Written by dietetic intern Kylee Henschel and Orli Rabin of ThrivingBiome
Intermittent fasting (IF) has been a popular weight-loss method for years, but for women in the cycling age (getting their period), especially those dealing with chronic gut and/or hormone-related symptoms, it’s not a one-size-fits-all solution or something we typically recommend to our clients. In this post, we’ll walk you through why this is, IF benefits and risks, and what the latest science reveals about its impact on women's hormones and overall health.
So what is intermittent fasting?
IF is an eating pattern that cycles between periods of eating and fasting rather than focusing on what you eat. It’s a relatively flexible approach (doesn't really put an emphasis on WHAT to eat) that has been shown (in smaller studies, mostly based on animals and men) to benefit weight management, metabolic health, and hormonal balance.
Popular methods include the 16:8 method (fasting for 16 hours, eating within an 8-hour window) and the 5:2 method (eating normally 5 days/week and reducing calorie intake on 2 non-consecutive days).
Potential benefits:
When used in an appropriate context, research has found some potential benefits to intermittent fasting such as the ones briefly mentioned above: weight loss, support with cognitive and heart health, improved blood sugar regulation, aid in microbiome healing, longevity boosting, and various other health benefits. Let's take a look at some of the most popular and researched populations benefiting from intermittent fasting:
Cancer:
A recent comprehensive literature review on fasting and its role in cancer prevention and treatment revealed that integrating prolonged periodic fasting with conventional cancer treatments enhanced survival outcomes, improved treatment effectiveness, and minimized side effects in cancer patients. By fasting, cancer patients activated the autophagic process, leading to a variety of mechanisms that resulted in cell death. This was the body’s way of cleaning out damaged or unnecessary cellular components. Increased autophagy helped remove the damage and oxidative stress that led to mutations and cancerous growth. It also enhanced chemotherapy effectiveness by promoting cellular clearance.
PCOS:
Polycystic ovarian syndrome is a hormonal disorder where women produce excess hormones called androgens. This leads to anovulation, fertility issues, and the development of small cysts on the ovaries. Studies showed that intermittent fasting decreased high levels of estrogen, testosterone, DHEA, and metabolized cortisol, leading to improvement in PCOS symptoms. A study of 15 women with PCOS following a 16:8 fast over 6 weeks showed improvement in menstruation, insulin resistance, chronic inflammation, and weight/body fat. The most common symptom of PCOS is irregular periods, and 70% of the women in the study reported an improvement in menstruation. Unfortunately, this was a small study, so we could not be certain how all women would respond. While there were possible benefits to intermittent fasting in PCOS, it is important to look at the big picture, especially when considering a symptomatic population with co-morbidities and/or a history of caloric restriction and yo-yo dieting. The female physiology is sensitive to dynamic changes in metabolism. Restriction of energy intake often negatively impacts this delicate biology. To shift the body’s mode from survival to healing, it needs practices that enhance its feeling of safety and provide it with the essential building blocks for functions like metabolism, energy production, fertility, and detoxification. Additionally, for those with a history of restrictive eating, IF often triggers a return to restrictive behaviors and compromised macro- and micronutrient intake, causing deficiency and physical exhaustion.
Obesity:
Another study examined obese individuals on an 8 am-2 pm eating schedule and found that after just 4 days, participants had lower ghrelin levels and reported fairly low hunger. Researchers determined that not eating for a duration of time prevented insulin secretion and blood sugar fluctuations, so less hunger was experienced. The purpose of this study was to show that during fasting, hunger levels did not increase but rather decreased. This was beneficial for individuals who were overweight or obese by reducing hunger and calorie intake, ultimately supporting weight loss. However, one danger with recommending IF for obesity is the tendency to overcompensate for a missed meal. Often, if people skip breakfast, they consume excess calories during their lunch or dinner, which negates the weight loss and causes a variety of other issues.
Heart Health:
A systematic review conducted in 2024 evaluated randomized controlled trials conducted within the past five years to assess the impact of various fasting methods on cardiovascular health. IF was associated with significant reductions in low-density lipoprotein (LDL) cholesterol and triglyceride levels, alongside increases in beneficial high-density lipoprotein (HDL) cholesterol. Additionally, IF contributed to improved blood pressure regulation, reduced body weight, enhanced insulin sensitivity, and decreased inflammatory markers—all crucial for reducing cardiovascular disease risk. The authors concluded that these fasting regimens generally improved cardiovascular risk factors, notably by lowering LDL cholesterol, blood pressure, and body weight. Despite these promising findings, they emphasized the necessity for larger-scale, long-term clinical trials to confirm and better understand the extent and mechanisms of fasting's cardiovascular benefits.
Potential risks:
People often blindly start intermittent fasting, without the knowledge to do so safely, in a way that supports their own unique needs, goals, and metabolism. Before we get into that though, it’s worth noting that, as mentioned briefly above, most of the studies conducted on the topic of IF have very small sample sizes, and have been done on animals or men. This is important because men and women have distinct metabolic needs, hormonal profiles, and physiological differences, which impact how they tolerate fasting. This leaves significant gaps in understanding how IF truly impacts female physiology.
Beyond the research limitations, many people turn to intermittent fasting not because it’s the best approach for them, but because it’s an easy shortcut out of a stressful morning routine—it's far simpler to skip breakfast, grab a coffee for a quick energy boost, and rush out the door than to take the time to plan, prepare, and eat a nutritious meal. However, the common boast of having no morning appetite often signals a struggling metabolism, relying on “fake fuel” like caffeine, stress, and elevated cortisol, which provide a temporary sense of alertness before ultimately leading to a crash. Other potential risks to intermittent fasting include binge eating, metabolic disruption, lost menstrual periods, early onset menopause, anovulation, and a variety of hormonal irregularities. As noted above, the female physiology is distinct and unique from that of a male. Significantly more research is needed to know exactly what IF does in the female body. The next section will dive deeper into the specifics of intermittent fasting and women’s health, exploring how it interacts with hormones, metabolism, and long-term well-being.
How does IF affect women’s hormones?
Women’s bodies are highly sensitive to both the frequency and quantity of energy intake, and maintaining a consistent fuel supply is essential for proper hormonal function. Unlike men, whose bodies can often adapt more easily to shifts in metabolism, women’s reproductive systems are particularly vulnerable to energy fluctuations. When food is consumed, the body prioritizes where that energy is directed, and for women, reproduction is near the bottom of the list. If energy intake is inconsistent or insufficient, hormonal function is one of the first systems to be affected. Unfortunately, diet culture has glorified under-eating, making many women feel proud of skipping meals and restricting food intake. Regardless of their weight, most women are not consuming enough high-quality nutrients to support optimal health. Intermittent fasting, while appealing for its simplicity, can easily become a socially acceptable excuse for not eating, leading to chronic energy deficits and metabolic disruption.
One specific hormone commonly disrupted by fasting is gonadotropin-releasing hormone (GnRH). This hormone is released in pulses by the hypothalamus as part of the hypothalamic-pituitary-gonadal (HPG) axis, a critical system that regulates reproductive function. GnRH release is highly sensitive to energy intake, exercise levels, and environmental factors. Research has shown that just three days of fasting can disrupt its delicate balance. GnRH plays an essential role in signaling the pituitary gland to release luteinizing hormone (LH) and follicle-stimulating hormone (FSH), two key regulators of the female reproductive cycle. Without proper LH and FSH function, follicle development in the ovaries and ovulation cannot occur. In other words, when fasting places too much stress on the body, the reproductive system may shut down entirely as a survival mechanism.
Understanding the unique metabolic and hormonal needs of women is crucial when considering intermittent fasting. We aim to communicate a safe environment in the body to allow for healing in our clients. Consistent food intake provides the necessary nutrients for survival, healing, and thriving. While IF may work for some, it carries significant risks, particularly for reproductive health in women.
How to fast safely:
Given the potential downsides of intermittent fasting, anyone who chooses to follow it should ensure they do so in the safest way possible. It is also essential to know when your body is reacting negatively. Symptoms of irregular periods, constantly feeling cold, issues sleeping, changes in digestion, changes in hair and skin, mood swings, and brain fog indicate that you should immediately stop fasting and resume a normal diet.
How we implement IF in practice:
As counterintuitive as it sounds given the above information, our clients (and anyone who is eating during regular hours) typically follow a small fast by default. Nighttime fasting is done by avoiding food intake a few hours before bed. Ideally, this creates a 12-14 hour fasting window between dinner and breakfast, without risking the loss of a meal. We always encourage clients to prioritize breakfast and if needed, eat dinner earlier. Research suggests that avoiding food for the three hours before bed may improve sleep quality and support overall health. This type of fast puts emphasis on maintaining consistent energy intake to prevent the dangers of fasting.
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Conclusion:
As with anything in nutrition, intermittent fasting is nuanced. In general, we would not recommend intermittent fasting to people with chronic disease, hormonal disruption, high-stress life, etc. In these cases especially, the priority needs to be getting adequate nutrients into the body to provide the building blocks for healing. Intermittent fasting has promising research but it also has risks for many populations. At this point in research, we lack predictability and have learned that everyone’s bodies will respond differently. There is room for it in certain areas and danger in others. Our primary goal with our clients is to communicate safety to the body to allow for healing. The best way to achieve this is by building up the body through sufficient food intake and nutrient consumption.
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