What is Keto Flu?

posted in: Enhancing Ketosis

In simple terms, Keto Flu is the group of symptoms usually encountered during the Induction Phase of ketogenic diets, although some may occur until through the Transition Phase.  Typical symptoms include:

In this article, we will discuss the symptoms, why they occur, and how to deal with them if you experience them.  Unfortunately, you may run into them whenever you are getting back into ketosis after cheating (if you cheat long enough or hard enough) in addition to your initial Induction

For a more detailed discussion around the physiology changes that occur during the different phases, see: “What happens to the body during keto?”.  In short, as your body burns through its carbohydrate stores (glycogen), a lot of water is released and eliminated. 

This is why people tend to lose a lot of weight in the first week on keto… most of it is water.  In fact, for every gram of glycogen burned, two grams of water will be released and eliminated!


Many people starting a ketogenic diet will get headaches during the first couple of days due to dehydration.  By drinking enough water, it is possible to avoid getting a headache for that reason.  However, low sodium can cause headaches as well, so you may need to supplement with salt for the first 4 days. 

You may also get a headache in the first 1-3 days due to the switch to ketosis itself.  As the body burns through its carbohydrate stores and starts trying to use ketones for fuel, you can get transient drops in blood glucose levels, which can also cause headaches. The treatment for this is controversial. 

Some people recommend using exogenous ketones to help increase the ketone levels in your body until the liver can get acclimated and producing enough ketones on its own.  See “Should I take exogenous ketones?” for more discussion and why we don’t recommend exogenous ketones for ketogenic diets.

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Constipation is another symptom of dehydration.  Without enough water in your system, your body conserves water by removing it from your colon, thus making it more likely to cause constipation. 

Additionally, many people will decrease the amount of fiber they are eating when switching to keto, which can also lead to constipation.  Make sure to add plenty of fiber to your diet to help keep you regular. 

Finally, the amount of food you eat will probably decrease.  Fat is much higher in calories, so you don’t need as much of it to get the same amount of energy from carbs or proteins. 

Without as much going in, there isn’t as much to come out.  With less needing to come out, it stays in your body longer and the longer it stays, the more water is removed from the colon, thus causing constipation.  The best treatment for keto induced constipation is eating enough fiber and drinking enough water.

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Muscle cramps

As your body removes excess fluid during the conversion to ketosis, it also loses sodium, magnesium and potassium.  This is because electrolytes follow water.  This is most significant for sodium and potassium, but magnesium is lost as well.  As a result, you may need to use an electrolyte supplement while on keto.

Sodium loss is directly related to the loss of water in the urine.  Additionally, as your insulin levels drop during Induction (usually resolved by day 4), sodium loss is even higher.  This can all be easily replaced by adding a little salt to your diet. 

This should be added sparingly if you have hypertension, or high blood pressure, and should discussed with your medical practitioner prior to altering your intake.  For most people, added salt will be necessary until you are done burning through your glycogen (carbohydrate) stores.  Please see headache section, as salt loss can cause headaches as well.

Potassium loss is also related to the increased water loss.  It is also a little more difficult to get from your diet, so supplementation may be necessary. 

Note: if you have kidney or heart problems, check with your medical provider before adding additional potassium to your diet. 

The best keto friendly dietary source of potassium would be avocado, which should be a staple of your ketogenic diet anyway.  Low potassium levels can cause muscle cramping, spasms, aches and stiffness, possibly even tingling/numbness.

Magnesium loss is another cause of muscle cramps, twitching and muscle weakness.  Magnesium can be a little difficult to get through ketogenic diets, so supplementation is probably your best option to keep your levels up. 

Magnesium supplementation isn’t as high-risk as potassium supplementation for people with kidney / heart problems, but if you have any issues, you should speak with your medical practitioner before adding magnesium supplements.

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Diarrhea is another symptom, although it is not as common.  There are two main reasons you may get diarrhea when you first start ketosis.  First, as you change your diet, you are changing the food available for the bacteria in your intestines. 

As the food changes, so does the bacteria.  This change in microbiome can lead to diarrhea, but it is usually over in a couple of days.  Additionally, during Induction, your body is not accustomed to eating as much fat as the diet requires. 

Until you are fat adapted (after Transition Phase is complete), your body may be overwhelmed by the amount of fat ingested.  Any undigested fat gets passed into the intestines, which draws in water and causes liquid stools. 

This typically stops once through the Transition Phase.  Continued diarrhea beyond the Transition Phase should be addressed by a medical practitioner, as it could be a symptom of something else.

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Generalized weakness

Generalized weakness also has a combination of causes.  Electrolyte imbalances (primarily potassium and magnesium, but sodium can cause fatigue as well), dehydration, variations in blood glucose levels during Induction (usually resolves around day 4), and not enough calories are all possible causes of generalized weakness. 

The last is particularly important for those people that try to do a Very Low Calorie Ketogenic Diet, as fat is somewhat limited, thus decreasing the amount of calories eaten every day.  If you are on a VLCKD and continue to have weakness and fatigue after the Transition Phase, let your prescriber know, as they may need to adjust your diet. 

Once fully transitioned (all people, not just VLCKD), you should have MORE energy, not less. Keto flu is a group of symptoms typically seen during the Induction Phase of ketogenic diets.  Most of the issues can be fixed by staying well hydrated and by taking a daily supplement for magnesium and potassium as well as by adding salt to the diet.  

Keto flu isn’t a long-term concern and is typically over in the first week or two.  If symptoms continue after two weeks into ketosis, it may be something other than the diet and should be looked into further.

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*These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure or prevent any disease.

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