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Are you considering intermittent fasting but are concerned about digestive woes? You’re not alone! Many people have experienced unpleasant gastrointestinal symptoms related to intermittent fasting, such as cramping, indigestion, and diarrhea. 

While these may be enough to dissuade some from trying intermittent fasting, understanding how fasting may cause gastrointestinal disturbances can help ease your concerns and make informed decisions – so let’s get right into the gastro-gritty details of why this form of dietary control affects digestion in such an extreme way!

Can You Get Diarrhea From Not Eating – Exploring the Fasting Paradox

The gastrointestinal (GI) system is an intricate network that’s pivotal in the digestion and absorption of nutrients. It’s also home to a diverse microbiome, a community of microbes that assist in breaking down food, synthesizing vitamins, and guarding against harmful pathogens.

When you fast, you break your GI system from its usual tasks. Initial research suggests fasting could have health benefits, such as reduced inflammation. But, the absence of food can also result in a few adjustments within the GI system that could lead to diarrhea.

  • Bile Acid Malabsorption

The liver continuously produces bile, which helps digest fats. Bile may not be reabsorbed properly without food, leading to diarrhea.

  • Microbiome Changes

Fasting could alter the gut microbiome. An imbalance or shift in these microbial communities could result in gastrointestinal issues like diarrhea.

  • Hormonal Fluctuations

The GI system is also subject to hormonal control. Fasting can cause fluctuations in hormones like gastrin and motilin, which can speed up the motility of the gut and lead to diarrhea.

  • Psychological Factors

Stress and anxiety about fasting can also play a role. Stress hormones like cortisol can directly affect bowel movements.

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Digestive Challenges You May Face When Intermittently Fasting

Intermittent fasting (IF) has gained substantial attention for its benefits, including weight loss, improved metabolic health, and enhanced cognitive function. However, any significant alteration to your eating pattern can come with digestive challenges. Let’s break down some of those challenges and the underlying science.

Common Digestive Challenges

Bloating and Gas

If you go without eating for a long time, your gut microbiota could experience alterations. The bacteria responsible for fermenting carbohydrates could overact when you finally eat, leading to bloating and gas.

Diarrhea or Constipation

As discussed in the previous question, your digestive system is complex. Alterations in bile acid composition, microbiome shifts, and hormonal fluctuations can lead to either diarrhea or constipation during intermittent fasting.

Acid Reflux or Heartburn

Fasting may increase gastric acidity in some people, mainly if coffee is consumed during fasting. This heightened acidity can cause or exacerbate acid reflux and heartburn.

Changes in Gut Motility

Fasting can alter gut motility, which refers to the time it takes for food to move through the digestive tract. Speeding up or slowing down gut motility can result in digestive discomfort.

Feelings of Nausea or Fullness

Not eating for prolonged periods can sometimes make you feel nauseous, particularly when you start eating again. This feeling may be due to slowed gastric emptying or changes in digestive enzymes.

Fasting and Diarrhea – Is There a Connection?

Fasting can affect your body in several ways, from lowering insulin levels to inducing autophagy—cells cleaning out damaged components. But how does it affect the digestive system? Your gastrointestinal tract is a highly dynamic system influenced by many factors, including what you eat, when, and your body’s internal rhythms.

When you fast, the liver produces bile acids necessary for fat digestion. Because no food can absorb these bile acids, they can accumulate in the colon, drawing in more water and potentially causing diarrhea.

Several hormones, including gastrin and motilin, regulate gut motility. Fasting could alter these hormones’ levels, resulting in increased bowel movements or diarrhea.

The stress of adopting a new eating routine or fasting schedule may produce stress hormones like cortisol, which can influence gut motility and potentially lead to diarrhea.

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Does Water Fasting Cause Diarrhea? Diving Into the Fluid Facts

During fasting, your digestive system breaks from constantly breaking down and assimilating food. 

There’s no conclusive evidence to suggest that water fasting directly causes diarrhea. Fasting is more commonly associated with constipation due to decreased bowel movement.

However, it’s necessary to mention symptoms of diarrhea to look out for while fasting:

1. Breaking the Fast. Some people experience diarrhea when they break their fast and reintroduce food into their system. This is sometimes referred to as “refeeding diarrhea.” Too many high-fiber or fatty foods too quickly can overwhelm your digestive system, which has been dormant.

2. Electrolyte Imbalance. Consuming large amounts of water without food could lead to an electrolyte imbalance, which in extreme cases could cause diarrhea, among other symptoms.

3. Pre-existing Conditions. Those with existing gastrointestinal issues, such as IBS or Crohn’s, may experience exacerbated symptoms during fasting.

How Dehydration Can Trigger Diarrhea During Fasting?

Dehydration happens when your body fails more liquids than it takes in, which can hinder normal bodily functions. Among other things, dehydration affects the balance of electrolytes in your system—these are essential minerals that impact everything from muscle function to pH balance and water regulation.

When dehydrated, your colon may not reabsorb water as efficiently as it should. In a well-hydrated state, the colon absorbs moisture from the stool, making it firm. However, dehydration can hamper this process, leading to loose stools or diarrhea.

Also, it can affect gut motility, which refers to the contraction of the muscles in the gastrointestinal tract. Impaired gut motility can result in diarrhea.

Moreover, if you attempt to correct dehydration rapidly by consuming large amounts of plain water, it can create an osmotic imbalance across your intestinal wall. This might draw water into the lumen of the intestine, contributing to diarrhea.

Does Fasting Make You Poop? The Scoop on Your Seat

Firstly, the relationship between fasting and bowel movements is complex. The digestive system is an intricate interplay of hormones, microbial activity, and muscle contractions that work harmoniously to process food, extract nutrients, and expel waste. 

Physiological Factors 

The gastrointestinal tract is regulated by complex hormonal signals, including ghrelin, which stimulates appetite, and leptin, which signals fullness. When you fast, ghrelin levels typically drop, which could slow down the rate at which your intestines move food and waste (motility), thereby reducing the frequency of bowel movements.

Microbiome Influence

Your gut microbiome, comprising various bacteria, also affects digestion and bowel movements. Fasting can affect the balance of these bacteria, changing how quickly food and waste are processed. Some may find that fasting leads to looser stools due to changes in microbial composition, although the evidence is not definitive on this. 

How Intermittent Fasting Affects Your Bowel Habits?

Hormonal Interplay

Firstly, it’s crucial to grasp that your gastrointestinal (GI) system doesn’t operate in isolation; it’s regulated by a variety of hormones like ghrelin (the “hunger hormone”) and leptin (the “satiety hormone”). Intermittent fasting can affect these hormonal levels. Specifically, fasting often lowers ghrelin levels, which generally decreases hunger but also can reduce the rate at which your intestines move waste (or its ‘motility,’ as it’s scientifically termed). Reduced motility often means reduced frequency of bowel movements.

The Fiber Factor

Let’s talk about fiber—the unsung hero in bowel regularity. If your eating window includes a balanced meal rich in fiber, it may counterbalance any decrease in fasting-induced motility. On the other hand, one of the best ways to adjust fasting practices for better digestive health

is if your limited eating window leads you to make less-than-optimal food choices low in fiber, you could experience reduced frequency and harder stools, aka constipation.

Impact on Gut Microbiota

Your gut is home to trillions of bacteria critical in digestion and waste elimination. Any drastic change in eating habits, like starting intermittent fasting, could result in a change in the composition of this microbiome. While the full extent of this is still under research, preliminary studies suggest that fasting states can alter gut microbiota, potentially affecting bowel habits.

Hydration Considerations

Don’t forget about water! If you are fasting, especially for more extended periods, you may consume less water than usual simply because you’re not eating (many people get a good amount of daily water from food). Reduced water intake could exacerbate any tendencies toward constipation. So, it’s advisable to maintain hydration even during fasting periods.

Intermittent Fasting Diarrhea: Unpacking the Unpleasant Side Effect

While fasting alone doesn’t cause diarrhea, there are two main reasons you may experience this.

  • Many people drink excessive water and liquids high in caffeine, such as tea and coffee, during the fasting window. Drinking too much of these can upset the digestive tract and cause diarrhea. Try drinking less water and/or less fluids with caffeine to see if this helps.
  • Digestive unease from fasting can occur after not eating for an extended time because the gastrointestinal tract (GI tract) loses its ability to function correctly. After all, it hasn’t been used for such a long time. This may mean that you are fasting for too long. Try fasting for a shorter period and see if the diarrhea disappears.

If you are still experiencing diarrhea after addressing both causes, you may want to stop fasting to see if this helps. Regardless, speak with your doctor because another underlying condition may be causing this, such as Cron’s disease, a nutrient deficiency, colitis, or an infection. 

Physiological Factors May Cause Diarrhea

  • Food intolerance or allergy
  • Lactose intolerance
  • Eating a high-fat diet
  • Eating lots of greasy foods

These factors, however, clearly have nothing to do with intermittent fasting but still are things to pay attention to if you are experiencing diarrhea.

Intermittent fasting shouldn’t cause loose stools, so pay attention to your bowel movements while fasting to ensure you aren’t compromising your overall health. Hopefully, this clarifies if intermittent fasting causes diarrhea. 

Diarrhea During Fasting or Why Timing is Everything?

The liver constantly produces bile, which is necessary for fat digestion. Fasting can cause bile to accumulate in the digestive system, as there’s no food to absorb it. Excess bile can cause the stool to loosen, leading to diarrhea. 

Your gut transit time—how long food takes to travel through your digestive system—can change when you fast. A faster transit time can result in diarrhea.

Fasting can alter your gut microbiota, potentially leading to loose stools or diarrhea as your digestive system adjusts. Oddly enough, dehydration can sometimes lead to diarrhea. When you fast, especially if it’s a dry fast with no fluid intake, you might become dehydrated, affecting your digestive system.

Finally, if you’re fasting but still consuming black coffee or tea, these beverages can act as mild laxatives and contribute to diarrhea.

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Steps to Prevent Diarrhea During Fasting Periods

1. Observe Patterns 

Pay attention to when you’re experiencing diarrhea. Is it right after breaking your fast or during the fasting period? Understanding the timing can help pinpoint the cause.

2. Hydrate

If you’re on a fast that allows fluid intake, ensure you’re hydrating adequately. Electrolytes can also help in maintaining a balance.

3. Slow Reintroduction of Foods

When you break your fast, consider reintroducing foods slowly, starting with easily digestible options. This can help your digestive system adjust more smoothly.

4. Adjust Fasting Schedule

If you find that diarrhea persists each time you fast, you may need to change your fasting routine or consider other forms of intermittent fasting.

5. Seek Immediate Medical Attention 

If diarrhea is accompanied by severe dehydration, high fever, or other symptoms, seeking immediate medical assistance is essential.

Yellow Stool While Fasting: What It Means & What to Do

Yellow stool while fasting can be a concerning symptom, and the reasons for its occurrence can be varied. Below are some of the most common factors:

  • Bile Secretion

Bile, produced in the liver and stored in the gallbladder, usually is greenish-yellow. It aids in the digestion and absorption of fats in the small intestine. When you fast, you’re not consuming food, and thus, bile may not be fully reabsorbed or metabolized into its usual by-products, causing your stool to appear yellow.

  • Gut Transit Time

Fasting could speed up or slow down the transit time of food through your gastrointestinal tract, affecting the color of your stool. Faster transit time can result in lighter-colored stools.

  • Change in Microbiota 

Fasting can affect the gut microbiota, the community of microorganisms in your digestive tract. This change can also result in variations in stool color.

  • Dehydration

By the way, can intermittent fasting cause diarrhea? Yes, particularly if extended, can lead to dehydration. Lack of fluids can sometimes alter the color and consistency of the stool.

  • Diet

Some people consume specific food or supplements before starting a fast, which could also be a contributing factor.

Diarrhea After Fasting: What’s Going On Down There?

Bile Secretion

Your liver constantly produces bile to help digest fats. During fasting, bile can accumulate in your digestive system. Once you start eating again, excess bile can lead to looser stools or diarrhea.

Gut Flora Changes

Fasting can lead to changes in your gut microbiota, and reintroducing food may cause a temporary imbalance, possibly resulting in diarrhea.

Sudden High-Fiber Intake 

How can I have diarrhea when I haven’t eaten? One of the ways is If you break your fast with high-fiber foods, especially fruits and vegetables, you may experience diarrhea. The sudden fiber can increase your stool’s bulk and water content.


The digestive system becomes more sensitive after fasting. Eating large meals right after can overwhelm it, leading to diarrhea.

Sensitivity to Foods

Fasting can make your gut more sensitive to certain foods, including foods you can tolerate. This increased sensitivity can result in diarrhea.

Diarrhea After Not Eating for 24 Hours – What’s Up With That?

  • Empty Stomach. Going for a long time without food can irritate your gastrointestinal tract. Your stomach continues to produce acid and digestive enzymes, which can lead to an imbalance when there is no food to process.
  • Stress Hormones. Fasting for 24 hours may cause stress to the body, leading to an upsurge in stress hormones like cortisol. Elevated cortisol levels can disrupt bowel movements and trigger diarrhea.
  • Body’s Defense Mechanism. The body could interpret prolonged fasting as a sign of stress or threat, triggering the “fight or flight” response. This may lead to sped-up digestion and bowel movements.
  • Refeeding. Why do I get diarrhea when I break my fast? If you break your fast by consuming a large meal or certain types of food (like those rich in fats or sugar), your gut may react by speeding up bowel movements, leading to diarrhea.
  • Existing Medical Conditions. If you have underlying health problems like IBS (Irritable Bowel Syndrome) or food intolerances, fasting may exacerbate these conditions.
  • Dehydration. Not eating often accompanies reduced fluid intake, leading to dehydration. A dehydrated body may produce loose stools, exacerbating diarrhea symptoms.

Also, readThe Metabolism Myth: What Your Body is Really Doing When You Eat

Why Diarrhea After Fasting 72 Hours?

Prolonged fasting can disrupt the regular function of your gastrointestinal system. Your gut continues to produce acids and digestive enzymes even when empty, potentially causing an imbalance that may lead to diarrhea.

When fasting, the body might release stress hormones like cortisol. When cortisol levels are high, it can cause several problems with digestion, such as diarrhea.

Extended fasting might also signal a threat to the body, activating the “fight or flight” response. This response can speed up digestion and bowel movements, which may manifest as diarrhea.

When you start eating again after fasting, consuming a meal that is high in carbohydrates or fats can cause what is known as “refeeding syndrome,” which often includes symptoms like diarrhea.

If you have pre-existing gastrointestinal conditions like IBS, Crohn’s Disease, or food intolerance, fasting could exacerbate symptoms, leading to diarrhea.

Your Go-To Guide on How to Stop Diarrhea From Intermittent Fasting

Stay Hydrated 

Dehydration can accompany diarrhea. Keep up your fluids with water or electrolyte solutions. 

Eat a BRAT Diet

Once your eating window opens, stick to simple, weak meals like Bananas, Rice, Applesauce, and Toast (BRAT) to soothe the digestive system.

Reduce Caffeine and Sugar 

Beverages like coffee and sugary drinks can worsen diarrhea. Opt for herbal teas or water instead.

Start with Smaller Meals 

Avoid large meals that overwhelm your digestive system when your eating window opens. Gradually reintroduce food.


Overall, experiencing stomach discomfort during intermittent fasting is a beautiful way to regulate body weight and use energy wisely. 

Knowing the science behind it may be helpful to understand what our body is going through during the process. While some might describe these bowel changes as uncomfortable or undesirable, they should generally not last long as you become accustomed to a period of fasting. Pay attention to how your body reacts to get the most out of your experience with temporary fasts. 

No matter where you are on this journey, remember that occasional bumps don’t make success impossible – learning about yourself and listening to your body go miles!

Clinical Project Manager and Writer

With a Ph.D. in nutritional biology, Dr. Richardson is an expert in the field of nutrition, particularly intermittent fasting. Her dissertation project was an intermittent fasting study in athletes, which is where she gained substantial knowledge on the topic. She has contributed to a number of peer-reviewed articles spanning a wide array of topics and works as a freelance writer trying to make scientific knowledge accessible to the public.

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