What is 'Keto Flu', and How do You Get Rid of It?
Table of Contents
The ketogenic (keto) diet, characterized by extremely low carbohydrate intake, has gained significant popularity for its potential benefits in weight loss and blood sugar management. However, the transition into a ketogenic state can be physically demanding. Many beginners experience a collection of unpleasant symptoms commonly referred to as the "keto flu." Despite the name, this is not a true viral infection but a temporary metabolic side effect as the body adjusts to burning fat for fuel instead of glucose.
What is it?
Keto flu is a term used to describe the collection of flu-like symptoms that appear when a person dramatically reduces carbohydrate intake to initiate nutritional ketosis. It is essentially a withdrawal period from glucose, the body's preferred and easily accessible fuel source. Once carbohydrate intake drops below a threshold—typically 20 - 50 grams per day—the body is forced to enter a state of ketosis, where it produces ketone bodies from fat as an alternative energy source.
- Not a Virus: It is crucial to understand that keto flu is not caused by a pathogen. It is a physiological reaction to the sudden and massive change in fuel source and the associated shift in fluid and electrolyte balance.
- Onset: Symptoms usually begin within 1 - 4 days of starting the very low-carb diet.
Symptoms
The symptoms of keto flu mimic those of a typical viral illness, which is why the name is fitting. However, they are all attributable to metabolic and electrolyte imbalances.
| Symptom Category | Common Manifestations | Underlying Cause |
|---|---|---|
| Physical Symptoms | Headache, nausea, muscle cramps, and sometimes dizziness. | Dehydration and loss of sodium, potassium, and magnesium. |
| Energy and Mental State | Fatigue, brain fog, difficulty concentrating, and irritability. | The brain's adjustment from burning glucose to burning ketones. |
| Digestive Issues | Constipation or diarrhea, bad breath (sometimes described as metallic or fruity). | Dietary shift (less fiber) and acetone (a ketone body) excretion through breath. |
Treatment
The keto flu is entirely manageable and preventable with targeted nutritional and lifestyle adjustments that correct the underlying causes: dehydration and electrolyte imbalance. There are no drugs required; simply correcting your fluid and mineral intake is often enough.
1. Increase Electrolyte Intake
This is the single most important and effective step. When you drastically cut carbs, insulin levels drop, signaling the kidneys to flush out excess sodium and water. This loss of sodium rapidly depletes other key electrolytes.
- Sodium: Add extra salt (Himalayan or sea salt) to your food, drink salty broth (bone broth is excellent), or consume electrolyte mixes. Aim for an additional 2 - 5 grams of sodium per day during the transition.
- Potassium: Include low-carb, potassium-rich foods like spinach, avocados, mushrooms, and salmon.
- Magnesium: Supplementation (e.g., magnesium citrate or glycinate) can alleviate muscle cramps, constipation, and aid sleep.
2. Hydrate Aggressively
The initial rapid water loss associated with glycogen depletion leads to dehydration. Drink significantly more water than usual, aiming for 2.5 - 4 liters per day, especially when you feel dizzy or have headaches.
3. Do Not Over-Exercise
During the first week, intense physical exercise can exacerbate fatigue and muscle pain. Focus on light activity like walking or stretching until the body has fully adapted to using ketones for fuel.
4. Adequate Fat Intake
Ensure you are consuming enough healthy fats (avocados, olive oil, fatty fish, nuts). When the body is forced into ketosis, it needs adequate fat intake to provide the raw material for ketone production, helping stabilize energy levels and reduce the brain fog.
Causes and risk factors
The root cause of the keto flu is the complex metabolic shift away from glucose dependency, but several factors increase the likelihood and severity of symptoms.
Primary Causes
- Glycogen Depletion and Diuresis: The body stores carbohydrates as glycogen, and each gram of glycogen holds approximately 3 - 4 grams of water. When you stop eating carbs, the body quickly burns through its glycogen stores, leading to a massive flush of water. This water loss is what carries essential electrolytes (sodium, potassium) out of the body, leading to the flu-like symptoms.
- Hormonal Adjustment: Reduced carb intake lowers insulin levels, which, while beneficial for ketosis, also changes how the kidneys manage salt and water, promoting the rapid diuresis mentioned above.
- Adaptation Period: The brain takes time to build up the necessary enzymes and transport systems to efficiently use ketones as fuel. During this transitional period, the brain's energy supply can temporarily fluctuate, causing brain fog and lethargy.
Risk Factors
- Abrupt Transition: People who switch suddenly from a high-carb diet to a very low-carb diet are more likely to experience severe symptoms than those who transition gradually.
- Inadequate Hydration: Failing to intentionally increase water and salt intake during the initial phase guarantees a more intense keto flu experience.
- High Intensity Exercise: Combining the keto transition with intense workouts can deplete electrolytes even faster and worsen physical fatigue.
Duration
The keto flu is a temporary adjustment phase. The duration varies significantly from person to person, but there are clear typical timelines:
- Typical Duration: For most people, symptoms begin within 1 - 4 days and last for about 3 - 7 days. If the symptoms are addressed quickly with hydration and electrolytes, they may resolve in just 2 - 3 days.
- Prolonged Symptoms: If symptoms last longer than one week, it is usually a sign that the individual is not correcting the electrolyte imbalance sufficiently, or they may not be adequately restricting carbohydrate intake to reach full ketosis.
- Post-Flu: Once the keto flu subsides, the body is considered "keto-adapted," and many people report increased energy, mental clarity, and reduced hunger.
Safety
For most healthy individuals, the keto flu is uncomfortable but not medically dangerous. It is a sign that the body's metabolism is changing as intended.
- When to Seek Medical Help: If symptoms are severe, such as vomiting, diarrhea that prevents fluid retention, or signs of severe dehydration (faintness, extreme confusion), consult a doctor immediately.
- Ketoacidosis vs. Ketosis: It is important not to confuse nutritional ketosis with diabetic ketoacidosis (DKA). Nutritional ketosis is a normal metabolic state that produces a safe level of ketones. DKA is a life-threatening complication of Type 1 diabetes (and sometimes Type 2) characterized by dangerously high blood sugar and extreme, uncontrolled ketone levels. The keto flu is not DKA.
Bottom line
The keto flu is the body's natural response to carbohydrate restriction, primarily driven by electrolyte depletion and dehydration. It is not an inevitable consequence of the diet, but rather a preventable side effect. By aggressively increasing fluid and electrolyte intake (especially sodium, potassium, and magnesium), minimizing intense exercise, and ensuring adequate fat consumption, most people can mitigate or entirely avoid the worst of the transition symptoms and achieve the state of ketosis smoothly.
Frequently Asked Questions about the Keto Flu
How can I tell if my symptoms are keto flu or a real flu?
The primary distinguishing factor is the presence of fever and congestion. Real flu (influenza) or a common cold is typically accompanied by a fever, sneezing, and coughing. The keto flu, on the other hand, is generally characterized by headaches, fatigue, muscle cramps, and "brain fog" without the infectious symptoms.
If your symptoms include a high fever (over 100.4°F or 38°C) or respiratory symptoms, it is almost certainly a viral infection and not the keto flu.
Does everyone get the keto flu?
No, not everyone experiences the keto flu. The severity of the symptoms depends heavily on how rapidly the body transitioned from a high-carb to a low-carb state, and how effectively the individual managed their hydration and electrolyte intake.
People who previously consumed a diet higher in salt or who transition slowly may experience very mild or no symptoms at all.
Will exercising make the keto flu worse?
Yes, intense exercise during the initial keto transition can significantly worsen the keto flu. Heavy sweating rapidly depletes the body's already low electrolyte levels, leading to more severe symptoms like muscle cramping, dizziness, and fatigue.
It is highly recommended to limit activity to light walking or yoga for the first one or two weeks until your energy levels stabilize and your body is fully keto-adapted.
Can I temporarily eat carbs to stop the keto flu?
While consuming carbohydrates will quickly stop the keto flu symptoms by returning your body to its preferred fuel state (glucose), it will also stop ketosis and simply postpone the entire adjustment process.
A better solution is to stick with the low-carb diet but aggressively treat the symptoms with sodium-rich broth and electrolyte supplements. This allows the body to complete the adaptation phase without reversing progress.
How we reviewed this article:
Our team continually monitors and updates articles whenever new information becomes available.
Written By
Ian Nathan, an MBChB Candidate, and the Healtharticles Editorial Team
Medically Reviewed By
Ian Nathan, an MBChB Candidate
