Intermittent Fasting for Women: A Complete Guide to How It Works, What to Expect, and the Science Behind It

Intermittent fasting has gone from a fringe practice to one of the most widely discussed nutrition strategies in the world, and for good reason. The research has grown, the clinical evidence has matured, and millions of people have integrated some form of fasting into their daily lives. But I get a lot of questions from women who are not sure whether it is right for them, how to do it safely, whether it affects hormones, and how to make it sustainable over time. This is the guide I wish I could hand to every woman asking me about intermittent fasting. I am going to walk you through what the research actually shows, the different fasting approaches, what is realistic, what works particularly well for women, and how I have used intermittent fasting in my own practice for over a decade.

What Intermittent Fasting Actually Is

Intermittent fasting is not a diet in the traditional sense. It does not tell you what to eat. It tells you when to eat. The premise is that the timing of your meals matters as much as the content of them, and that giving your body extended windows without food triggers metabolic shifts that support fat burning, blood sugar regulation, cellular repair, and a number of other downstream benefits. There are several different fasting approaches, but they all share the same core idea: structured periods of eating alternating with structured periods of not eating.

What separates intermittent fasting from disordered restriction is the structure. Intermittent fasting is planned, time-bound, and supported by adequate nutrition during eating windows. It is not skipping meals because you forgot to eat. It is not eating less in general. It is creating predictable rhythms that the body can adapt to and benefit from.

Humans evolved with intermittent fasting built into daily life. Our ancestors did not eat from 6 a.m. to midnight. They ate when food was available, which often meant long gaps between meals and overnight fasting that stretched 12 to 16 hours. Modern eating patterns, with constant grazing and late-night snacking, are the historical anomaly. Returning to a more traditional eating rhythm is the foundation of every intermittent fasting approach.

The Main Types of Intermittent Fasting

There is no single intermittent fasting protocol. There are several, and they suit different lifestyles and different goals. The most important thing is to find one that you can actually follow consistently. The best fast is the one you will sustain.

Time-restricted eating (the 16:8 method)

The most popular and the gentlest entry point. You eat all your daily food within an 8-hour window and fast for the remaining 16 hours. For most people, this looks like skipping breakfast, eating lunch around noon, and finishing dinner by 8 p.m. Some women do better with an earlier window, eating from about 8 a.m. to 4 p.m., which research suggests may be slightly more effective for metabolic outcomes. This approach requires almost no change to your weekly schedule and most people find it surprisingly easy to maintain.

5:2 fasting

Five days of regular eating per week and two non-consecutive days of significantly reduced intake, typically around 500 to 600 calories. The 5:2 structure is the framework I built the Bone Broth Diet around, where the two "fasting" days are anchored by bone broth and a few small protein-based meals. A 2025 systematic review and network meta-analysis published in the BMJ analyzed 99 randomized clinical trials with 6,582 adults and found that whole-day fasting approaches like 5:2 produced significant weight loss and cardiometabolic improvements, particularly in trials lasting 24 weeks or more1.

Alternate-day fasting

You alternate between regular eating days and significantly restricted days (usually 500 calories or less). More aggressive than 5:2. Effective for weight loss in the short term but harder to sustain for most people in the long term. I rarely recommend this as a starting point. The reduced eating days can be hard on women, particularly women in the second half of life, and the on-off pattern can feel disruptive.

OMAD (one meal a day)

One large meal per day, no other intake. Some people thrive on this. Most do not. OMAD is the most extreme of the commonly discussed approaches and I would not recommend it for women without specific guidance from a healthcare provider, particularly women who are still cycling, women with thyroid concerns, or women in perimenopause.

Extended fasting

Fasts lasting 24 hours, 48 hours, 72 hours, or longer. Extended fasting has specific therapeutic uses, particularly in research contexts around autophagy and metabolic reset, but it is not what most women asking about intermittent fasting are looking for. Extended fasts should only be done with medical supervision.

What the Research Actually Shows

The research on intermittent fasting has come a long way in the last decade. We now have hundreds of randomized clinical trials, multiple systematic reviews, and a clearer picture of what fasting actually does in the body. Here is what the most rigorous evidence currently shows.

Weight loss and body composition

The 2025 BMJ network meta-analysis is currently the most comprehensive synthesis of intermittent fasting research1. It analyzed 99 randomized clinical trials with 6,582 adults of varying health conditions. The findings: intermittent fasting strategies produced weight loss and cardiometabolic benefits broadly comparable to continuous calorie restriction. Whole-day fasting (the 5:2 framework) showed particular promise for trials lasting 24 weeks or more. Alternate-day fasting showed additional advantages for triglycerides and blood pressure. The bottom line: intermittent fasting works for weight loss, and it appears to work at least as well as traditional calorie restriction with the added benefit of a simpler structure that is easier for many people to follow.

Cardiometabolic markers

Intermittent fasting has been documented to improve fasting glucose, fasting insulin, triglycerides, blood pressure, and inflammatory markers across multiple meta-analyses1. The 2025 clinical trial of the Bone Broth Diet, which uses a 5:2 intermittent fasting structure, documented significant reductions in fasting glucose, a 0.43 mmol/L decrease in triglycerides, and improvements in quality-of-life measures across multiple domains3. These cardiometabolic benefits are often what motivates clinicians to recommend intermittent fasting alongside or instead of traditional calorie restriction.

Muscle preservation when done correctly

One of the concerns women raise about intermittent fasting is muscle loss. The research is clear: when intermittent fasting is paired with adequate protein intake, muscle mass is preserved. A 2024 meta-analysis in Clinical Nutrition ESPEN found that protein intakes of at least 1.3 grams per kilogram of body weight per day significantly preserved muscle mass during weight loss5. Additional research from the University of Pittsburgh published in 2024 documents how fasting interacts with protein metabolism at the level of bone remodeling and muscle preservation4. The takeaway: prioritize protein during your eating windows and muscle preservation is achievable.

Cellular and metabolic effects

During extended fasting windows, the body shifts from primarily burning glucose for fuel to burning stored fat, which is what produces the metabolic benefits associated with intermittent fasting. This metabolic switch typically begins around 12 hours of fasting and deepens over the next 12 to 24 hours. The body also activates autophagy, a cellular cleanup process where damaged cellular components are recycled. The research on autophagy in humans is still being clarified, but the mechanism is well-established in animal studies.

Intermittent Fasting and Women’s Hormones

This is the question I get asked most often, and it deserves a careful, honest answer. The concern is real. The conclusion is more nuanced than either side of the debate suggests.

The most rigorous review of intermittent fasting and reproductive hormones in women was published in 2022 in Nutrients by researchers at the University of Illinois at Chicago, the same group that has published much of the leading IF research2. The review examined the effects of various intermittent fasting protocols on female reproductive hormones in human trials. The findings: intermittent fasting did not affect estrogen, gonadotropin, or prolactin levels in cycling women. It did appear to decrease androgen markers like testosterone and the free androgen index, particularly in premenopausal women with obesity, and to increase sex hormone-binding globulin, which is generally a positive cardiometabolic signal.

The takeaway: in healthy adult women, the documented evidence on intermittent fasting does not show significant disruption of estrogen, menstrual cycles, or core reproductive hormones. The widespread concern about fasting harming female hormones comes primarily from a single rat study in which young rats were fasted for 24 hours every other day for 12 weeks. Rat physiology is not human physiology. Human studies do not show the same effects.

That said, here is who I would have go more slowly

• Women trying to conceive. If pregnancy is the goal, this is not the time for any restrictive eating pattern. Adequate caloric intake and nutritional density are the priorities.

• Women who are pregnant or breastfeeding. Avoid intermittent fasting during these phases.

• Women with a history of disordered eating. Intermittent fasting can become a trigger for anyone with this history.

• Women with thyroid issues, particularly hypothyroidism. Aggressive fasting can sometimes affect thyroid function. Go gently and work with your physician.

• Women who are highly underweight or athletes with low energy availability. More restriction is not what your body needs.

• Women with adrenal dysfunction or chronic stress. Adding the metabolic stress of fasting on top of an already taxed system is not always the right move.

For most women, here is what works

Start with the gentlest approach. A 12 to 14 hour overnight fast is something most women adapt to easily and that delivers metabolic benefits without significant stress. Move to 16:8 only if 12 to 14 hours feels comfortable and you have been doing it for a few weeks. Move to 5:2 only if 16:8 is working well. Building up gradually is more sustainable and gentler on the body than jumping straight to aggressive protocols.

During perimenopause and menopause, intermittent fasting can actually be particularly helpful for the metabolic shifts that come with hormonal change. Insulin sensitivity tends to decrease in midlife, and intermittent fasting is one of the most effective non-pharmaceutical strategies for improving it. The 5:2 framework I built into the Bone Broth Diet was designed specifically with women in midlife in mind.

What to Drink and Eat While Intermittent Fasting

During the fasting window

Strict fasting allows only water. But there is a wider window of options that most practitioners agree are acceptable for the metabolic and weight-loss goals most people are after.

• Water. Plain, sparkling, with a squeeze of lemon. The most important fluid during a fast.

• Black coffee. No cream, no milk, no sweetener. A small amount of caffeine does not break a fast and may actually support fat oxidation.

• Plain tea. Black, green, herbal. The same rules as coffee.

• Mineral salt or a pinch of sea salt in water. Particularly useful if you are doing longer fasts and want to support electrolyte balance.

Bone broth. This is the one that gets debated and the answer depends on your fasting goals. For metabolic fasting, where the goal is to stay in a fat-burning state, bone broth is generally considered acceptable because it does not significantly raise insulin or blood sugar. It also provides amino acids that support the body during the fasting window. For strict water-only fasting, bone broth does technically count as eating. I have written more on whether you can drink bone broth while fasting for a deeper answer.

Coffee questions in particular come up a lot. I have a dedicated piece on whether you can drink coffee while intermittent fasting that walks through what counts and what does not.

During the eating window

This is where intermittent fasting either succeeds or fails. The eating window is not a free-for-all. If you eat ultra-processed food, sugar, and refined carbohydrates during your eating window, intermittent fasting will not deliver the results you are hoping for. The structure of your meals matters as much as the timing.

• Prioritize protein. At least 25 to 35 grams per meal is the rough target for muscle preservation and satiety.

• Build your plate around non-starchy vegetables. Leafy greens, cruciferous vegetables, peppers, zucchini, asparagus, mushrooms.

• Include healthy fats. Avocado, olive oil, nuts, seeds, coconut oil.

• Be intentional with carbohydrates. If you are working out hard, you need them. If you are sedentary, you need fewer of them. Choose unrefined sources.

• Stay hydrated, including during your eating window. Many people confuse thirst for hunger.

I have written specifically on what to eat during intermittent fasting for a fuller meal-plan approach.

How to Start Intermittent Fasting Successfully

The first two weeks are the hardest. After that, most people find intermittent fasting becomes effortless. Here is how I would walk a new patient through starting.

• Pick your approach. For most women, I recommend starting with 12 to 14 hour overnight fasting. So if your last meal is at 8 p.m., your first meal the next day is at 8 to 10 a.m. This is gentle and almost effortless.

• Settle in for two weeks. Do not change anything else. Just establish the rhythm.

• Move to 16:8 only if you want to and only if 12 to 14 hours feels easy. The transition from 14 to 16 hours is usually the harder part. Use bone broth in the morning to soften the transition.

• Track how you feel, not just the scale. Energy, sleep, mood, digestion. The scale lags.

• Adjust if your body is unhappy. Female physiology is not linear. If your sleep gets worse, your cycle becomes irregular, or you feel persistently exhausted, scale back. Intermittent fasting is supposed to make you feel better, not depleted.

I have a separate piece on how to start intermittent fasting that walks through this more practically, and 6 ways to make intermittent fasting a breeze for the strategies that have helped my patients the most.

The Bone Broth Connection

I want to be transparent about why I am such a strong proponent of pairing intermittent fasting with bone broth. It is not coincidental that the Bone Broth Diet uses an intermittent fasting structure. Bone broth and intermittent fasting work together in a specific, complementary way.

When you are fasting, your body is using stored fat for fuel, which is the metabolic state you want. But fasting also produces amino acid demand, because the body still needs raw materials for the dozens of biochemical processes that do not stop just because you are not eating. Plain water fasting provides none of those raw materials. Bone broth provides them.

The amino acid profile of bone broth, particularly glycine, supports the body during the fasting window in measurable ways6. The minerals support electrolyte balance. The warm savory broth softens the experience of fasting and makes longer fasting windows easier to sustain. And crucially, bone broth does not significantly raise blood sugar or insulin, so the metabolic fasted state is largely preserved.

This is why I built the 5:2 structure of the Bone Broth Diet around bone broth as the daily anchor. The 2025 clinical trial of the Bone Broth Diet documented an average weight loss of approximately 15 pounds over 8 weeks with fat loss and muscle preservation, significant reductions in hunger and cravings, and improvements in quality of life3. If you want to combine intermittent fasting with bone broth as a structured program, the Bone Broth Diet is what I would point you to. If you want a more flexible approach, my piece on intermittent fasting with bone broth walks through the integration.

Common Mistakes to Avoid

I have seen the same mistakes derail intermittent fasting attempts over and over. Here are the ones to be aware of.

• Overeating during the eating window. The most common reason intermittent fasting does not work for someone is that they compensate during the eating window by eating dramatically more, often poorly. The eating window is not unlimited.

• Under-eating protein. Insufficient protein during the eating window leads to muscle loss, persistent hunger, and metabolic slowdown.

• Doing too much, too fast. Jumping straight to OMAD or aggressive protocols is a setup for failure. Build up.

• Ignoring sleep. Intermittent fasting and poor sleep do not mix well. If you are short on sleep, you will be hungry. Address sleep first.

• Combining with very high-intensity exercise. If you train hard and fast hard, something has to give. Pair intermittent fasting with moderate exercise, especially when starting out.

• Not adjusting for your cycle. Some women find that fasting is harder during certain phases of their menstrual cycle, particularly the week before a period. Lean into a shorter fasting window during the days when your body is asking for more support.

• Treating it as punishment. Intermittent fasting should never feel like deprivation as a moral act. It is a structural choice to support your health.

Tools to Support Your Fasting Practice

You do not need anything to do intermittent fasting except a clock and a willingness to commit. But there are tools that make the practice smoother, more sustainable, and more effective.

Bone broth as the daily anchor. A warm mug of Classic Chicken or Classic Beef bone broth is what gets most of my patients through their first few weeks of fasting. It is satisfying, soothing, and supportive without significantly disrupting the fasted metabolic state.

Capsules for travel and busy days. When you cannot drink a mug, the Bone Broth Capsules provide the same amino acid profile in a portable format.

A protein source for breaking the fast well. My Bone Broth Protein powder makes a fast breakfast or post-workout shake that supports muscle preservation without the bloat or crash that can come with whey.

A structured program if you want one. The Bone Broth Diet takes the principles in this article and turns them into a 21-day protocol with day-by-day meal plans. The 21-Day Bone Broth Diet Bundle has everything you need to begin.

Frequently Asked Questions

Is intermittent fasting safe for women?

Yes, for most healthy adult women. A 2022 review of intermittent fasting and reproductive hormones in humans found that intermittent fasting did not significantly affect estrogen, gonadotropins, or prolactin in cycling women, though it did appear to decrease androgen markers and increase sex hormone-binding globulin, generally positive cardiometabolic effects2. Women who are pregnant, breastfeeding, trying to conceive, have a history of disordered eating, have thyroid issues, or are highly underweight should be cautious or avoid intermittent fasting.

What is the best intermittent fasting window for weight loss?

The research suggests that whole-day fasting approaches like the 5:2 framework and time-restricted eating windows of 14 to 18 hours produce the most consistent weight loss results in trials of 24 weeks or more1. For most women, starting with a 14 to 16 hour overnight fast is the most sustainable entry point. I have a separate piece on the best intermittent fasting window for belly fat for more on the specifics.

Will intermittent fasting cause muscle loss?

Not if you eat adequate protein during your eating window. A 2024 meta-analysis found that protein intakes of at least 1.3 grams per kilogram of body weight per day preserved muscle mass during weight loss5. The Bone Broth Diet clinical trial specifically documented fat loss with muscle preservation3. Prioritize protein during your eating windows and muscle preservation is achievable.

Can I drink coffee while intermittent fasting?

Yes, black coffee with no cream, milk, or sweetener does not break a metabolic fast and may actually support fat oxidation during the fasting window. Adding cream, milk, or sweeteners technically breaks the fast. The same rules apply to tea.

How long does it take to see results from intermittent fasting?

Most people notice changes in energy, sleep quality, and bloating within the first two weeks. Visible body composition changes typically become clear by weeks 4 to 6. The 2025 BMJ network meta-analysis found that the most significant outcomes appear in trials of 24 weeks or more, suggesting that intermittent fasting is best thought of as a sustained practice rather than a short-term intervention1.

Is intermittent fasting good for perimenopause and menopause?

It can be particularly helpful in midlife because insulin sensitivity tends to decrease during perimenopause and menopause, and intermittent fasting is one of the most effective non-pharmaceutical strategies for improving it. The 5:2 structure I built into the Bone Broth Diet was designed with women in midlife in mind. Go gently, prioritize protein, and listen to your body. If something is not working, modify it.

Can I exercise while intermittent fasting?

Yes, and many people find that fasted exercise, particularly walking or light resistance training, actually feels good. Moderate exercise during the fasting window is generally well tolerated. Very high-intensity training is harder to do fasted, particularly for women. If you train hard, schedule your hardest sessions inside your eating window or break your fast 30 to 60 minutes before training.

Will intermittent fasting affect my menstrual cycle?

In most healthy women, the documented evidence does not show significant disruption of menstrual cycles from moderate intermittent fasting2. However, every woman is different. If you notice cycle irregularity, shorter cycles, or changes in menstrual flow after starting intermittent fasting, that is a signal to scale back. Listen to your body. Female physiology is sensitive, and you do not have to do this if it is not serving you.

Can I do intermittent fasting every day or do I need to take breaks?

Most people do best with daily time-restricted eating (16:8 or similar) as their baseline, with occasional breaks. Some women feel better cycling their fasting practice, fasting more aggressively for a few weeks and then easing off. There is no one-size-fits-all answer. The best practice is the one you can sustain.

Should I do intermittent fasting or follow a regular diet?

The 2025 BMJ network meta-analysis found that intermittent fasting and continuous calorie restriction produce broadly comparable weight loss results1. The advantage of intermittent fasting for many people is simplicity. You do not have to count calories or track macros if the eating window itself naturally moderates intake. Whichever approach you can actually follow consistently is the right one for you.

Where to Start Today

Intermittent fasting is one of the most flexible, sustainable, and well-researched approaches to eating in modern nutrition. It is not magic, and it is not for everyone, but for most healthy adult women it offers real metabolic benefits with relatively little disruption to daily life. Start gently. Build slowly. Use bone broth as a daily anchor during your fasting windows. Prioritize protein during your eating windows. Listen to your body and adjust. The goal is not perfection. The goal is a sustainable rhythm that supports your health for years, not weeks. If you want a structured way to begin, the Bone Broth Diet combines intermittent fasting with bone broth in a 21-day program that has been clinically tested. If you want to ease in more gently, start with a 14 to 16 hour overnight fast, add a daily cup of bone broth, and see how you feel after two weeks.

References

1. Semnani-Azad Z, Khan TA, Chiavaroli L, et al. Intermittent fasting strategies and their effects on body weight and other cardiometabolic risk factors: systematic review and network meta-analysis of randomised clinical trials. BMJ. 2025;389:e082007. Available at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC12175170/

2. Cienfuegos S, Corapi S, Gabel K, Ezpeleta M, Kalam F, Lin S, Pavlou V, Varady KA. Effect of Intermittent Fasting on Reproductive Hormone Levels in Females and Males: A Review of Human Trials. Nutrients. 2022;14(11):2343. Available at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9182756/

3. Doma KM, Moulin M, Al-Wahsh H, Guthrie N, Crowley DC, Lewis ED. An open-label clinical trial to investigate the safety and efficacy of a bone broth diet on weight loss in adults with obesity. Clin Nutr Open Sci. 2025;61:231-240. (Funded by Veyl Ventures LLC; conducted by KGK Science Inc., an independent clinical research organization; ClinicalTrials.gov NCT05740670.) Available at: https://www.sciencedirect.com/science/article/pii/S2667268525000488

4. Amorim T, Kumar NGV, David NL, et al. Methionine as a regulator of bone remodeling with fasting. JCI Insight. 2024;9(11):e177997. Available at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC11383369/

5. Kokura Y, Ueshima J, Saino Y, Maeda K. Enhanced protein intake on maintaining muscle mass, strength, and physical function in adults with overweight/obesity: A systematic review and meta-analysis. Clin Nutr ESPEN. 2024;63:417-426. Available at: https://www.sciencedirect.com/science/article/abs/pii/S2405457724001761

6. Soh J, Raventhiran S, Lee JH, Lim ZX, Goh J, Kennedy BK, Maier AB. The effect of glycine administration on the characteristics of physiological systems in human adults: A systematic review. GeroScience. 2024;46(1):219-239. Available at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10828290/

These statements have not been evaluated by the Food and Drug Administration. Dr. Kellyann products are not intended to diagnose, treat, cure, or prevent any disease. Intermittent fasting is not appropriate for everyone. Results may vary. Consult your healthcare provider before starting any new dietary or fasting program, especially if you are pregnant, nursing, trying to conceive, taking medication, or have a medical condition including diabetes, thyroid disease, or a history of disordered eating.