Can keto make you feel bad? Understanding the "Keto Flu"

December 17, 2025 •

4 min read

According to research, up to 25% of people who attempt a ketogenic diet experience a cluster of flu-like symptoms during the initial transition period. This phenomenon is known as the "keto flu" and directly answers the question, "Can keto make you feel bad?" as the body shifts from burning glucose to fat for fuel.

Quick Summary

The keto flu is a common but temporary side effect of starting a ketogenic diet, caused by carb withdrawal and changes in electrolyte balance. Symptoms like fatigue, headache, and nausea are normal and can be managed with proper hydration, electrolyte replacement, and rest. This period usually resolves in a week or two as the body adapts to burning fat for energy.

Key Points

Keto Flu is Normal: A cluster of flu-like symptoms often occurs in the first weeks as your body adapts to burning fat for energy instead of glucose.
Electrolytes are Key: Drastically cutting carbs can lead to dehydration and a loss of electrolytes like sodium, potassium, and magnesium, causing headaches, fatigue, and muscle cramps.
Replenish Electrolytes: Counter keto flu by drinking plenty of water, consuming bone broth, and adding salt to your food to replace lost minerals.
Ease into the Diet: Transitioning to a ketogenic diet more gradually, rather than all at once, can help minimize the severity of symptoms.
Fatigue and Brain Fog are Temporary: Initial low energy and mental cloudiness are common as your brain adjusts to its new fuel source; these symptoms typically subside within a few weeks.
Digestive Issues are Common: Expect potential constipation or diarrhea as your gut adapts to lower fiber and higher fat intake.
Rest and Moderate Exercise: Listen to your body and prioritize rest. Opt for gentle exercise like walking or yoga during the adaptation period instead of strenuous workouts.
Consult a Professional: Always speak with a doctor or registered dietitian before beginning a ketogenic diet, especially if you have pre-existing health conditions.

The Metabolic Shift Behind Feeling Unwell on Keto

When you drastically reduce your carbohydrate intake on a ketogenic diet, your body undergoes a significant metabolic shift known as nutritional ketosis. Instead of its usual fuel source—glucose from carbs—your body must learn to burn fat for energy. This can be a challenging adjustment for many, triggering a range of unpleasant side effects often collectively referred to as the "keto flu". These symptoms are not caused by an infection but are a direct result of your body's adaptation process. Understanding the underlying mechanisms is the first step toward managing them effectively and avoiding the temptation to quit the diet prematurely.

The Role of Electrolyte Imbalance and Dehydration

One of the primary drivers of keto flu symptoms is a rapid change in fluid and electrolyte balance. When you cut carbs, your insulin levels drop, causing your kidneys to release more sodium and water. This diuretic effect can lead to dehydration and a loss of crucial electrolytes like sodium, potassium, and magnesium. The resulting imbalance can manifest as headaches, muscle cramps, dizziness, and fatigue. Staying well-hydrated is key during this period, but simply drinking more water isn't always enough. You must also actively replenish lost electrolytes through food or supplements.

The Shock of Carbohydrate Withdrawal

For those accustomed to a high-carbohydrate diet, the brain relies heavily on glucose for energy. When this primary fuel source is removed, the brain must adapt to running on ketones, a process that can take a few days. During this transition, a temporary state of mental fatigue and brain fog can occur as the brain's energy supply is disrupted. This is often accompanied by intense cravings for carbohydrates, as the body still remembers its old energy source. For some, this feels like a genuine withdrawal period, similar to giving up caffeine or other substances the body has grown dependent on. However, once fully in ketosis, many people report enhanced mental clarity and sustained energy.

Digestive System Changes

The change in your macronutrient composition can also impact your digestive system. Switching from a high-fiber, carb-rich diet to a low-carb, high-fat one can cause issues like constipation or diarrhea. Constipation is common due to the reduced fiber intake from grains, fruits, and starchy vegetables. Conversely, some may experience diarrhea, especially if they are not used to digesting such large quantities of fat. The gut microbiome also shifts with these dietary changes, which can contribute to temporary digestive upset as it adjusts to the new food sources.

Managing Your Transition for Fewer Ailments

Fortunately, it is possible to mitigate or even avoid the worst of the keto flu symptoms with a few key strategies. Taking a gradual approach to carbohydrate reduction, rather than going "cold turkey," can give your body more time to adapt. Additionally, proper hydration and electrolyte intake are essential. Many find relief by adding a pinch of salt to water, drinking bone broth, or using electrolyte supplements. Getting enough rest and avoiding strenuous exercise in the initial weeks can also help your body conserve energy for the metabolic transition.

Comparison Table: High-Carb vs. Keto Transition


Feature	High-Carbohydrate Diet Transition	Ketogenic Diet Transition (Keto Flu)
Primary Fuel Source	Body remains primarily glucose-dependent.	Shifts from glucose to fat (ketones) for energy.
Energy Levels	Generally stable; minor fluctuations possible with diet quality changes.	Possible fatigue, weakness, and decreased endurance initially.
Mental Acuity	Stable; depends on consistent carb intake.	Temporary brain fog and mental fatigue possible.
Electrolyte Balance	Largely stable, assuming adequate intake from a balanced diet.	Dehydration and electrolyte loss are common due to water shedding.
Gastrointestinal Issues	Can vary based on fiber changes; generally fewer acute issues.	Constipation or diarrhea are common as gut adapts to high fat, low fiber.
Duration of Discomfort	Minimal or none.	Usually a few days to a couple of weeks.

Conclusion

While the keto diet can make you feel unwell during the initial adaptation phase, this is a normal and temporary process for many. The "keto flu" is primarily caused by carbohydrate withdrawal, electrolyte imbalances, and dehydration as the body switches its primary fuel source. By being prepared for these symptoms and taking proactive steps to manage them—such as focusing on hydration, supplementing electrolytes, and easing into the diet gradually—you can make the transition more comfortable. However, as with any major dietary change, it is important to consult a healthcare professional to ensure it is the right path for your individual needs and to rule out any underlying health issues.

[For more information on optimizing your diet transition and a list of keto-friendly electrolyte sources, consult a registered dietitian.]

Frequently Asked Questions

The 'keto flu' is a term for the flu-like symptoms—including headaches, fatigue, nausea, and irritability—that some people experience when first starting the ketogenic diet. It happens as the body adapts to burning fat for energy (ketosis) instead of glucose from carbohydrates.

For most people, keto flu symptoms are temporary and last a few days to a couple of weeks as their body adjusts. If symptoms persist or worsen, it's advisable to consult a healthcare provider.

Yes, dehydration is a major contributor to keto flu symptoms. When you cut carbs, your body sheds water and electrolytes, which can lead to lightheadedness, fatigue, and headaches.

Muscle cramps, particularly in the legs, are often caused by an electrolyte imbalance, specifically a loss of sodium, potassium, and magnesium during the initial phase of ketosis. Electrolyte supplements or consuming foods rich in these minerals can help.

No, not everyone experiences the keto flu. Symptoms and severity vary greatly from person to person, with some adapting to ketosis with few to no side effects.

You can reduce symptoms by staying well-hydrated, replenishing electrolytes (through supplements or food), easing into the diet gradually, and resting during the initial transition period.

Bad breath, sometimes called 'keto breath,' is caused by the body producing ketones, including acetone, which is excreted through the breath. This side effect usually diminishes over time.