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Why did I gain weight after a 24 hour fast?

There are a few possible reasons why you may have gained weight after completing a 24 hour fast:

Water retention

When you fast for an extended period of time, your body can hold on to more water than usual. This is because when carbohydrate stores are depleted, the body will release insulin less frequently. Since insulin helps kidney’s excrete excess sodium and water, water retention can occur during a fast. This water retention can lead to an increase on the scale. However, this weight is not true fat gain, but just extra water weight that will subside after resuming normal eating.

Overeating after the fast

It’s common to feel extra hungry after completing a 24 hour fast. There’s even a phrase for this phenomenon called “post-fast gorging.” If you eat significantly more calories than needed after a fast, you can gain fat very quickly. Even a single day of drastic overeating can add a pound or two of fat gain. It’s important to resume normal healthy eating after a 24 hour fast and not binge.


Fasting can lead to constipation, which causes extra waste and food to hang around in your digestive tract. This can cause water retention and bloating, showing up as increased weight on the scale. The constipation should subside once you resume normal eating.

Muscle loss

During an extended fast, your body can break down muscle tissue for energy. The lost muscle is then replaced with water. So although you may be losing protein mass, the water replacing it on the scale makes it appear as though you gained fat. However, you’re just losing muscle and gaining water.

Eating more carbs

After a fast, some people react by eating more carbohydrate-rich foods. This restocks glycogen stores, but it also pulls water into the muscles and liver. Each gram of stored glycogen binds with 3-4 grams of water. So if you gain 300 grams of glycogen, that’s an extra 1.2 pounds (0.5 kg) of water weight.

Why gaining weight after fasting is normal

Gaining a few pounds after completing a 24 hour fast is completely normal and expected. Here’s why:

Glycogen and water fluctuations

Glycogen is the stored form of glucose (sugar) used for energy. It’s mostly stored in the liver and muscles. Each gram of glycogen binds with 3-4 grams of water. So when glycogen is burned for energy during fasting, water is excreted too.

Then after the fast, glycogen is replenished by eating carbs. This brings water back into the body. These fluctuations in glycogen and water can temporarily increase your weight after fasting. But it’s not actual fat gain.

Food volume

When fasting, your stomach and intestines empty out completely. So when you start eating again, your digestive tract needs to re-adjust to having food inside it again. This extra food volume alone can account for some weight gain.

Waste buildup

Not eating anything for 24 hours allows waste and stool to build up in your digestive tract. Once you start eating again, this extra buildup contributes to feelings of fullness and additional weight on the scale.

Hormone changes

Fasting causes short-term hormone changes. Insulin drops while cortisol and ghrelin (the “hunger hormone”) go up. This mobilizes stored energy. When you resume eating, insulin shoots back up allowing glycogen and fat storage again. These shifts can lead to an increase in scale weight.

How much weight gain is normal after a 24 hour fast?

It’s normal to gain up to 5 pounds (2.5 kg) of scale weight in the days following a 24 hour fast. However, this is mostly just water weight and food volume.

True fat gain from a single 24 hour fast is typically minimal. One study found people only gained 0.7 lbs (0.3 kg) of fat after fasting for 2 days (48 hours). Other studies show no fat gain with daily 16-20 hour fasts.

So if you gained 2-3 lbs (1-1.5 kg) after your 24 hour fast, it’s likely just water, glycogen, and food volume. These fluctuations resolve within a few days. Only minor fat gain occurs with short 24 hour fasts.

Tips to minimize weight gain after fasting

Here are some tips to help minimize rapid water weight gains after completing a 24 hour fast:

Avoid carb binging

Eating a very high carb meal after fasting causes rapid spikes in glycogen and water weight. Limit high-carb meals and snacks.

Increase protein intake

Protein stimulates insulin less than carbohydrates. Eating more protein can help reduce water weight fluctuations.

Stay hydrated

Drink plenty of water and electrolytes during your fast. This helps reduce water retention as you transition back into eating.


Light activity after fasting helps normalize fluid regulation and prevents bloating.

Be patient

Understand that initial weight gain is temporary glycogen, water, and food volume. It does not reflect actual fat gain. The number on the scale will decline again within a few days.

When to worry about weight gain after fasting

In general, gaining a few pounds after completing a 24 hour fast is not concerning. However, you may want to reassess your diet and fasting regimen if:

– You gain over 5 lbs (2.5 kg) after fasting

– You consistently gain weight after fasting

– The weight persists longer than 3-5 days

– You experience frequent binge eating after fasting

Rapid and persistent weight gain after fasting could indicate you need longer, less frequent fasts. It could also mean your diet is too high in calories, carbs or sodium when not fasting.

Should I be concerned about metabolism slowing?

There is a myth that fasting slows your metabolic rate, often called “starvation mode.” However, studies show fasting for 24-48 hours does not slow metabolism.

For example, one study compared a 36-hour fast to eating normally. The fast did not decrease the number of calories burned over 36 hours. Other research reveals fasting increases fat burning while preserving lean mass.

Therefore, a single 24 hour fast is unlikely to cause long-term metabolic adaptation or slow your metabolism. Fasting periods up to 48 hours seem to have no negative effect on metabolic rate.

How to resume eating after a 24 hour fast

The way you break your fast can impact changes in weight, energy and digestion. Here are some tips:

Break your fast gradually

Start with broth, non-starchy vegetables or a small snack. Don’t go straight for a large meal.

Eat nutrient-dense foods

Focus on eating whole, minimally processed foods like lean proteins, veggies and healthy fats after fasting.

Limit refined carbs

Avoid added sugars and refined grains to minimize glycogen/water fluctuations.

Stay hydrated

Drink plenty of water and electrolytes like broth to normalize fluid balance.

Listen to your body

Go slowly, keep portions small and stop eating if you feel discomfort. Hunger and fullness signals may be altered after fasting.

Should I be concerned about muscle loss?

Some people worry that fasting may cause loss of muscle mass. However, research shows lean mass is well-preserved with short fasts up to 48 hours.

In one study, participants did 22-hour fasts for 21 days. They lost fat mass but retained their lean mass and strength. Other studies reveal fasting increases human growth hormone, which helps preserve muscle.

As long as you’re fasting for 24-48 hours infrequently, muscle loss should not be a concern. Make sure to eat enough protein and lift weights to maintain and build muscle. Longer fasts of several days may require more caution to prevent excessive muscle breakdown.

Health benefits of intermittent fasting

Despite some temporary weight fluctuations, short term fasting provides powerful health benefits:

Weight and fat loss

Intermittent fasting is linked to fat loss, especially around the midsection. Fasting stimulates lipolysis to burn fat.

Insulin sensitivity

Fasting periods improve insulin sensitivity for better blood sugar control.


Studies associate fasting with longer lifespan through beneficial impacts on health and aging.

Brain function

Fasting may enhance cognition, focus, mood and memory.

Immune system

Short fasts boost immune regulation and help fight inflammation.

Heart health

Some research indicates intermittent fasting protects cardiovascular health.

Cancer prevention

Intermittent fasting shows promise for reducing cancer risk and assisting cancer treatment.

Should I continue intermittent fasting if it causes weight gain?

Mild weight gain immediately after fasting is normal. It’s typically just water fluctuations that subside after a few days.

However, if you consistently gain weight after every fast, reassess your regimen. Troubleshoot by adjusting the frequency and duration of your fasting periods. Also, examine what and how much you eat during non-fasting times.

In most cases, occasional 24-72 hour fasts should not cause fat gain. The metabolic and health benefits far outweigh temporary weight fluctuations. Continue fasting while working to minimize post-fast binging.


Gaining a few pounds after completing a 24 hour fast is common. It’s usually just water weight and food volume, not actual fat. Rapid shifts in glycogen, hormones, digestion, and water balance after fasting can temporarily increase your weight.

Aim to minimize carb binging and stay hydrated after fasting. Understand that initial weight gain will subside within a few days once your body adapts. Unless you gain considerable fat over several weeks, occasional fasting is beneficial for health, not harmful. Be patient through the adjustment period.