Thoughts on <Fast Like a Girl>

Opening

Having heard from many sources that intermittent fasting helps improve physical condition, I started skipping breakfast entirely in September. Since childhood, my parents' philosophy of "you must eat a hearty breakfast" had made breakfast feel mandatory. But once I dropped it, I found I had a bit more morning time and the psychological pressure of needing to eat something disappeared — I felt lighter in multiple ways. Because I hadn't started with a specific goal in mind, I didn't feel dramatic changes. But one thing I noticed clearly was that I never once felt the urge to nap after lunch in the afternoon. That felt significant enough that I came to believe intermittent fasting was a better routine than not.

While sharing this update with a friend, I heard that women, due to their more complex hormonal systems, actually benefit more from a slightly different approach to fasting. That made sense when I thought about it, so I bought the recommended book Fast Like a Girl and read it right away.

About the Book

This book begins from the fact that women have a cycle of three major sex hormones — estrogen, progesterone, and testosterone — and therefore need to fast differently. The book first explains the types of fasting and why fasting matters, then explains how women's bodies differ, centered on hormones. It then introduces two dietary types and presents a 30-day fasting plan aligned with the menstrual cycle. I've summarized a few key ideas below.

Types of Fasting

I found it interesting that fasting can be categorized by effect.

1. Intermittent fasting (12–16 hours): After 8 hours of fasting, the body begins producing ketones.
2. Autophagy fasting (17–72 hours)
3. Gut reset fasting (24+ hours): The starting point at which the body produces stem cells; these stem cells seek out damaged cells and revive them.
   1. About 90% of microbes live in the gut; extending fasting to 24 hours is critically important for the immune system.
   2. It also energizes the microbes that produce neurotransmitters — the ones that make the brain happy and calm, and maintain focus.
4. Fat-burning fasting (36+ hours): The way to flip the fat-burning switch that shorter fasting periods can't reach.
   1. At this stage, ketones are continuously produced and cholesterol and inflammation decrease. Fat loss around the abdomen is especially noticeable.
      1. How it works: Cholesterol is produced in the liver; when large amounts of sugar, inflammatory fats, and toxins enter the liver, cholesterol levels rise. The liver also produces ketones — 36-hour fasting enhances the liver's ketone-production ability, stopping it from overproducing cholesterol.
   2. Doing a 36-hour fast about once a month can help address cholesterol problems.
5. Dopamine reset fasting (48+ hours)
   1. Fasting repairs dopamine receptor sites, creates new dopamine receptors, and improves dopamine pathways.
   2. Anxiety is managed by the amygdala. The amygdala's job is to keep you safe — but when it's in control, you see only what's wrong in your life. To solve this, you need to stimulate the prefrontal cortex and produce GABA; 48-hour fasting helps the brain accomplish both.
   3. Even once a year can produce benefits.
6. Immune reset fasting (72 hours): Commonly known as a 3–5 day water fast.
   1. The reason to fast up to 5 days: stem cells regenerate in a 72-hour fasting state; the goal is to maximize stem cell production.
   2. Many experts believe that doing a 3-day water fast once or twice a year can help eliminate cancer cells that may have accumulated in the body.

Women's Bodies

This is what makes this book special. Unlike women, men's hormones are much simpler — testosterone, the primary male hormone, follows a 24-hour cycle that moves in and out of the body every 15 minutes. That's why when men and women fast identically, men often see benefits while women experience side effects. I mapped out the hormonal system in diagrams below.

Hormone secretion system

Why stress is the root of all illness...

• Key sex hormones for women: estrogen, testosterone, progesterone
  • Estrogen: rises when insulin is low
    • Detoxification is critically important for this hormone.
    • Estrogen that isn't broken down and excreted clings to tissue and can cause breast cancer, breast tenderness, night sweats, low mood, weight gain, and other PMS symptoms.
  • Progesterone: rises when cortisol (triggered by stress) is low.

1. Phase 1 (Days 1–10): All three sex hormones at their lowest.
2. Phase 2 (Days 11–15): Estrogen levels gradually rise, peaking around day 13.
   • Estrogen contributes to collagen production and is a precursor to serotonin, dopamine, and noradrenaline — so mood improves, thinking becomes clearer, communication skills improve, and life looks more optimistic.
   • Testosterone is added, producing a surge of strong energy — a good time to increase the duration and intensity of strength training.
3. Phase 3 (Days 16–18): All hormones drop again.
   • Unlike Phase 1, the body focuses on preparing to produce progesterone rather than estrogen.
   • Progesterone: prepares the uterine lining; lowers aggression and calms.
4. Phase 4 (Day 19 through menstruation): Progesterone peaks 6–8 days after ovulation.
   • Fasting applies appropriate stress to the body — the cortisol spike from fasting stress can cause a sharp drop in progesterone.
   • Progesterone requires glucose to trigger menstruation, so fasting can mean failing to supply the fuel progesterone needs.
   • It's best not to fast during this phase.

The Advanced 30-Day Fasting Reset

The book provides beginner and advanced 30-day fasting reset plans with day-by-day fasting types. Since I've been doing intermittent fasting, I mapped out the advanced plan below to suit my own cycle. I'm currently on day 9 — I want to go through a full month, a full cycle, and then check whether it suits my body.

• Power phase: Ketobiotic diet
  • Days 1–6: Intermittent fasting — 15 hours
  • Day 7: Gut reset fast — 24 hours
  • Days 8–11: Autophagy fasting — 17 hours
• Manifestation phase: Hormone feasting diet
  • Days 12–17: Intermittent fasting — 15 hours
• Power phase: Ketobiotic diet
  • Day 18: Gut reset fast — 24 hours
  • Days 19–22: Autophagy fasting — 17 hours
• Nourishing phase: Hormone feasting diet
  • Days 23–33: Intermittent fasting — 13 hours

Closing

I've summarized a few key points above, but if you want to fully understand and put the content into practice, I recommend reading the book directly. The appendix includes Q&A on fasting and multiple recommended recipes for each dietary type. It's been nearly a week since I read it, and so far I'm still making an effort to watch what I eat and vary my fasting each day according to the plan. I hope it doesn't become a three-day wonder — I want to find the rhythm that puts my condition at its peak. Starting the new year with my own fasting routine feels like it could make for an especially energetic year ahead. If I genuinely feel the benefit, I'd also love to build a service that makes maintaining such a routine consistently easier. A motivating book on multiple levels 😉