Diarrhea after fasting: Why does it happen and how to avoid it?

Fasting can offer a lot of benefits, including weight loss and autophagy. It also gives the deserved rest to your digestive system, giving it time to heal and recover from daily digestion. However, when you suddenly reintroduce food to your rested gut, it can react adversely, causing side effects like diarrhea.

Most often, diarrhea after fasting appears because of a sudden microbiome change, digestive overload, bile dump, and certain food choices. It’s not always a cause for concern, but rather a normal physiological reaction. To prevent it, it’s important to break your fast gradually and thoughtfully, choosing easy-to-digest meals and sticking to smaller portions.

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Fasting is voluntary abstinence from food for a certain period of time. It can be complete fasting, when you don't eat (and sometimes drink) for a day or a few. Or it can also be intermittent fasting, when you have fixed fasting and eating windows every day.

Depending on its type, fasting can have a range of benefits, from weight loss to autophagy. But it can also have a few side effects. In particular, some people may experience diarrhea after fasting. In this guide, we’ll look at the main reasons that cause it and discuss tips that can help prevent this side effect.

Digestive Processes Associated With Fasting

In order to understand what might be causing diarrhea after breaking fast, it’s important to understand what happens in your gut when you fast, in general.

The first thing you need to understand is that fasting gives you digestive rest. When you don’t consume any food for some time, you eliminate the typical load on your stomach and intestines because you don’t give them anything to process. During this time, your gut recovers from constant digestion and heals.

In the process of resting, your gut also activates the migrating motor complex (MMC). In a nutshell, it’s a cyclic motility process that takes place after 10+ hours of fasting. It’s meant to reduce inflammation, clear out undigested material, and stabilize gut bacteria.

Also, it’s important to note that fasting typically causes a metabolic reset. When your body doesn’t receive food to take energy from, it starts taking energy from stored fat. This process is known as ketosis.

All these processes are normal physiology. They happen in response to food abstinence and are aimed at supporting your digestive tract and gut health. Yet, while they all bring certain health benefits, they can also trigger adverse effects when you return food.

What Causes Diarrhea After Fasting?

As you now know, fasting triggers a number of changes in your gut. So when you reintroduce food after it, it’s possible to experience diarrhea. The most common causes include:

Sudden Gut Microbiome Change

Multiple studies confirm that fasting affects the richness, diversity, and composition of gut microbiota. And when you break a fast, the food that gets into your stomach creates a sudden change in your microbiome. This can alter gut motility and water absorption, and potentially contribute to diarrhea.

Digestive System Overload

Sometimes, when breaking a fast, people make a common mistake and reintroduce food rapidly or in overly large amounts. When this happens, your digestive system (that has been resting during the fasting period) may get overwhelmed. This triggers the process called the gastrocolic reflex – when your stomach stretches from food ingestion and stimulates colon evacuation. In addition, if you have insufficient enzymes after fasting, it can also cause malabsorption and loose stools.

Bile Dump

While fasting, your body concentrates bile in the gallbladder. When you reintroduce food (especially if it’s fatty), this can trigger the so-called bile dump. This is when excessive bile that has been stored earlier flushes. Bile salts can act as a laxative, which might be another cause of diarrhea.

Other Triggers

In some cases, diarrhea after breaking fast may appear not because of the fasting-related changes in your gut, but rather because of the specific food that you eat. For example, reintroducing high-fiber and fructose-rich foods can cause excessive water in your bowel. Fatty or very sweet foods can act as laxatives, especially if you have a sensitive gut.

It’s also possible to experience diarrhea if you have food intolerances or an existing gastrointestinal tract condition, such as celiac disease, inflammatory bowel disease, or another condition.

What Causes Watery Diarrhea After Breaking Fast?

Watery diarrhea typically refers to very loose, high-fluid stool. Experiencing this kind of diarrhea after fasting can be incredibly discomforting and cause a lot of concern, too. But it’s possible.

Watery diarrhea might result from one of the following causes:

• Osmotic shifts (or fluid shifts) stand for the movement of liquid across semipermeable cell membranes. They appear when the solute concentration in your body changes. Fasting is known to cause changes in aquaporins in the peritoneum. Respectively, it can cause osmotic shifts and draw a larger amount of water to your intestines, causing loose stools.
• Bile salt tends to accumulate during fasting. When you reintroduce food, it can flush, causing a laxative effect and sometimes resulting in watery diarrhea.
• Rapid motility can result when you reintroduce food after prolonged fasting. In some cases, it can result in watery diarrhea, though it can also cause slowed digestion and constipation.

It’s important to note that the choice of food for breaking a fast has a direct impact on your digestive comfort and the possible side effects. In particular, sugary, fatty, or large meals can contribute to watery diarrhea.

Practical Tips to Prevent Diarrhea After Breaking Fast

As was mentioned earlier, certain food choices (as well as portion sizes) can contribute to diarrhea after fasting and overall poor well-being. With this in mind, it’s important that you break your fast the right way. Here are a few practical tips that should help:

• Reintroduce food gradually. Since your digestive system has been resting during your fast, a sudden reintroduction of food can overwhelm it and cause adverse effects. Start slowly with low-calorie and easy-to-digest meals, such as cooked vegetables, broth, lean protein, etc.
• Avoid heavy foods and drinks that can contribute to diarrhea. You should steer clear of alcohol, excessive caffeine, fatty meals, sugary foods and drinks, spicy meals, and similar products that can irritate your gut.
• Keep your portions small. Consuming too much food with your first meal can also overwhelm your gut and contribute to discomfort and diarrhea. Start with small portions and increase them gradually.
• Keep an eye on your fluid balance. Consuming a sufficient amount of water and keeping a good electrolyte balance is crucial to prevent rapid dehydration after you break a fast.

When to Seek Professional Help?

As you can see, diarrhea after fasting isn’t uncommon or odd. It can happen for a number of reasons and often shouldn’t be a cause for concern. 

If you notice mild, short-lived diarrhea that lasts for 24-48 hours, it might be normal. It’s also okay if it’s accompanied by mild bloating or urgency.

However, in some cases, diarrhea might require medical attention. Look out for the following red flags:

• Persistent diarrhea and urging that lasts for 48+ hours.
• Diarrhea accompanied by severe cramping, nausea, and/or vomiting.
• Blood or mucus in stool.
• Fever or severe dehydration that comes with dizziness and dark urine.

If you notice these symptoms, it might be wise to see your healthcare provider to ease them and detect underlying causes.

FAQ

Why do I get diarrhea after breaking a fast?

Although it can be discomforting and cause a lot of concern, diarrhea after fasting isn’t uncommon. It can appear when you reintroduce food after a period of fasting due to one of the following reasons:

• Sudden gut microbiome change
• Digestive system overload
• Bile dump
• Certain food choices or existing intolerances

When should I be concerned about diarrhea after breaking fast?

In many cases, diarrhea after breaking fast should not be a cause for serious concern. If your symptoms are mild and short-term, there is no need to get worried. However, it’s important to keep an eye on your condition and possibly seek professional help if you notice blood or mucus in your stool, experience severe cramping or nausea, or have other disturbing symptoms that persist for over 48 hours.

What foods to avoid when breaking a fast?

In order to prevent diarrhea and general gut discomfort, it’s recommended to break your fast thoughtfully. The top foods to avoid include sugary foods (and drinks), dairy, fatty foods, processed foods, and spicy foods. It’s also important to avoid large portions because they can overwhelm the rested digestive system. Alcohol and excessive caffeine should also be avoided as they can contribute to diarrhea.

Conclusion

So, what causes diarrhea after fasting? In simple terms, it’s caused by a sudden reintroduction of food after a period of rest. During fasting, your digestive system relaxes and takes time to heal. Its microbiome changes. So when you suddenly start eating again, this can overload the gut and lead to adverse effects.

Luckily, diarrhea after fasting is not necessarily a cause for concern. If it’s mild and short-lived, it’s just a normal reaction of your body. Still, you can avoid or ease it by following simple rules, such as breaking your fast gradually, choosing easy-to-digest products, and keeping your portions small.

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