Why Do I Feel So Bad on Keto? The Truth About the 'Keto Flu'

October 26, 2025 •

4 min read

Approximately one-third of people who begin a ketogenic diet report experiencing flu-like symptoms. This phenomenon, known as the 'keto flu,' is a common reason why many find themselves wondering, "Why do I feel so bad on keto?".

Quick Summary

The unpleasant symptoms experienced when starting a keto diet are commonly known as the keto flu. They are caused by your body transitioning from burning glucose to fat for energy, leading to temporary electrolyte imbalances and dehydration.

Key Points

Keto Flu Causes: The primary culprit is the body's metabolic switch to burning fat instead of glucose, combined with dehydration and electrolyte imbalances.
Electrolyte Replenishment: Increase intake of sodium, potassium, and magnesium through salt, bone broth, avocados, and leafy greens to combat fatigue and cramps.
Stay Hydrated: Drink plenty of water to counteract increased fluid loss from glycogen depletion and the diuretic effect of ketones.
Ease into It: Transitioning slowly into keto by gradually reducing carbs can minimize the severity of withdrawal symptoms.
Adequate Fat Intake: Ensure you are consuming enough healthy fats to provide a sufficient energy source for your body, as a calorie deficit can worsen fatigue.
Prioritize Rest: Allow your body to adapt by resting more and avoiding intense physical activity during the initial adjustment period.
Address Digestive Issues: Counter constipation by eating plenty of low-carb, high-fiber vegetables like leafy greens and broccoli.
Consult a Professional: Speak with a doctor or registered dietitian before starting the keto diet, especially if you have pre-existing health conditions.

Understanding the 'Keto Flu': The Metabolic Shift

When you dramatically reduce your carbohydrate intake, your body is forced to switch from using glucose (sugar) to using fat and ketones for energy. This metabolic change is what defines ketosis, and the transition period is often what causes you to feel unwell. Your body is simply unaccustomed to running on this new fuel source, and it can take days or even weeks to adapt fully. This phase is often marked by various uncomfortable side effects, from headaches to fatigue, that are collectively referred to as the keto flu.

The Primary Causes of Feeling Bad on Keto

Dehydration and Electrolyte Imbalance

One of the most significant factors behind feeling terrible on keto is the rapid loss of water and essential minerals, or electrolytes. When carbohydrate intake is low, your insulin levels drop, signaling your kidneys to excFcrete more sodium. As sodium leaves the body, so does water. This process can quickly lead to dehydration and an imbalance of key electrolytes like sodium, potassium, and magnesium. Without proper replenishment, symptoms such as headaches, muscle cramps, and fatigue are common.

Carbohydrate Withdrawal

For many people, the body has been running on a steady supply of glucose for years. Suddenly cutting this off can feel like withdrawal. Symptoms like brain fog, irritability, and intense cravings are a direct result of your body and brain adjusting to the lack of its preferred fuel. Your brain, in particular, relies heavily on glucose initially and needs time to become efficient at using ketones for energy.

Nutritional and Digestive Adjustments

Transitioning to a high-fat, low-carb diet can also cause digestive distress. The sudden shift can disrupt your gut microbiome and alter bowel movements, leading to either constipation or diarrhea. Additionally, many nutrient-dense foods rich in fiber, vitamins, and minerals (like whole grains and many fruits) are restricted on keto. This can lead to deficiencies in important micronutrients like selenium, magnesium, and vitamins B and C, which contributes to overall malaise.

How to Combat the 'Keto Flu' and Feel Better

Replenish Electrolytes: When you feel unwell, try drinking a glass of water with half a teaspoon of salt or a quality sugar-free electrolyte powder. Consuming electrolyte-rich foods like avocados, leafy greens, and bone broth is also crucial.
Stay Hydrated: Drink plenty of water throughout the day, aiming for at least 2.5 liters, especially during the first few weeks.
Ease the Transition: Gradually reducing your carb intake over a week or two can lessen the severity of withdrawal symptoms.
Get Enough Healthy Fats: Ensure you are eating enough fat for fuel. Not eating enough calories, especially fat, can exacerbate fatigue. Incorporate sources like coconut oil and MCT oil, which can be quickly converted into ketones for energy.
Prioritize Sleep and Rest: The body is under stress during adaptation, so listen to it and get extra rest. Avoid high-intensity workouts during the initial phase.
Include Fiber: To combat constipation, make sure you're getting enough fiber from keto-approved vegetables like broccoli, spinach, and cauliflower.

Comparison Table: Keto Flu vs. Actual Flu


Symptom	Keto Flu	Actual Flu
Onset	Occurs within 2-7 days of starting keto diet	Rapid, often within 1-4 days of infection
Cause	Body’s metabolic shift and electrolyte imbalance	Viral infection
Fever	No fever (unless an unrelated illness occurs)	Often involves a fever
Cough/Sore Throat	Not typically present	Common respiratory symptoms
Duration	Usually resolves within a week, up to a month	Typically lasts 3-7 days
Remedy	Electrolyte replenishment, hydration, rest	Rest, hydration, OTC medications

Conclusion: Navigating the Early Stages of Keto

Feeling unwell during the initial stages of the ketogenic diet is common but temporary, signaling your body's adaptation. The discomfort is mainly due to dehydration and electrolyte imbalances as your body switches from burning carbs to fat. Staying hydrated, replenishing electrolytes, and resting can mitigate these side effects. While challenging, understanding the causes and using simple strategies can help you push through and potentially experience increased energy and clarity. Always consult a healthcare professional before starting any significant dietary change like keto. A registered dietitian can provide detailed nutritional guidance.

Potential Long-Term Concerns

While the keto flu is short-lived, potential long-term concerns include impacts on heart and kidney health, and nutrient deficiencies. High saturated fat intake may increase 'bad' LDL cholesterol, and the diet's restrictive nature can lead to micronutrient and fiber deficiencies. Monitor your health and listen to your body throughout your keto journey.

The Mental and Emotional Toll

Beyond physical symptoms, the restrictive nature of keto can affect mental and emotional well-being. Lack of variety can cause frustration, and social eating can be difficult. Some may experience mood swings, irritability, or anxiety during adjustment. Being aware of these effects and finding support can help. Consult a mental health professional if you struggle.

Keto's Effect on Sleep

Sleep patterns can change when starting keto; some experience insomnia, others improved sleep. Electrolyte balance, especially magnesium, influences sleep. Adequate magnesium and a consistent sleep routine can be beneficial.

Frequently Asked Questions

The 'keto flu' is a collection of temporary, flu-like symptoms that can occur when first starting a ketogenic diet. Symptoms often include headache, fatigue, irritability, and brain fog, and are caused by the body's adaptation to using fat for fuel instead of glucose.

For most people, the symptoms of the keto flu last for a week or less, though in some cases, they can persist for up to a month as the body fully adapts to ketosis.

Yes, dehydration is a major cause of feeling bad on keto. The reduction in carb intake leads to increased urination and loss of fluids, which can cause symptoms like headaches, dizziness, and fatigue.

Yes, electrolyte imbalances are very common on the keto diet. Reduced carb intake lowers insulin levels, causing the body to excrete more sodium, potassium, and magnesium, which can lead to cramps and fatigue.

You can get electrolytes by liberally salting your food, drinking bone broth, and consuming foods rich in potassium (avocado, spinach) and magnesium (nuts, seeds). Sugar-free electrolyte supplements are also an option.

Digestive issues like constipation or diarrhea are common when starting keto due to the change in macronutrient intake and a potential lack of fiber from restricted foods. Increasing your water and fiber intake from low-carb vegetables can help.

Yes, it is very normal to experience low energy and fatigue during the initial transition period. This is because your body is still learning to efficiently use fat and ketones for fuel. Energy levels typically improve once you are fully adapted.

Cravings are a normal part of carb withdrawal. Ensure you are eating enough healthy fats to feel satiated, and try consuming more fiber-rich vegetables. Some find that supplementing with MCT oil can also help manage energy levels.

No, not everyone experiences the keto flu. Some people are more 'metabolically flexible' and can transition with minimal side effects. The severity of symptoms can vary widely depending on the individual and their diet before starting keto.