Alternate Day Fasting for Women: What the Research Really Shows

A woman in our community asked me something last month that I keep thinking about. She had been fasting for almost two years, comfortable in a 16 or 18 hour window, and she wanted to know if she was ready to try alternate day fasting. Not because a headline told her to. Because she had spent two years building enough trust with her own body to be curious about what came next.

That is really the right way to arrive at this question. Alternate day fasting, or ADF, means moving between roughly 36 hours without food and a 12 hour window of eating, over and over. It is also one of the more heavily researched forms of fasting we have, which is a nice change of pace from most nutrition trends that show up with a lot of confidence and not much data underneath them. So let's walk through what the studies actually found, including the parts that do not flatter the practice. You deserve the full picture before you decide whether this belongs anywhere near your life.

What Your Body Is Doing During Those 36 Hours

Every time you eat, your body runs on the glucose from that meal. Somewhere between the 12th and 16th hour without food, glucose stores start to thin out and your liver begins converting stored fat into ketones for fuel. That shift from sugar burning to fat burning is the whole game. A shorter fast gets you there briefly. A 36 hour fast keeps you in that state long enough for your body to go looking for sugar it stored away years ago, tucked into your liver, your muscle, your fat tissue.

Some women like to watch this shift happen in real time rather than take my word for it. A continuous glucose monitor will show you the exact moment your blood sugar flattens and stabilizes as you move deeper into a fast. I still find it satisfying to watch, even after years of doing this myself, and it is one of the reasons I point people toward a CGM like Nutrisense or Levels when they want data instead of guesswork.

This is also where autophagy has time to deepen. Autophagy is your cells' internal recycling system, the process that clears out damaged proteins and cellular debris so healthier cells can take their place. A 12 hour fast barely touches this system. A 36 hour fast gives it real working time, which is part of why alternate day fasting keeps showing up in longevity research and not only in weight loss research.

The Study I Keep Coming Back To

In 2019, a group of Austrian researchers ran the largest clinical trial to date on strict alternate day fasting in healthy, non obese adults. Participants alternated 36 hours of zero calorie fasting with 12 hour eating windows for one month. The results held up well. They stayed in mild ketosis even on their eating days. LDL cholesterol dropped. A marker of systemic inflammation dropped too. And the question I hear most often, whether a fast this long starts eating into muscle or bone, got answered directly in this trial. It did not. Bone health and immune function stayed intact throughout (Cell Metabolism, 2019).

If you want to confirm you are actually in ketosis rather than assuming it, a blood ketone meter takes the guesswork out. I keep a Keto Mojo in my kitchen drawer for exactly this reason. It is a small thing, but there is something reassuring about seeing the number instead of relying on how you feel, especially in the first few times you try a longer fast.

I break this same study down in more depth here, if you would rather see it explained with the research pulled up on screen:

https://www.youtube.com/embed/R5z_EmK0E1w

Where the Data Gets Less Flattering

A good education includes the research that does not support the conclusion you were hoping for. A 2017 randomized clinical trial published in JAMA Internal Medicine put alternate day fasting up against a standard daily calorie restricted diet in adults with obesity, tracked over a full year. Alternate day fasting did not outperform simple daily calorie restriction on weight loss, weight maintenance, or cardiovascular risk markers, and dropout was actually a bit higher in the ADF group (JAMA Internal Medicine, 2017).

I have spent enough years in this field to know fasting gets treated like a miracle sometimes, and I understand the impulse behind it. Alternate day fasting does not get you around the basic energy math of the body. What it seems to offer is a different route to some of the same destinations, one that reaches cellular aging markers and insulin sensitivity in ways daily calorie cutting does not reach quite as efficiently. That is a more modest claim than the internet usually makes for it. I also think it is the honest one, and honest claims tend to hold up better over years of practice than exciting ones do.

What Happens to This Picture After Menopause

Here is the part of the research that surprised me the most. A pooled analysis combining three separate 12 week ADF trials looked specifically at whether menopausal status changed how women responded. Premenopausal women, postmenopausal women, and men all lost a comparable percentage of body weight, with fat mass, insulin resistance, and blood pressure improving similarly across every group (Lin et al., pooled analysis, 2021). Being postmenopausal did not blunt the response the way a lot of women have been led to assume it would.

A separate six month trial found that postmenopausal women actually lost more weight on ADF than premenopausal women did, roughly 11 percent compared to 6 percent, with no adverse effect on bone metabolism in either group (Barnosky et al., Nutrition and Healthy Aging, 2017). If a doctor or a diet culture voice ever told you your body stops responding to fasting once you are through menopause, I would want you to sit with this data for a minute.

There is a gap worth naming here too. Nearly every one of these trials excluded perimenopausal women outright. Researchers tend to leave that group out of clinical trials because hormones are shifting too unpredictably during that transition to control for cleanly in a study design. Which means we simply do not have rigorous ADF data for the exact years so many of you reading this are living through right now. I bring this up not to talk you out of trying it, but because I think you make better decisions when you know where the evidence actually ends and where clinical experience has to fill in the rest.

Why I Teach ADF Differently Depending on Where You Are

If you still have a menstrual cycle, even an irregular one, your progesterone responds to the cortisol rise that comes with a long fast differently than your estrogen does. I have watched this play out with thousands of women over the years. Leaning into longer fasts like ADF in the first half of your cycle, and again briefly around ovulation, tends to work with your biology. Pulling back in the second half, when your body wants more nourishment and stability to support progesterone production, tends to work with it too.

Push alternate day fasting through the entire month during your reproductive years and some women find themselves sleeping worse, feeling more irritable, or watching their cycle become less predictable. Your body is not malfunctioning when this happens. It is telling you something useful about timing.

Once you are postmenopausal, that monthly rhythm is gone, and I suspect this is part of why postmenopausal women in these trials tolerated ADF so consistently and saw such strong results. There is simply more room to stay in it without a hormonal cost working against you at the same time.

If you decide to try a longer fast, especially your first 36 hour stretch, your electrolytes matter more than people expect. Sodium, potassium, and magnesium all get depleted faster than most women anticipate once you pass the 24 hour mark, and that depletion is behind a lot of the headaches and fatigue people blame on the fast itself. I reach for LMNT on longer fasts for this reason. It is a small habit that makes a long fast feel considerably more manageable.

None of this requires you to have every variable mapped out before you start. If you have not yet spent real time in a 16 or 18 hour window, or worked your way up to an occasional 24 or 36 hour fast, that is simply the more useful place to begin. Your body benefits from practice moving between fed and fasted states before it takes on something like ADF.

If You Want to Feel This in Your Own Body

Reading about the metabolic switch is one kind of learning. Noticing it happen in your own body, the mental clarity that shows up around hour eighteen, the way hunger changes shape after hour twenty four, is a different kind entirely. That is the experience we build together in Fast Training Week, the free training I run each month, with a different fasting style as the focus each time. If alternate day fasting has been on your mind, that week is a good place to explore it with some guidance instead of figuring it out alone.

And if you finish that week wanting to keep learning, the Reset Academy is where a lot of women land next. It is the community I built for people who want ongoing support, current research explained as it comes out, and other women asking the same questions you are asking right now.

This article is for educational purposes and is not a substitute for personalized medical advice. If you have a history of disordered eating, are pregnant or breastfeeding, or manage a chronic health condition, talk with your doctor before starting any extended fasting protocol.