Will I Burn Calories by Not Eating? The Truth About Fasting and Metabolism

December 31, 2025 •

5 min read

According to a 2009 study on caloric restriction, participants who consumed significantly fewer calories per day experienced a major decrease in their metabolic rate over several months. This highlights that while the body constantly burns calories, its response to not eating is a complex survival mechanism.

Quick Summary

The body continuously burns calories to function, but not eating triggers a metabolic slowdown to conserve energy. This can hinder long-term weight loss as the body shifts from burning stored fat to potentially breaking down muscle tissue.

Key Points

Metabolic Rate: Your body burns calories even when not eating, a process governed by your basal and resting metabolic rates.
Fat Burning During Fasting: After exhausting carbohydrate stores, the body enters a phase of metabolic switching, using stored fat for energy.
Starvation Mode: Prolonged and severe calorie restriction triggers adaptive thermogenesis, a survival response that significantly slows your metabolism.
Muscle Loss: Extreme calorie deprivation can lead to the breakdown of lean muscle tissue for energy, which further reduces your metabolic rate.
Weight Regain: The suppressed metabolism caused by starvation makes it difficult to maintain weight loss and often leads to rapid weight regain.
Risks of Starvation: Beyond metabolic issues, long-term starvation can cause nutrient deficiencies, weakened immunity, organ damage, and psychological harm.
Healthy Alternatives: Sustainable weight loss relies on a moderate calorie deficit, adequate protein, strength training, and proper hydration to preserve muscle and maintain metabolic health.
Seek Professional Advice: Always consult a healthcare professional before undertaking significant dietary changes, especially fasting.

Your Body's Constant Calorie Burn

Your body is a complex engine that is always running, even when you are completely at rest. The number of calories your body burns just to keep you alive and functioning is called your Basal Metabolic Rate (BMR) or Resting Metabolic Rate (RMR). These are the calories used for basic, life-sustaining functions such as breathing, blood circulation, cell repair, and maintaining body temperature.

When you don't eat, your body's source of fuel from recent meals runs out, but it doesn't just shut down. Instead, it begins to tap into its stored energy reserves. In the short term, this process is known as metabolic switching, where your body first uses up stored carbohydrates (glycogen) and then transitions to burning stored fat for energy. So, yes, you do burn calories and fat by not eating for short periods.

The Stages of Energy Use During Fasting

For those who practice forms of intermittent fasting, the body cycles through different energy-burning phases.

Fed State: Occurs for several hours after eating. Your body is digesting and absorbing nutrients, and insulin levels are high.
Early Fasting State: Starts around 3-4 hours after eating and lasts up to 18 hours. Blood sugar and insulin levels drop, and the body converts stored glycogen back into glucose for fuel.
Fasting State: After 18 hours, liver glycogen is depleted. The body intensifies lipolysis—the breakdown of fat cells—and begins producing ketone bodies from fat for fuel, entering a metabolic state called ketosis.
Starvation State (Prolonged Fasting): Typically begins after 48 hours without food. The body significantly slows its metabolism to conserve energy, and muscle breakdown may occur as the body seeks protein for glucose synthesis.

The Problem with Severe Calorie Restriction

While short-term fasting can prompt your body to burn fat, intentionally starving yourself for an extended period is not a sustainable or healthy weight loss strategy. Your body is hardwired with survival mechanisms to cope with perceived famine, and these adaptations can work against your weight loss goals.

This phenomenon is known as adaptive thermogenesis or “metabolic adaptation”. In response to severe and prolonged calorie deprivation, your body reduces its resting metabolic rate to conserve energy. This means it burns fewer calories just to exist, making further weight loss incredibly difficult. Once you resume eating normally, this suppressed metabolism can cause rapid weight regain, a cycle often seen in so-called 'crash diets'.

Metabolic Effects of Starving vs. Sustainable Weight Loss

To illustrate the metabolic differences, here is a comparison of what happens to your body during severe calorie restriction versus a healthy, sustainable approach.


Feature	Severe Calorie Restriction (Starving)	Sustainable Weight Loss (Moderate Calorie Deficit)
Metabolic Rate	Significantly and unsustainably suppressed due to adaptive thermogenesis.	Minimally affected, with more control over energy expenditure via diet and exercise.
Hormonal Response	Increased hunger hormones (ghrelin) and decreased satiety hormones (leptin) lead to intense cravings and rebound eating.	Balanced hormonal response with a gradual, controllable increase in hunger cues over time.
Primary Fuel Source	Initially glycogen, then fat. But as starvation continues, it shifts to breaking down valuable muscle tissue for energy.	Sustainable fat burning while preserving lean muscle mass, especially with adequate protein intake and resistance training.
Body Composition	Loss of both fat mass and metabolically active lean muscle mass, further slowing your metabolism.	Targeted fat loss while maintaining or even building lean muscle mass.
Nutritional Intake	High risk of nutrient deficiencies, fatigue, and other severe health issues.	Ensures adequate intake of essential vitamins, minerals, and macronutrients.
Mental Health	Can lead to mood swings, brain fog, and obsessive thoughts about food due to hormonal and nutritional imbalances.	Supports better focus, mood, and a healthier relationship with food.

The Risks of Prolonged Starvation

Attempting to lose weight by not eating for extended periods poses significant health risks far beyond a slower metabolism. The body's shift to burning muscle tissue for energy can lead to severe muscle wasting, a weaker immune system, and an increased risk of infections. Critical bodily functions, including heart and brain activity, are compromised due to a lack of fuel and essential nutrients. Over time, this can lead to severe malnutrition, organ failure, and even death. Psychological effects are also common, including depression, irritability, and the development of disordered eating behaviors. It is a dangerous and ineffective path to weight loss.

Healthy Alternatives to Severe Calorie Restriction

Instead of resorting to extreme measures, sustainable weight loss is achieved through a balanced approach that combines a moderate calorie deficit with proper nutrition and physical activity. A reasonable calorie reduction of 300 to 500 calories per day is often recommended, as it encourages fat loss without triggering the drastic metabolic slowdown associated with starvation.

Prioritize Protein Intake: High protein consumption helps preserve muscle mass during weight loss, which is crucial for maintaining a healthy metabolic rate. Protein also increases satiety, helping you feel fuller for longer.
Incorporate Strength Training: Building and maintaining lean muscle mass through resistance training helps boost your metabolism because muscle tissue burns more calories at rest than fat tissue.
Stay Hydrated: Drinking plenty of water is essential for optimal metabolic function, digestion, and nutrient absorption.
Manage Stress and Sleep: Lack of sleep and chronic stress can disrupt metabolic hormones, leading to weight gain. Prioritizing quality sleep and stress management supports a healthy metabolism.

Conclusion

While your body does burn calories when you're not eating, relying on starvation for weight loss is a dangerous and counterproductive strategy. The human body is designed to survive periods of famine by slowing its metabolism and breaking down muscle, which makes long-term weight management more difficult. Sustainable and healthy weight loss is a marathon, not a sprint. By focusing on a balanced, nutrient-dense diet, regular exercise, and preserving muscle mass, you can effectively burn fat while supporting your overall health and well-being. Always consult a healthcare professional or registered dietitian before making significant changes to your diet to ensure a safe and personalized approach. For more information on the metabolic effects of fasting, you can explore resources like the National Institutes of Health (NIH) website.

Frequently Asked Questions

Yes, short-term fasting, like in intermittent fasting, can lead to weight loss by causing your body to use stored fat for energy. However, it's not a healthy or sustainable method when taken to extremes, and a balanced diet with a moderate calorie deficit is often more effective long-term.

Severely and consistently restricting calories or skipping meals can cause your metabolism to slow down, a phenomenon known as metabolic adaptation. This is your body's survival response to conserve energy, making weight loss more challenging over time.

After your last meal, your body first uses stored glucose (glycogen) for energy. This stored glycogen is typically depleted within 18-24 hours, after which the body begins converting fat into fuel, a process known as ketosis.

'Starvation mode' is a popular term for adaptive thermogenesis, a state where the body drastically reduces its metabolic rate in response to extreme, prolonged calorie restriction. It's an energy-saving survival mechanism that can hinder weight loss.

Stopping eating for extended periods is dangerous because it can lead to severe malnutrition, muscle wasting, organ damage, and a drastically slowed metabolism. It also increases the risk of developing disordered eating behaviors.

Initially, your body will primarily burn stored glycogen and then fat. However, during prolonged periods of starvation, once fat reserves are significantly depleted, your body will begin breaking down muscle tissue for energy.

To lose weight safely, create a moderate calorie deficit (e.g., 300-500 calories less than you burn) through a balanced diet and regular physical activity. Prioritizing protein and incorporating strength training helps preserve muscle mass and maintain a healthy metabolism.