Keto flu: what to expect with the keto diet

Claims of increased energy levels, weight loss, and other health benefits may have led you to try the keto diet for yourself. But you might not have expected to be hit with flu-like symptoms after getting started on the high-fat, low-carbohydrate diet. 

What is the so-called “keto flu”? It isn’t a genuine illness, even if it can feel like it is. Keep reading to understand what to expect if you experience these symptoms, along with strategies for dealing with them. 

The keto flu describes flu-like symptoms that some people develop after beginning a ketogenic diet—a diet that’s high in fat, low in carbohydrates, and moderate in protein. Not everyone who starts a keto diet experiences these side effects. The symptoms range from mild to severe, depending on the person.

Typically, symptoms happen within the first week of beginning a keto meal plan, subsiding with time. 

Does ketosis cause the keto flu?

When carbohydrate intake is low, metabolism changes to use a process called ketosis, where your body burns fat for energy and produces ketones.

Both the brain and central nervous system use carbohydrates (glucose) for fuel. In a low-carbohydrate diet, your metabolism adjusts to using ketones to replace carbs (Masood, 2020). 

The keto flu likely is caused by the transition from using carbs to fat and ketones as the primary energy source. The keto flu has never been formally studied, so there is a lot still unknown, including why it affects some people more than others.  

How long does the keto flu last?

For most people, the keto flu side effects will last for 5–7 days but could last for up to a month (Bostock, 2020). 

Whether or not you feel symptoms of the keto flu and how long they last depend on a few things. Your metabolism, genetics, and how well your body adapts to using fat as its primary energy source are all factors. Suppose you start eating carbs again and get out of a state of ketosis. In that case, you will possibly experience the keto flu symptoms again if you restart the keto diet.

The symptoms of the keto flu range from mild to severe, depending on the person. Here are some of the common symptoms reported with the keto flu:

• Nausea and vomiting

• Constipation or diarrhea

• Headache

• Fatigue

• Irritability, difficulty concentrating, and brain fog

• Weakness

• Muscle cramps and soreness

• Weakness

• Dizziness

• Stomach pain

• Difficulty sleeping

• Sugar cravings

The keto flu can leave you feeling pretty miserable. We don't have research on the keto flu, so most of the information on helping with the keto flu comes from reports from people who have tried the diet themselves.

These tips could help lessen keto flu symptoms, and they may help your overall health. 

Drink enough water

The keto diet increases fluid loss (Masood, 2020). Increasing your water intake after starting a low-carb diet helps maintain hydration and potentially helps with keto flu symptoms. 

It’s especially important to drink plenty of water if you’re experiencing vomiting or diarrhea during the keto flu because these will lead to further fluid loss. If vomiting or diarrhea become severe or last for more than a day or two, be sure to contact your healthcare provider.

Limit intense exercise

Your body creates energy for exercise in two ways: with oxygen or without oxygen. 

When your body can use oxygen in metabolism, like in low-intensity exercises, fat is the primary energy source used. This type of metabolism takes more time which is why it is best suited for low-intensity exercise.

During intense exercise and quick movements (like weight lifting, sprinting, and high-intensity interval training), your body uses a faster option to produce energy using carbs or glucose as the energy source (Harvey, 2019).

So when you first start a keto diet, your body needs to adapt to no longer using carbs as its fuel source for intense exercise. Doing intense workouts could leave you feeling more fatigued without carbs or glycogen stores to help provide energy. Doing low-intensity exercise is a better option during this time (Harvey, 2019). 

Replace electrolytes

Some of the keto flu symptoms, like vomiting and diarrhea, could lead to dehydration and an imbalance of your body’s electrolytes.

An electrolyte imbalance can cause many symptoms similar to the keto flu, like headaches, difficulty concentrating, muscle cramping, and muscle weakness (Shrimanker, 2020). 

Many foods that naturally contain electrolytes (fruits and starchy vegetables) aren't allowed on the keto diet. Eat leafy green vegetables, broccoli, avocado, and other keto-friendly vegetables naturally high in potassium and magnesium. Eating phosphorus-rich foods, like meat, and adding small amounts of salt to food will help replace those electrolytes. 

Get enough sleep

Poor sleep leaves most people feeling tired and unfocused. Insufficient sleep on top of the keto flu only compounds the effect. 

Tips for sleeping better include (Yazdi, 2016):

• Going to bed and waking up at the same time each day

• Sleeping in a quiet, cool, and comfortable room

• Limiting caffeine, nicotine, and alcohol before bed

Consider adjusting your diet

There are mixed reports on whether changing your diet to include more healthy fats or reintroduce small amounts of carbs could help with keto flu symptoms. 

For some people, eating more fat provides more calories, potentially helping with their symptoms. 

Adding some carbs back for a slower transition from a high-carb diet to a keto meal plan may help to reduce keto flu symptoms. But some people claim this prolongs keto flu symptoms by moving you in and out of ketosis. More vigorous research is needed for keto diet participants to confidently know which path to take to reduce keto flu symptoms.

The keto flu seems like an unfortunate part for many people who wish to follow a ketogenic diet. Speak with your healthcare provider if you have any specific concerns or if your symptoms last.

If your symptoms continue for longer than a week, it might be time to check in with your healthcare provider to make sure something else isn’t going on. The flu-like symptoms of the keto flu could be masking the symptoms of an illness, or you may need to adjust your approach to the keto diet.

Talk with your healthcare provider to get your questions answered and get medical advice.

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Current version

April 27, 2021

Written by

Ashley Braun, RD, MPH

Fact checked by

Steve Silvestro, MD

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About the medical reviewer

Steve Silvestro, MD, Ro

Dr. Steve Silvestro is a board-certified pediatrician and Associate Director, Clinical Content & Education at Ro.