue, decreased bone density, and poor recovery.
And here’s the kicker: during perimenopause, distinguishing between expected hormonal changes and the signs of RED-S can be difficult.
If you’re experiencing fatigue, poor recovery, or irregular periods (which many women won’t even be able to tell if they’re using birth control that alters their period), it could be a sign that you’re not fueling enough to support your workouts and overall health.
Adequate fueling, especially with carbohydrates, is especially important in midlife to support both training and hormonal health.
Conclusion: Fasting for Women—A Personalized Approach
Fasting may work for some women, but it’s important to remember that, when it comes to weight loss, it’s just one method of calorie restriction and it hasn’t been proven to be more effective than other dietary strategies.
For active women, self-awareness and paying attention to how you feel during workouts and recovery are crucial. Before considering a fasting routine, make sure your body is well-fueled.
A balanced intake of carbohydrates and protein is essential—especially to support your workouts, recovery, and overall well-being. Under-eating can make fasting counterproductive—increasing stress on the body rather than supporting health.
And if you’re very active or an athlete with goals—working toward a PR in your lifts or training for a competition or race—you may want to leave fasting in the dust as it can decrease your performance and inhibit your recovery.
There’s no one-size-fits-all approach. The best strategy is well-thought-out and supports your energy, performance, and long-term health—not one that leaves you depleted.
And be aware that there is a growing number of “doctors” and experts out there who are speaking about fasting (and profiting from it) even though it is very much out of their own scope of practice. So before diving in head first, be sure you check what kind of doctor they are and verify that you only take advice from qualified nutrition experts.
If you’re unsure where to start, consider getting support from a registered dietitian or nutritionist who specialize in working with women in your specific population (active, endurance athletes, and/or perimenopausal women) to help tailor a plan that works for you. —Karin
References
- Vitality Pro. (2019). “Intermittent Fasting: A Search Trend.” Vitality Pro Nutrition Statistics. Link
- Cienfuegos, S., et al. (2022). “Intermittent Fasting and Reproductive Health: The Potential Effects.” Nutrients.
- Correia, M., et al. (2020). “Intermittent Fasting and Aerobic Performance.” Journal of Sports Sciences.
- Batitucci, G., et al. (2022). “The Impact of Fasting on Muscle Mass and Body Composition.” Frontiers in Nutrition.
- Zouhal, H., et al. (2020). “Effects of Fasting on Exercise Performance: A Review.” Sports Medicine.
- Mellin, A. (2025). “Carbohydrate Availability and Menstrual Health in Active Women.” Journal of Sport Nutrition and Exercise Metabolism.
- Aird, L., et al. (2018). “Fasted vs Fed Exercise: The Impact on Fat Metabolism and Performance.” International Journal of Sports Physiology and Performance.
- Slater, J., et al. (2016). “Female Recreational Exercisers at Risk for Low Energy Availability.” International Journal of Sport Nutrition and Exercise Metabolism.
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