What Is Keto Flu? (And How To Prevent It)

If you’re not feeling your best on your first week of keto, you’re not alone. Many low carb dieters experience a range of symptoms—headaches, cramps, fatigue, and digestive problems—known as keto flu. 

This can be frustrating. You’re following your macros, but the symptoms don’t seem to notice. 

Fortunately, keto flu isn’t a keto requirement. It’s not a price you have to pay. In most cases, a few simple tweaks will dramatically increase your energy and get you feeling great again. 

We’ll cover how to prevent keto flu at the end. Right now, let’s define what it is.

What Is Keto Flu?

Keto flu is a label for a range of symptoms associated with low-carb dieting. Keto flu symptoms include:

  • Headache
  • Tiredness
  • Weakness
  • Muscle cramps
  • Irritability
  • Insomnia
  • Diarrhea
  • Constipation

Comparing these symptoms to influenza (flu) is a stretch. Unlike real flu symptoms, keto flu symptoms are fairly low-grade. Nobody is bed-ridden with keto flu. There’s no virulent infection, no cough, no sore throat, and no fever. 

But keto flu is catchier than “keto malaise”, “keto fatigue”, or “keto enervation”—so it stuck. 

Keto flu symptoms tend to arise in the early stages of low-carb dieting, and sometimes go away on their own. But time doesn’t always cure keto flu. Why not? Because there are different potential causes. 

What Causes Keto Flu?

At the highest level, eating a keto diet causes keto flu. Let’s explore this at a high level then we’ll drill down. 

When you eat a keto diet, you diligently restrict carbohydrates to below 10% of calories. This carb limit triggers a series of metabolic changes that culminate in fat-burning. 

The most important change is a reduction in the hormone insulin. When insulin is low, it signals body fat to break apart, travel to your liver, and oxidize (burn) to make energy and ketones. 

But these changes don’t happen in a vacuum. Other bodily processes—like your ability to retain water and electrolytes—change too. The symptoms of keto flu are a product of these changes. 

Okay, time to drill down. Here are five specific causes of keto flu symptoms:

#1: Not enough electrolytes

Electrolytes are charged minerals that regulate fluid balance and conduct electricity in your body. They’re essential for maintaining energy levels. 

Eating a keto diet increases your risk of electrolyte deficiency. Why? First, because the keto diet restricts many electrolyte-rich foods like some fruits and sweet potatoes. Second, because your kidneys excrete more electrolytes in a low-insulin state. 

Less electrolytes are coming in and more are going out. This combination often results in a sodium or potassium deficiency. The resulting symptoms—muscle cramps, fatigue, and headache—look eerily familiar to keto flu. 

#2: Dehydration

Closely linked to low electrolytes on keto is dehydration on keto. The mechanism is similar. With insulin suppressed on a low-carb diet, your kidneys don’t retain fluids as well. In other words, going keto has a diuretic effect

If fluids aren’t replaced, you may become dehydrated. Common dehydration symptoms include headache, tiredness, muscle cramps, and nausea. Dehydration can also cause constipation, since water loosens stool. 

#3: The brain is in transition

On the keto diet, your liver burns fat to produce ketones. Ketones are tiny molecules that can fuel your entire body, but their number one job is to fuel your brain. 

Normally, your brain runs entirely on glucose. But in ketosis (a state of elevated ketones), ketones start to shoulder the burden. This appears to have cognitive benefits

But the switch from glucose to ketones isn’t always a smooth one. The brain may protest for a few days as it adapts to the new fuel source. 

It’s like switching to a better bank. It’s a pain to make the transition, but once your accounts are linked up, your life runs smoother. 

#4: Fiber intake

If you’re constipated on the keto diet, you might not be eating enough fiber. Fiber is the indigestible plant material from fruits, nuts, seeds, and vegetables that keeps things moving along down there. 

“Indigestible” just means fiber isn’t absorbed as a source of calories. It is digested, however, by gut bacteria. To reward you for sending them fiber, they produce anti-inflammatory compounds like butyrate. 

Diarrhea is less common on keto, but in those cases, it’s probably wise to limit fiber for a few days. Fiber can feed bad, diarrhea-causing bacteria along with the good. 

#5: Sugar withdrawal

Sugar has addictive properties and researchers believe that eating sugar—since it raises blood sugar and insulin levels—stimulates dopamine receptors in the brain. When the sugar is removed, the dopamine receptors get lonely and cause withdrawal symptoms. 

If you weren’t eating much sugar before you went keto, sugar withdrawal probably isn’t your problem. If you were, don’t worry, the withdrawal symptoms are temporary. 

Tips for Preventing Keto Flu

The remedies for keto flu follow directly from the causes listed above. Here are the big ones:

1. Give it a week. The brain takes time to utilize ketones for fuel and your body may be in carb withdrawal. These problems shouldn’t last longer than a week, provided you’re following the other tips. 

2. Take electrolytes. Keto flu is often driven by electrolyte deficiencies. Get enough sodium and potassium by salting your food, eating potassium-rich, low-carb foods like spinach, and taking a zero-sugar electrolyte supplement. 

3. Drink enough fluids. Drinking to thirst should be sufficient to prevent dehydration. If you think you need more fluids, consume them with electrolytes to prevent electrolyte deficiencies.  

4. Eat fiber. Non-starchy veggies are a great source of fiber, and should be filling your keto plate to prevent constipation. Nuts and seeds are also keto-approved sources of fiber. (Try this delicious keto granola for 6 grams fiber per serving). 

5. Get plenty of sleep. More sleep equals less stress which your body will thank you for through out your keto transition.

5. Take MCT oil. Medium chain triglycerides (MCTs) are readily converted to ketones, and may ease the transition to a very low-carb diet. 

Follow these tips and you’ll be in a great position to ward off keto flu. And that will make going keto a whole lot easier. 


Brian Stanton is the author of Keto Intermittent Fasting, a certified health coach, and a leading authority on the keto diet. Follow Brian’s work by visiting his website at www.primalsapien.com

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