Most health advice is written for men. It’s frustrating. For decades, clinical trials basically ignored the female endocrine system because it was "too complicated" to account for shifting hormones. Then Dr. Mindy Pelz released Fast Like a Girl, and suddenly, thousands of women realized why their keto diet or 16:8 fasting window was making them lose hair, feel exhausted, or miss their periods.
Fasting isn't just about not eating. It's about timing.
If you're a woman, your body isn't a stagnant pond; it’s a tidal wave. Dr. Pelz’s core argument is that the "one size fits all" approach to fasting is actually damaging to the female metabolism. Men can fast every day. Their testosterone stays relatively stable. Women? We have a 28-day (ish) dance between estrogen, progesterone, and testosterone. If you try to power through a 24-hour fast when your progesterone is trying to build up, you’re basically screaming at your thyroid to shut down. It’s stressful. It’s counterproductive. Honestly, it’s a recipe for burnout.
The Problem With "Traditional" Fasting For Women
You’ve probably seen the biohacking bros on YouTube. They talk about autophagy and OMAD (one meal a day) like it’s a religion. But here’s the thing: estrogen loves low insulin. It thrives when you’re fasting. Progesterone? She hates it. Progesterone is the "keep the baby alive" hormone. She wants snacks. She wants complex carbs. She wants you to be calm.
When you fast aggressively during the week before your period—the luteal phase—your cortisol spikes. Since progesterone and cortisol are made from the same raw material (pregnenolone), your body will choose survival (cortisol) over reproduction (progesterone) every single time. This is why you get "hangry." It’s why you can’t sleep. Fast Like a Girl breaks this down into specific zones based on where you are in your cycle.
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It’s not just about weight loss. It’s about not fighting your own biology.
Fasting According to Your Hormonal Map
Dr. Pelz categorizes the month into three main phases.
The Power Phase (Day 1-10)
This is when your period starts. Estrogen is low but climbing. This is your time to shine. You can do the longer fasts here—15 hours, 17 hours, even 24 hours if you’re experienced. Your body is more resilient to stress right now. You’re basically a fasting machine. This is also when "Keto-Green" eating works best. Think high healthy fats, low carbs.
The Manifestation Phase (Day 11-15)
Ovulation is happening. Estrogen is peaking, and testosterone makes a cameo. You might feel amazing, but you need to be careful. Dr. Pelz suggests keeping fasts shorter here, maybe 13-15 hours. Why? Because the liver is working overtime to process all that estrogen. If you stress it too much with long fasts, you might end up with hormonal acne or mood swings. Eat "hormone-building" foods. Think cruciferous vegetables like broccoli or kale to help the liver detoxify that used-up estrogen.
The Nurture Phase (Day 19 to Bleed)
Stop. Just stop fasting. This is the week before your period. Progesterone is the star of the show. She needs your blood sugar to be a bit higher. If you try to do intermittent fasting now, you’re going to feel like garbage. Dr. Pelz calls this "Pause Phase." You should be eating more complex carbohydrates—sweet potatoes, squashes, beans. If you don't, your body will likely demand sugar in the form of a late-night pint of ice cream because it's desperate for glucose to build that uterine lining.
What Most People Get Wrong About Fast Like a Girl
A lot of people think this is just another diet book. It’s not. It’s a manual for metabolic flexibility.
One of the biggest misconceptions is that you have to be "perfect" with the days. Look, cycles vary. If you’re stressed, your ovulation might be late. If you’re perimenopausal, your cycle might be a total mystery. Pelz acknowledges this. She suggests that if you don't have a regular cycle, you track your symptoms or follow the moon phases (it sounds woo-woo, but historically, women’s cycles tracked with the lunar month).
Another big mistake? Neglecting the "break-fast." What you eat to end a fast is just as important as the fast itself. If you break a 17-hour fast with a sugary donut, you’ve just sent your insulin on a roller coaster that will ruin your energy for the rest of the day. Start with protein. Or bone broth. Give your gut a chance to wake up.
Real Science: Autophagy and Glucose
The book leans heavily on the concept of autophagy. This is basically your body's "cellular recycling" mode. At around 17 hours of fasting, your cells start cleaning out the junk—misfolded proteins and damaged mitochondria.
For women with PCOS (Polycystic Ovary Syndrome), this is huge. PCOS is often driven by insulin resistance. By using the Power Phase to lean into longer fasts, many women find their insulin sensitivity improves, which can actually help restore regular ovulation. But again, you can't live in a fasted state. You have to feast to fast.
The Perimenopause Pivot
If you’re in your 40s, the rules change. Progesterone starts dropping first, which is why your sleep starts sucking and you feel more anxious. Fasting can be a godsend for the "middle-age spread," but it can also be a disaster if it’s too aggressive.
Dr. Pelz often discusses the importance of the 13-hour "Circadian Fast" for perimenopausal women. It’s long enough to give the digestive system a break but short enough that it doesn’t trigger a massive cortisol spike that causes belly fat storage. It’s a delicate balance.
Actionable Steps to Start Today
Don't just jump into a 24-hour fast. That’s a mistake.
- Track your cycle. Use an app like Clue or Flo, or just a paper calendar. You need to know what day you are on. Day 1 is the first day of your period.
- Start with 13 hours. This is the "Goldilocks" zone. If you finish dinner at 7 PM, don't eat until 8 AM. Anyone can do this.
- Identify your Nurture Phase. If you are within 5-7 days of your period, stop the long fasts. Increase your carb intake slightly with whole foods. Notice how your cravings actually decrease when you give your body what it wants.
- Vary your foods. Don't eat the same three things every day. Your gut microbiome needs diversity to help metabolize hormones.
- Listen to your hair and skin. If your hair starts thinning or your skin gets incredibly dry, you are likely fasting too much or not eating enough protein. Back off.
Fasting "like a girl" means acknowledging that we are cyclical beings. It’s about working with your body’s natural ebb and flow rather than trying to beat it into submission with a rigid, male-centric schedule. When you align your eating patterns with your hormones, the "weight loss" becomes a side effect of a body that finally feels safe and regulated.