How to Fast for Fat Loss, Hormones, and Better Sex | Dr. Mindy Pelz

What It Covers

Dr. Mindy Pelz explains how the body operates on two energy systems — glucose-burning and fat-burning — and how women can optimize fasting, exercise, and nutrition across their 28-32 day hormonal cycle. She outlines specific windows for intense training, longer fasts, and recovery based on estrogen, testosterone, and progesterone fluctuations.

Key Questions Answered

• •Two Energy Systems: The body switches from glucose-burning to fat-burning (ketogenic) mode after 8-10 hours without food. This produces ketones that suppress hunger, elevate GABA for calm, and sharpen mental clarity. People who eat continuously never activate this second system, which accelerates aging and causes every calorie-focused diet to eventually fail regardless of food quality.
• •Fasting Training Protocol: Start with a consistent 12-hour fast daily rather than jumping into extended fasts. When hunger or fatigue appears, return to eating and repeat the next day, gradually extending the window. This mirrors athletic training — the discomfort signals cellular repair (hormetic stress), and the body adapts over weeks, not days, before the energy and clarity benefits emerge.
• •Women's Cycle-Based Fasting Windows: Days 1-10 (estrogen-building phase): fast longer, eat low-carb, train hard. Days 11-15 (ovulation): testosterone surges — lift heavy weights, keep fasting minimal. Days 16-19: fast longer again, up to 36-48 hours if desired. Days 20 onward (progesterone phase): stop fasting, increase carbohydrates, reduce workout intensity to yoga or hiking only.
• •Cortisol-Insulin-Hormone Hierarchy: Cortisol controls insulin sensitivity, which controls sex hormone balance. Elevated cortisol from stress, over-fasting, or excessive exercise makes women insulin resistant, disrupting estrogen and progesterone. Oxytocin sits at the top of this hierarchy — social connection lowers cortisol, which improves insulin sensitivity, which then allows sex hormones to regulate naturally, making lifestyle interventions like diet and exercise actually work.
• •Eating Timing vs. Food Composition: A 300-pound food-addicted patient lost 13 pounds in month one eating the same unhealthy foods — 12 sodas and buffalo wings daily — simply by compressing consumption into a 10-12 hour window. Over subsequent months, fasting progressively reshaped microbiome cravings toward healthier foods. Total weight lost reached 175 pounds, demonstrating that when eating occurs often matters more than what is eaten initially.
• •Birth Control and Hormonal Disruption: Long-term oral contraceptive use prevents the brain-ovary communication pattern from developing, particularly when started shortly after a first period. It also decimates gut microbiome diversity needed to produce serotonin, regulate immunity, and break down estrogen. Women who stop birth control after years of use often require months or years for natural hormonal rhythms to re-establish, which directly affects fertility and cycle regularity.