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Diarrhea During Fasting: Why it Happens and What to Do

Fasting is a popular tool for weight loss and overall health. It involves going without food for an extended period of time — usually anywhere from 16 hours to several days. 

Intermittent fasting can be great for you. There are many benefits of fasting, including balanced blood sugar, stable insulin levels, protection against insulin resistance, and a decreased risk of heart disease. 

However, some people experience side effects from fasting. One of the less common side effects is diarrhea. 

This article will cover the causes of diarrhea during fasting, whether or not you should worry about it, and how you can prevent diarrhea during fasting.

Is Diarrhea During Fasting Normal?

It’s rare to get diarrhea during fasting, but it does happen.

Diarrhea is caused by food and water moving too quickly through your digestive system. The food passes through undigested and you don’t absorb its nutrients or energy content. 

It’s not normal to have diarrhea or watery stools during fasting. If you get diarrhea in the middle of a water fast (when you’re having only water, no food), you should end your fast immediately. Diarrhea during fasting can be dangerous. 

Causes of Diarrhea During Fasting

Fasting doesn’t usually cause diarrhea directly. Because you aren’t eating, there’s no food in your digestive tract, which makes it unlikely that you’ll have diarrhea. 

It’s more common to experience diarrhea or other unusual bowel movements after you break your fast. This is especially true after multiple days of fasting.

Your digestive system slows down when you stop eating. If you break a long fast with a large, rich meal, your digestive system may not be able to digest it, causing diarrhea.

That said, there are a few possible causes of diarrhea during fasting. 

Too Many Electrolytes

Electrolytes can help prevent dehydration during fasting. A lot of people like to take magnesium, potassium, calcium, and sodium during fasting periods. 

However, excess electrolytes can cause diarrhea, especially when you take them on an empty stomach[*]. 

Your body tries to maintain electrolyte balance between the fluid inside your cells and the fluid outside your cells. 

If you take too many electrolytes, you may excrete them with excess water through your gastrointestinal tract, causing diarrhea[*]. 

Coffee, Tea, and Other Caffeinated Drinks

Caffeine can also cause diarrhea, especially if you drink a lot of it.

A lot of people drink coffee during fasts because it contains no calories and may suppress appetite. 

If you get diarrhea after drinking coffee or tea, try leaving it out of your fasting routine. 

Zero-Calorie Sweeteners

If you’re drinking zero-calorie drinks during your fast, they may contribute to diarrhea. 

Zero-calorie sweeteners like aspartame, sucralose (Splenda), and sugar alcohols (erythritol and xylitol) can cause digestive distress. 

Side Effects of Diarrhea During Fasting

Diarrhea during fasting can cause severe dehydration. 

When you fast, you lose several pounds of water weight. This happens because your body is burning through your glycogen (sugar) stores, which releases water.

As a result, you’re at risk of being dehydrated. Any time you’re fasting, it’s a good idea to increase your water intake. 

Diarrhea causes dehydration too, and diarrhea during fasting can dehydrate you to dangerous levels. 

Watch for the following symptoms:

  • Blurry vision
  • Dry mouth
  • Dizziness
  • Nausea
  • Headache
  • Cramping
  • Bloating

If you experience any of these symptoms, drink lots of water and add some Himalayan pink salt or sea salt to balance your electrolytes. You may also want to end your fast early with an easy-to-digest food like bone broth. 

Other Reasons to Stop Your Fast

Diarrhea alone is not necessary cause for concern, as long as you drink plenty of fluids. 

However, chronic diarrhea may be a symptom of a bigger digestive issue. If it persists for multiple days, stop fasting and visit your doctor. 

You should also stop fasting and see a doctor if you have any of the following symptoms:

  • Severe abdominal pain
  • Fever
  • Bloody stool
  • Loss of consciousness
  • Persistent fatigue

These symptoms could be a sign of a more serious health issue. 

How to Prevent Diarrhea During Fasting

There are a few things you can do to prevent diarrhea during fasting. 

  • Drink lots of water.
  • Take electrolytes like magnesium, sodium, calcium, and potassium (follow the recommended doses, as excess electrolytes can cause diarrhea).
  • Avoid caffeine from coffee, tea, and energy drinks. 
  • Avoid zero-calorie sweeteners like aspartame, sucralose, erythritol, and xylitol. 

When you break your fast, eat a small, easy-to-digest meal instead of a heavy or high-fat one. A small meal will stimulate your digestive system and bring it back to full function. 

A couple of hours after you break your fast with a small meal, you should be okay to eat whatever you want. 

The Takeaway

Diarrhea is a rare side effect of fasting, but it does happen to some people. 

Usually, diarrhea during fasting comes from caffeine, zero-calorie sweeteners, or taking electrolytes on an empty stomach. It’s typically not dangerous as long as you drink plenty of fluids to prevent dehydration. 

By and large, fasting is a valuable tool for weight loss and overall health. 

Most people fast with few to no side effects, and if you’re trying to get in better shape, you may want to add intermittent fasting to your daily routine. This complete guide to intermittent fasting has everything you need to get started.