Can Cutting Down on Carbs Make You Feel Sick?

December 29, 2025 •

4 min read

According to research published in Frontiers in Psychology, the so-called 'keto flu' can be a real phenomenon for those transitioning to a very low-carbohydrate diet. Many people wonder if the uncomfortable symptoms they experience, such as headaches and fatigue, are normal when reducing their carbohydrate intake.

Quick Summary

This article explains why abruptly reducing carbohydrate intake can trigger a temporary period of adjustment known as the 'keto flu'. It explores the underlying physiological causes, common symptoms, and provides actionable strategies to mitigate these unpleasant effects during the transition.

Key Points

Metabolic Adjustment: Sickness from cutting carbs is due to your body switching from burning glucose to fat for energy, a process called ketosis.
Electrolyte Loss: The flush of stored water during the initial phase of carb reduction also depletes key electrolytes, leading to fatigue and muscle cramps.
Common Symptoms: Expect temporary headaches, fatigue, brain fog, nausea, and irritability, collectively known as the 'keto flu'.
Gradual Transition: Easing into a lower carb diet, rather than making a sudden cut, can help minimize the shock to your system and reduce symptoms.
Stay Hydrated and Supplement: Drinking plenty of water and replenishing lost electrolytes, either through food or supplements, is crucial for managing symptoms.
Symptoms Are Temporary: For most people, the unpleasant side effects of carb withdrawal only last for a week or two as the body adapts.

Understanding the 'Keto Flu' and Why It Happens

When you significantly reduce your carbohydrate intake, especially when aiming for ketosis, your body undergoes a major metabolic shift. Your primary source of fuel switches from glucose (derived from carbohydrates) to ketones (produced from fat). This metabolic pivot is a significant change for your body and can cause a collection of side effects often referred to as the "keto flu" or "carb flu".

The Physiological Shift: Glucose to Ketones

For years, your body's cells have been powered predominantly by glucose. When you remove that reliable energy source, your body has to adapt. The symptoms you experience are part of this adaptation process. Your body first depletes its stored glucose (glycogen). Glycogen is stored with water and electrolytes, and as your body uses it up, it releases that water and flushes out important electrolytes like sodium, potassium, and magnesium. This loss is a major contributor to feeling unwell, causing dehydration and electrolyte imbalances that can lead to headaches, fatigue, and muscle cramps.

Electrolyte Imbalance and Dehydration

The most common and immediate cause of feeling sick is dehydration and the subsequent imbalance of electrolytes. Symptoms like headaches and dizziness are often directly related to a rapid loss of fluids and key minerals. Without enough sodium, your blood pressure can drop, making you feel lightheaded. Lack of potassium can contribute to muscle cramps, while magnesium deficiency is associated with fatigue and sleep issues.

Common Symptoms of the Low-Carb Transition

The symptoms of the "carb flu" can vary widely in severity and type, though most people report similar experiences. They typically begin within a few days of starting a low-carb diet and can last anywhere from a few days to a few weeks, depending on the individual.

Common 'Keto Flu' Symptoms:

Headaches: Often caused by dehydration and electrolyte loss, or a direct response to reduced blood sugar.
Fatigue and Weakness: The body is used to a fast-acting glucose fuel source and may feel sluggish as it adapts to using fat for energy.
Brain Fog: Reduced glucose to the brain can cause a temporary period of mental fogginess and difficulty concentrating.
Nausea and Stomachache: Some people experience gastrointestinal distress, including nausea, bloating, or stomach cramps as their digestion adapts.
Constipation or Diarrhea: A shift in dietary fiber intake and changes in gut bacteria can disrupt digestion.
Irritability and Mood Swings: Hormonal and blood sugar fluctuations can cause mood changes and increased irritability.
Muscle Cramps: Depletion of electrolytes, especially potassium and magnesium, can lead to painful muscle spasms.

Managing and Mitigating Symptoms

Fortunately, these uncomfortable symptoms are temporary and can be effectively managed with a few simple strategies. Easing into the diet is the best approach to minimize the shock to your system.

Actionable Steps to Feel Better:

Hydrate, Hydrate, Hydrate: Drink plenty of water throughout the day to combat dehydration caused by reduced glycogen stores and increased urination.
Replenish Electrolytes: Actively replace lost electrolytes by salting your food generously and consuming foods rich in potassium and magnesium. You can also use a sugar-free electrolyte supplement.
Ease into the Diet Gradually: Instead of cutting carbs completely overnight, try a more moderate low-carb approach first. Gradually reduce your intake over a week or two to allow your body to adapt more smoothly.
Increase Healthy Fat Intake: As you reduce carbs, ensure you are getting enough calories from healthy fats. This helps provide a steady energy source and can reduce fatigue and cravings.
Listen to Your Body and Rest: Don't push yourself through intense exercise during the initial transition. Opt for lighter activities like walking or yoga and prioritize getting plenty of rest.

Comparison of Carb Withdrawal and the Flu

While the symptoms can feel similar, it is important to distinguish between the temporary "keto flu" and an actual viral infection.


Feature	'Keto Flu' (Carb Withdrawal)	Viral Flu Infection
Cause	Metabolic shift from glucose to fat for fuel, often accompanied by dehydration and electrolyte imbalance.	Infection by the influenza virus, triggering an immune response.
Duration	Typically lasts a few days to a couple of weeks, subsiding as the body becomes fat-adapted.	Usually lasts for 7-10 days, though cough and fatigue can linger.
Key Symptoms	Headaches, fatigue, brain fog, irritability, nausea, constipation, muscle cramps.	Fever, body aches, sore throat, cough, runny or stuffy nose.
Treatment	Staying hydrated, replenishing electrolytes, consuming healthy fats, and getting rest.	Rest, fluids, and potentially antiviral medication in severe cases.

Conclusion

It is common and normal to feel sick when cutting down on carbs, particularly if the reduction is sudden and severe. The "keto flu" or "carb flu" is a temporary, but manageable, set of symptoms caused by your body's metabolic adjustment, dehydration, and electrolyte imbalances. By transitioning gradually, staying hydrated, and focusing on replenishing key minerals, you can significantly reduce the severity and duration of these effects. If symptoms persist for more than a couple of weeks or become debilitating, it's wise to consult a healthcare provider to rule out any other issues. With a proper approach, the initial sickness is a passing phase on the way to a more fat-adapted metabolic state.

This article is intended for informational purposes only and does not constitute medical advice. For any dietary changes, please consult with a qualified healthcare professional or registered dietitian. For more in-depth information on the keto flu, see the Healthline article The Keto Flu: Symptoms and How to Get Rid of It.

Frequently Asked Questions

For most people, the flu-like symptoms from cutting carbs, known as the 'keto flu,' last anywhere from a few days to about two weeks. The duration depends on your body's ability to adapt to using fat for fuel.

When you stop eating carbs, your body loses its primary and most readily available fuel source, glucose. The fatigue and weakness are a sign that your body is in the process of shifting its energy source to ketones derived from fat, which takes time.

Yes, dehydration is a major contributor to the symptoms. When you cut carbs, your body flushes out water and electrolytes, which can lead to headaches, dizziness, and fatigue if you don't actively rehydrate and replenish your mineral intake.

Electrolytes are essential minerals like sodium, potassium, and magnesium that help regulate body functions. When cutting carbs, your body loses these minerals, and replenishing them is vital to prevent muscle cramps, headaches, and general malaise.

If you feel severely ill, you can add some healthy carbs back slowly to ease the transition. A better long-term strategy is to focus on hydration and electrolytes, as a gradual reduction is often more sustainable than quitting entirely.

The 'keto flu' is a collection of temporary, diet-related side effects, unlike the viral flu which is caused by an infection and often includes a fever and cough. The keto flu is the result of metabolic change, not a pathogen.

Focus on high-fat, high-electrolyte foods. Consume plenty of broth for sodium, avocado and leafy greens for potassium, and nuts and seeds for magnesium. Increasing healthy fat intake can also help maintain energy.