Some people who follow a keto diet may experience diarrhea and other gastrointestinal (GI) problems as side effects.

In some cases, diarrhea or GI issues may be short term while the person’s body adjusts to the new diet. However, these problems can sometimes be more long term and even last the duration of the keto diet.

In this article, we explain what a keto diet is, why it can cause GI problems, other possible symptoms, how to relieve symptoms, and when to see a doctor.

What is a keto diet?

Consuming a greater amount of dairy when beginning a keto diet may lead to diarrhea and other GI problems.
A ketogenic diet, or keto diet, is a diet that is high in fat, moderate in protein, and very low in carbohydrate.

Typically, the body uses carbohydrates as an energy source. When there are insufficient carbohydrates in the body, it enters a state called ketosis. During ketosis, the body breaks down stored fat to create molecules called ketones, which it uses as fuel instead of carbohydrates.

Research examining the keto diet has found that it can successfully help people lose weight by improving fat oxidation. Fat oxidation is a process in which the body breaks down fatty acids.

Additional evidence from human studies suggests that the keto diet may have beneficial effects on blood sugar levels and insulin resistance, among other markers of health. However, more long-term research is necessary to understand exactly how the keto diet affects overall health.

Despite the evidence to support the benefits of a keto diet, other research suggests that eating this way can have some adverse effects. These may include:

• diarrhea
• constipation
• vomiting
• abdominal pain
• Causes of GI problems

As with any extreme dietary changes, starting on the keto diet can have an effect on a person’s stomach and intestines, causing GI problems.

Some people may continue to have GI problems throughout the keto diet. A person on a keto diet must consume a lot of fat and little carbohydrate to reach and maintain ketosis. Some people may find that their body struggles to adjust to the high fat content of this diet.

There are a few other reasons why people on a keto diet may experience GI problems. These include:

Microbiota
Gut microbiota refers to microorganisms in the GI tract. These mainly consist of bacteria that are important to health in many ways, including helping the body absorb nutrients.

A 2019 study found that diets high in fat might cause inflammation of the digestive tract, a decrease in healthful fatty acids, and unfavorable changes in the gut microbiome.

However, other studies have shown that a keto diet may have positive effects on gut bacteria. More high quality research is necessary to determine the effects of the keto diet on gut health.

High fat
To break down fat, the liver needs to produce bile. A diet high in fat requires the liver to release extra bile. As bile is a natural laxative, an excessive amount may push waste through the digestive tract quicker than usual, leading to diarrhea.

The high fat and low carb content of the keto diet may also lead to other gastrointestinal symptoms, such as nausea and bloating.

Artificial sweeteners
During a keto diet, people may consume more artificial sweeteners and sugar alcohols than usual if they try to find low carb alternatives to items that they previously consumed. For example, they may wish to replace the sugar in tea and coffee.

Research has found that excessive consumption of certain artificial sweeteners can cause a laxative effect, leading to diarrhea.

Other symptoms
Diarrhea may not be the only symptom that manifests when a person begins a keto diet. People may also experience the following symptoms:

• constipation
• cramping
• abdominal bloating
• nausea
• abdominal pain
• vomiting
• Tips to relieve symptoms

If someone on the keto diet is experiencing GI problems, they can try to relieve the symptoms by:

• Eating more fiber: Fiber can help relieve constipation or diarrhea. Greens, cucumber, broccoli, and cabbage are some high fiber foods that are suitable as part of a keto diet.
• Eating less dairy: As whole dairy products are high in fat, people starting on a keto diet may consume more of these than they previously did. This dietary change may reveal an intolerance to dairy, common symptoms of which are diarrhea, bloating, and gas.
• Easing into the diet: A sudden change in the diet may disrupt the GI bacteria. Easing into a keto diet by slowly reducing carbs and increasing fats may help the body adapt better.
• Staying hydrated: Drinking can help improve constipation. If a person is experiencing diarrhea, it is important to replace fluids. Drinking water and sugar-free electrolyte beverages can help replace these fluids and any lost electrolytes.
• Adding probiotics and prebiotics: Taking a probiotic supplement or increasing the intake of probiotic- and prebiotic-rich foods may effectively support the gut bacteria during a keto diet. Doing this may reduce the GI effects of a keto diet.
• Eating fermented foods: Eating fermented foods, such as sauerkraut and kimchi, can help improve digestion, possibly reducing constipation or diarrhea.
• Decreasing the intake of sugar substitutes: Cutting back on the intake of foods and beverages that contain keto-friendly sugar substitutes may help relieve gastrointestinal symptoms.

When to see a doctor
Before making any significant lifestyle or dietary changes, it is always worth consulting with a healthcare professional.

Anyone who is experiencing diarrhea that is severe or lasts longer than a week should consult their doctor.

Similarly, if constipation is long lasting or extremely painful, it is best to seek medical attention.

For most people, following an extreme diet such as the keto diet is not necessary for weight loss or to improve health.

People interested in trying a low carb diet may wish to consider working with a qualified dietitian to see what dietary changes would work best for their lifestyle. Less restrictive dietary patterns can often produce similar benefits without the risk of uncomfortable side effects.

Summary
A keto diet is a diet that largely consists of foods that are high in fat and low in carbs. During a keto diet, the body goes into a state of ketosis in which it uses fat instead of carbohydrates for energy.

Research has shown that a keto diet may aid weight loss. However, as the diet can lead to changes in the digestive tract, it can also cause adverse GI effects, such as diarrhea and constipation.

To relieve diarrhea and constipation that is due to the keto diet, people should consider taking steps such as eating more fiber, taking probiotic supplements, and easing into the diet.

If the symptoms of GI upset are severe, long lasting, or painful, it is best to seek medical attention.