Fast Like a Girl

Dr. Mindy Pelz argues that women have been failed by one-size-fits-all approaches to both dieting and fasting. She contends that fasting, when timed to a woman's menstrual cycle and paired with the right foods, is the most powerful free tool for reversing the metabolic damage behind rising rates of chronic disease.

At 19, Pelz was diagnosed with chronic fatigue syndrome and told by a specialist to drop out of school and try experimental medications. Her mother instead took her to a holistic doctor, who prescribed a diet resembling today's ketogenic diet, a low-carbohydrate, high-fat eating plan. Within three weeks, her energy, mood, and mental clarity returned, launching her career studying how food and fasting affect health. She cites Centers for Disease Control and Prevention (CDC) data showing that 60 percent of Americans have at least one chronic disease, and a University of North Carolina at Chapel Hill study finding that only 12 percent are metabolically healthy, meaning their bodies can regulate blood sugar, blood pressure, and cholesterol without medication.

In Part I, Pelz identifies "the failed five," the systemic reasons diets have failed women. Calorie restriction lowers the body's metabolic set point, the caloric range at which it maintains weight; she cites the Minnesota Starvation Experiment of the 1960s, in which participants regained all lost weight plus 10 percent upon refeeding. The low-fat food movement led manufacturers to replace fat with sugar and chemicals, driving obesity from under 14 percent in the 1960s to nearly 40 percent today. She defines insulin resistance, a condition in which cells can no longer use the hormone insulin to absorb blood sugar, as the root cause of metabolic syndrome, a cluster of conditions including high blood sugar, high blood pressure, and abnormal cholesterol. Rigid diets and overexercise spike cortisol, the body's stress hormone, which in turn raises insulin. Obesogens, fat-inducing chemicals in plastics and food additives, accumulate as fat because the body cannot break them down. Each sex hormone also has different dietary needs: Estrogen thrives on low carbohydrates, while progesterone requires more, so women who eat the same way all month work against their hormonal design.

Pelz presents the science behind fasting by explaining two fuel systems: a sugar-burning system activated by eating, and a fat-burning (ketogenic) system that activates roughly eight hours after the last meal, when the liver produces ketones, an alternative fuel derived from fat. A 2019 meta-analysis in The New England Journal of Medicine of over 85 studies concluded that intermittent fasting should be a first-line treatment for obesity, diabetes, cardiovascular disease, and neurodegenerative conditions. Healing mechanisms triggered during fasting include autophagy, a cellular self-cleaning process that removes damaged components (recognized by Dr. Yoshinori Ohsumi's 2016 Nobel Prize); increased growth hormone; and more sensitive dopamine receptors. She cites Dr. Valter Longo's finding that 72 hours of water fasting regenerates the immune system through stem cell release, and a 2016 Journal of the American Medical Association study showing that daily fasts of 13 or more hours reduced breast cancer recurrence by 64 percent.

Pelz outlines six fasting lengths: intermittent fasting (12 to 16 hours) for weight loss and energy; autophagy fasting (starting at 17 hours) for detox and hormonal balance; gut-reset fasting (24 hours) to repair the intestinal lining; fat-burner fasting (36 hours) for stubborn weight-loss resistance; dopamine-reset fasting (48 hours) to reboot mood; and immune-reset fasting (72-plus hours) for chronic conditions and anti-aging. She introduces metabolic switching, the alternation between sugar-burning and fat-burning states, as the central mechanism behind fasting's benefits. Its effects include cycling between autophagy and mTOR (cellular growth triggered by eating), creating hormetic stress that forces cells to adapt, repairing mitochondria (the energy-producing structures in cells), and regenerating neurons.

A full chapter addresses why women must fast differently than men. Pelz walks through the menstrual cycle in four stages: days 1 through 10, when estrogen builds; days 11 through 15, when estrogen and testosterone peak during ovulation; days 16 through 18, when hormones dip; and day 19 through bleeding, when progesterone rises. Progesterone is highly sensitive to cortisol and requires higher glucose, making fasting and low-carbohydrate diets counterproductive the week before a period. Pelz describes a hormonal hierarchy: Oxytocin, produced through connection and physical affection, calms cortisol; cortisol spikes insulin; and insulin suppresses sex hormones. Hormonal surges also release stored toxins from tissues, so triggering autophagy when hormones peak can double the detox burden. Bridget, a 42-year-old executive, developed palpitations, insomnia, and hair loss after six months of consistent fasting; her symptoms resolved once she varied her fasts to match her cycle.

In Part II, Pelz builds a food framework around four principles: Ingredient quality matters more than calorie count, glycemic index determines a food's effect on blood sugar, dietary diversity feeds beneficial gut microbes, and cycling food choices to match hormonal phases maximizes hormone production. She introduces two eating styles: Ketobiotic eating limits net carbohydrates (total carbohydrates minus fiber) to 50 grams per day, protein to 75 grams, and derives over 60 percent of calories from good fats to support estrogen and fat burning; hormone feasting allows up to 150 grams of net carbohydrates from natural sources like root vegetables, squashes, and fruits to support progesterone. Microbiome health depends on what she calls the "three Ps": probiotic-rich fermented foods, prebiotic-rich fiber foods, and polyphenol-rich plant foods.

The Fasting Cycle divides the menstrual cycle into three phases. During Power Phases (days 1 through 10 and 16 through 19), when sex hormones are lowest, she recommends longer fasts with ketobiotic eating. During the Manifestation Phase (days 11 through 15), when estrogen and testosterone peak, she limits fasts to under 15 hours and shifts to hormone feasting foods. During the Nurture Phase (day 20 through bleeding), she recommends no fasting, reduced exercise, and hormone feasting to protect progesterone.

Part III provides the 30-Day Fasting Reset for women new to fasting, women without a regular cycle, and postmenopausal women. A two-week pre-reset removes harmful oils, refined sugars, and chemical-laden ingredients while gradually compressing the eating window to 13 hours. The reset uses three fasting lengths (13, 15, and 17 hours) across the cycle phases, with an advanced version adding 24-hour gut-reset fasts. Pelz recommends monitoring blood sugar with a finger-prick meter or continuous glucose monitor, targeting a fasting reading of 70 to 90 milligrams per deciliter. She cites an 80-year Harvard University study on relationships and health to reinforce her emphasis on community support.

Pelz addresses how to break a fast and troubleshoot common obstacles. She presents four strategies for the first post-fast meal: probiotic, prebiotic, or polyphenol foods to support the microbiome; protein to build muscle; fat to extend fat burning; or simply following cravings. For fasts over 48 hours, she recommends gradual refeeding. Her advice covers hunger (mineral packets and fasted snacks, small fat-based foods like nut butter), detox symptoms (varying fasts, sweating, binders like activated charcoal), and blood sugar management. She addresses fasting during pregnancy (avoid fasting to prevent toxin transfer), nursing (limit to 13-hour fasts), diabetes (monitor blood sugar under medical supervision), and eating disorders (involve a doctor and stop if fasting triggers restrictive behavior).

The appendices provide condition-specific protocols, most structured as month-long plans. These cover infertility (a two-month protocol emphasizing insulin sensitivity), autoimmune conditions, thyroid disorders, chronic fatigue, type 2 diabetes, brain health including memory loss and depression, adrenal fatigue (a gentle entry over up to six months), and immune system reset (including a 72-hour water fast).

Pelz closes by returning to Lani, a patient diagnosed with metastatic breast cancer at 40 who extended her life to 11 years through lifestyle changes. In her final months, Lani asked Pelz about fasting, sparking the research behind this book. Pelz connects fasting to the COVID-19 pandemic, arguing that poor metabolic health left people vulnerable, and calls for the emergence of "The Empowered Woman," one who takes control of her health through fasting and builds community with other women.