Written by: Michelle Severs, MS
Reviewed by: Emily J., MSc RD
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Intermittent fasting has potential benefits for weight loss and stabilized glucose levels, but emerging research suggests it may affect women differently than men. Learn what the science says — and how to incorporate intermittent fasting into your routine.
One of the most popular diets you’ve likely heard about recently is intermittent fasting (IF). But even if you’ve seen social media posts about it or know friends who’ve tried it, you may still be wondering exactly what it is .
Intermittent fasting is a form of time-restricted eating that limits meals to a defined window of time every day. Unlike calorie restriction, it focuses on when to eat as opposed to what to eat.
The term has a polarizing response in the scientific community: some experts point to the health benefits of fasting — such as improved blood glucose control, more efficient metabolism, and weight loss — while others believe it can affect women differently than men.
New research suggests women may indeed be more sensitive to intermittent fasting, and experience effects on their glucose regulation, menstruation, and hormones. Here, we’ll explore how IF affects women differently, the signs it’s working (or not working), and how to incorporate it into your life in a way that works best for your body.
Intermittent fasting has been shown to improve blood sugar levels, reduce insulin resistance, and promote weight loss in both men and women . A few randomized clinical trials in humans suggest that weight loss associated with IF may be due to reducing calorie and energy intake by 10% to 30% compared to eating a diet that isn’t limited to a window .
But early research presents a more complicated view of intermittent fasting when it comes to women. One small study from 2011 focusing largely on women revealed that fasting throughout the day and eating at night resulted in weight loss, smaller waist circumferences, and improved blood glucose and HbA1C levels .
However, another small study from 2012 indicated that while IF improved insulin sensitivity in men, it actually worsened blood sugar control in women .
During a three-week trial of alternate-day fasting, non-obese women experienced impaired glucose response after each meal, while men’s glucose response didn’t change and their insulin sensitivity improved. This study only looked at the alternate-day fasting method, so we can’t draw conclusions about all types of fasting based on this research. However, these results suggest that alternate-day fasting has a clear effect on women’s metabolic health, and it may be too severe a fasting method for some women.
Intermittent fasting may also influence cortisol, a stress hormone that helps break glucose down to maintain blood sugar .
Cortisol has a number of standard side effects: it slows down digestion, increases blood pressure, and raises blood sugar. But in women, chronically high levels of cortisol can delay ovulation and menstruation — or even cause you to miss it altogether .
This happens because cortisol, which is produced by your adrenal glands, diverts your body’s attention and resources to focus on survival — including shifting hormone production directly related to menstruation .
In fact, one study found that increased cortisol levels were associated with functional hypothalamic amenorrhea, or FHA, in women (i.e., missed periods due to stress) . FHA accounts for about 30% of cases where women who have already begun to menstruate miss their periods for 3 or more months.
Cortisol levels start to rise within 12 hours of fasting and remain higher than non-fasting levels for the duration of IF .
It’s unclear whether alternating fasting every other day or fasting once a week leads to long-term effects, but the point is the same: if you are practicing intermittent fasting and start to miss your period, the fasting plan you’re following is likely too severe.
You may not be getting enough calories and nutrients, are developing high cortisol, or both. If this occurs, speak with your healthcare provider, and try a less restrictive fasting plan.
Studies in female rats suggest that IF is associated with higher levels of testosterone, smaller ovaries, and irregular reproductive cycles .
According to a recent study in humans, IF may impact dehydroepiandrosterone (DHEA), a hormone integral to egg quality and ovarian function . In both pre- and post-menopausal women who stuck to 4-6-hour eating windows, DHEA dropped by 14%. DHEA levels were still within normal ranges despite the drop and other reproductive hormones like estrogen and progesterone were unaffected, but the results suggest that fasting impacts reproductive structures and processes.
Studies also show that during the luteal phase of your menstrual cycle, or the phase right before your period, some women burn more calories at rest, due to the increased energy needs at this part of the cycle . While the amount of energy burned varies (anywhere from 1.7% to 10% more energy burned), if you feel especially fatigued and sluggish before your period, this may be a time to avoid fasting and focus on adequate nourishment and rest, which helps to keep cortisol levels low.
Signs that intermittent fasting is working are steady weight loss (0.5-2 pounds per week is considered healthy), a shrinking waist circumference, and more stable glucose levels — which you can monitor using a continuous glucose monitor .
If you are practicing intermittent fasting and notice a disruption in your menstrual cycle, experience disturbed sleep or fatigue, or have nausea and dizziness, your body might not be adapting to IF.
These symptoms tend to occur when your body doesn’t get enough calories. If you notice any of these symptoms, you should pause IF and return to the eating regimen that previously worked best for you.
If you’re pregnant, breastfeeding, or have a history of disordered eating, check in with your medical provider before trying any form of intermittent fasting.
A more relaxed approach to IF may be best for women in order to optimize the potential health benefits and minimize any adverse effects. Some ways IF can work for women is to have longer eating windows or fewer fasting days. For example, you may choose to a 10-12 hour window of eating, rather than 4-6 hours, which is more extreme. Or you can elongate the fasting time between each meal.
If your body tolerates these relaxed approaches to IF, then you might consider stepping up to more advanced methods.
Some IF methods that have worked for women include:
Also known as Leangains protocol, the 16:8 method is fasting for 16 hours a day and creating an eating window for the remaining 8 hours. Some fitness professionals recommend that women start with 14-hour fasts at the beginning to make sure their bodies respond positively to the fasting protocol before moving on to 16 hours . To practice the 16:8 method, try eating only between 10am and 8pm, and fasting for the remaining 10 hours.
This form of IF involves fasting for one day (24 hours) once or twice a week. Typically, this 24-hour fast is continuous, meaning you do all 24 hours in one go. But since cortisol levels increase within 12 hours of not eating, it may be best to spread out the 24 hours across two or three days to start so you’re eating something every day — which will help keep your cortisol low .
In other words, start by fasting for 8 hours three times per week (this might mean eating a hearty breakfast around 10am, skipping lunch, and having dinner at 6pm). Once you’ve tried this for at least one month and noticed no effect on your period or health, try going the full 24 hours.
It’s important to check in with yourself as you’re trying these methods. Ask yourself if you’re experiencing any of the signs that IF is working for you, such as healthy weight loss, more energy, or better glucose levels. Be mindful also of signs that it’s not working for you, such as nausea, unusual tiredness, or disrupted menstrual cycles. There is no one-size-fits-all approach, and learning what’s best for you is essential.
Intermittent fasting can be an effective tool to regulate blood sugar levels and lose weight, but it’s not for everyone — and women in particular should start with milder forms of IF and gauge how their body tolerates fasting before moving on to more extreme forms.