Do You Gain Fat When Fasting? The Surprising Truth Behind the Scale

December 20, 2025 •

4 min read

According to a study involving intermittent fasting, some participants experienced initial weight gain, not fat gain. So, do you gain fat when fasting? The simple answer is that while true fat gain is rare during a fast, several factors can cause your scale to go up, leading to confusion and frustration.

Quick Summary

This article explores why weight can increase while fasting and debunks the myth that fasting directly causes fat accumulation. It covers critical factors like diet, activity, and metabolism that influence body composition and scale readings during a fasting regimen.

Key Points

Fat gain during fasting is a misconception: You don't directly gain fat during the fasting period; weight fluctuations are typically due to water retention and glycogen changes.
Calories still matter: Overeating during your eating window will negate the calorie deficit from fasting, leading to weight and potential fat gain.
Metabolic adaptation can occur: If you severely restrict calories for too long, your metabolism can slow down, making weight gain easier when you do eat.
Mindful eating prevents overconsumption: Eating nutrient-dense, whole foods mindfully during your feeding window is crucial to avoid a caloric surplus and support metabolic health.
Lifestyle factors are critical: Stress, sleep, and activity levels all impact your hormonal balance and metabolic function, affecting your weight loss results.
Strength training is beneficial: Incorporating strength training helps preserve muscle mass, which keeps your metabolism high and promotes a healthier body composition.

Understanding the Fundamentals: Calories In vs. Calories Out

At its core, weight management comes down to a simple principle: if you consume more calories than your body burns, you will gain weight, and if you burn more than you consume, you will lose weight. This rule doesn't magically disappear when you start fasting. The issue for many is that the practice of intermittent fasting can indirectly lead to a caloric surplus during their feeding windows.

The All-Important Eating Window

Many people make the mistake of overcompensating during their designated eating period. After fasting for 16, 18, or even 20 hours, the temptation to indulge in large, calorie-dense meals is very strong. Fasting is not a free pass to binge on high-sugar, high-fat, or heavily processed junk food. If you consume more calories in your 8-hour window than you typically would in a full day, you will gain weight, and a significant portion of that will be fat. Success in fasting depends heavily on making nutritious, mindful food choices to remain in a healthy caloric deficit.

The Role of Water and Glycogen Retention

One of the most common reasons for seeing the number on the scale increase, particularly in the first week, is not fat, but water weight.

Glycogen depletion and replenishment: During the fasting state, your body first uses up its stored glucose, known as glycogen, for energy. Glycogen is stored with water, so as your body burns through these stores, you experience a rapid loss of water weight. When you break your fast and begin eating carbohydrates again, your body replenishes its glycogen stores, which also causes it to retain water again. This process can lead to a quick rebound in weight that is not fat.
Electrolyte imbalances: Fasting can cause a diuretic effect, leading to the loss of fluids and key electrolytes like sodium. When you reintroduce food and fluids, your body's electrolyte balance shifts, and it retains water. This can cause temporary bloating and weight gain on the scale, but it is not true fat.

Hormonal and Metabolic Changes

Fasting is known to trigger several hormonal shifts that can impact weight, though not always in the way people expect. While short-term fasting can increase your metabolic rate, prolonged and excessive calorie restriction can have the opposite effect.

The Impact of Prolonged Caloric Restriction

If your total daily calorie intake is consistently too low for an extended period, your body may enter a state of metabolic adaptation. Sensing a potential period of starvation, your body slows down its metabolism to conserve energy. This means you burn fewer calories at rest, and when you do eat, your body becomes more efficient at storing those calories as fat.

Why You Could be Gaining Weight During Fasting


Mistake	What Happens to Your Body	Potential Resulting Fat Gain?
Overeating in the eating window	Consuming a caloric surplus, nullifying the fasting period's deficit.	Yes, high likelihood
Eating unhealthy foods	Consuming high-sugar, high-fat, and processed foods that negate metabolic benefits.	Yes, high likelihood
Excessive stress (Cortisol)	High cortisol levels trigger fat storage, particularly in the abdominal area.	Yes, likely
Lack of adequate sleep	Disrupts hunger hormones (ghrelin/leptin) and impairs metabolic function.	Yes, likely
Reduced physical activity	Not burning enough calories to maintain a deficit, even with a reduced eating window.	Yes, likely
Prolonged severe calorie deficit	Metabolism slows down, and the body becomes more efficient at storing calories.	Yes, with prolonged practice

Making Your Fast Work For You

If you find that the scale is moving in the wrong direction while fasting, several strategies can help get you back on track.

Prioritize food quality: Focus on nutrient-dense, whole foods during your eating window. Include lean proteins, healthy fats, and plenty of fiber from fruits and vegetables. This will keep you full and satisfied, making you less likely to overeat.
Stay active: Regular physical activity, especially strength training, helps preserve and build muscle mass. Muscle is more metabolically active than fat, meaning it burns more calories at rest. This can counteract any minor metabolic slowdown and help with body recomposition.
Mindful eating: Be aware of your body's hunger and fullness cues. Don't feel pressured to stuff yourself during the eating window just because it's available. Slow down and savor your meals to give your brain time to register fullness.
Manage stress: High stress levels increase cortisol, a hormone that promotes fat storage. Incorporate stress-management techniques like meditation, yoga, or deep breathing to mitigate this effect.
Stay hydrated: Drink plenty of water throughout the day, especially during your fasting period. This helps flush out excess fluid and keeps you hydrated, preventing headaches and fatigue.

Conclusion

The perception that you gain fat when fasting is largely a myth stemming from scale fluctuations caused by factors other than genuine fat storage. While fasting is a powerful tool for promoting weight loss and metabolic health, it is not a cure-all. Success hinges on what and how much you eat, your activity level, sleep, and stress management. By focusing on high-quality nutrition and addressing these other lifestyle factors, you can maximize the benefits of fasting and achieve your body composition goals without the frustration of unexpected weight changes.

For more detailed information on metabolic adaptations, read this study from the National Institutes of Health (NIH): https://pmc.ncbi.nlm.nih.gov/articles/PMC5959807/.

Frequently Asked Questions

No, fasting itself does not cause you to gain fat. The process of fasting leads your body to burn stored energy, including fat. Weight gain that occurs while fasting is almost always due to overeating during your eating window, eating unhealthy foods, or water weight fluctuations.

Initial weight gain in the first week of fasting is often due to water retention, not fat. When you reintroduce food after a fast, your body restocks its glycogen stores, which hold water. This is a normal physiological process and the weight gain is temporary.

Intermittent fasting can lead to overeating if you feel overly hungry or if you have the misconception that you can eat anything you want during your eating window. Practicing mindful eating and choosing satiating, nutrient-dense foods can help prevent this.

Focus on nutrient-dense whole foods such as lean proteins (fish, chicken), healthy fats (avocado, nuts), fiber-rich fruits and vegetables, and whole grains. This will keep you full and satisfied without a huge calorie load.

Short-term fasting can actually boost your metabolic rate. However, severe and prolonged calorie restriction can cause your body to adapt and slow down your metabolism to conserve energy. This is why it's important to find a balanced, sustainable fasting schedule.

High stress levels increase the hormone cortisol, which promotes fat storage, especially in the abdominal area. Unmanaged stress can also lead to emotional eating and disrupt your fasting routine.

Yes, it is possible to achieve body recomposition with fasting, especially when combined with strength training. Studies show that men who combined resistance training with intermittent fasting lost fat while maintaining muscle mass. Eating enough protein is key for muscle preservation and growth.