Key Takeaways
- Most weight lost during the first 72 hours of a water fast is water and glycogen, not body fat. The body stores approximately 400-500g of glycogen, each gram binding roughly 3g of water.[3]
- Actual fat loss is limited by your metabolic rate. A person with a TDEE of 2,000 kcal can lose roughly 0.26 kg (0.57 lbs) of fat per day at most.[1]
- Metabolic rate decreases during extended fasting — the body conserves energy by reducing basal metabolic expenditure by approximately 5-10% after the first few days.[2]
- After resuming normal eating, water weight returns within 2-3 days as glycogen stores replenish. Only the fat loss component represents lasting change.
How This Estimator Works
This calculator uses the Mifflin-St Jeor equation to estimate your Basal Metabolic Rate (BMR), then applies a sedentary activity multiplier (1.2) to determine your Total Daily Energy Expenditure (TDEE) during a zero-calorie period.[1]
Since caloric intake is zero, your entire TDEE becomes your daily energy deficit. Fat loss is calculated by dividing this deficit by 7,700 kcal per kilogram of body fat (the commonly accepted energy density of adipose tissue).
Water and glycogen losses are modeled separately based on published data on glycogen storage capacity (~400-500g) and the water-to-glycogen binding ratio of approximately 3:1.[3]
Metabolic Phases During Extended Fasting
Hours 0-24 (Glycogen Depletion): The body first depletes liver and muscle glycogen stores for glucose. This releases substantial bound water, causing rapid initial weight loss on the scale that is mostly non-fat.
Hours 24-72 (Transition): Remaining glycogen is depleted. The liver begins producing ketone bodies from fatty acids. Gluconeogenesis (creating glucose from amino acids) peaks during this phase.[3]
Days 3+ (Ketosis Adaptation): The brain increasingly relies on ketone bodies for fuel, reducing the need for gluconeogenesis. Metabolic rate begins to decline as the body conserves energy.[2]
Understanding Fat Loss vs. Scale Weight
The scale can show dramatic numbers during a fast — 1-2 kg per day in the first 72 hours — but most of this is water and glycogen depletion, not body fat. True fat loss is constrained by your energy expenditure and follows the energy balance equation.
This is why the calculator provides both “Total Scale Loss” and “Estimated Fat Loss” as separate figures, along with a post-refeed weight estimate that accounts for water weight returning.
Frequently Asked Questions
Sources & References
- A New Predictive Equation for Resting Energy Expenditure in Healthy Individuals. Mifflin MD, St Jeor ST et al.. American Journal of Clinical Nutrition (1990)
- The Biology of Human Starvation. Keys A, Brozek J, Henschel A et al.. University of Minnesota Press (1950)
- Fuel Metabolism in Starvation. Cahill GF Jr.. Annual Review of Nutrition (2006)
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NASM-certified fitness and nutrition coach with over 10 years of hands-on experience helping people build strength, lose fat, and live healthier lives. Specializing in gym-based workouts with a strong focus on lifting technique, biomechanics, and practical exercise science. Through FitLifeRegime, sharing the tools, tips, and insights that have worked for hundreds of clients — helping you start your own fitness journey with confidence and clarity.
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Certified Personal Trainer & Sports Nutritionist from the National Academy of Sports Medicine
Disclaimer: This tool is for informational and personal journaling purposes only. Results are estimates based on published formulas and may vary based on individual factors such as genetics, training history, and measurement technique.