Key Takeaways
- The ketogenic diet may cause the keto flu as your body adjusts to using ketones for energy.
- Not everyone who follows the ketogenic diet gets the keto flu.
- Signs of the keto flu vary but often mirror those of influenza (the flu).
Keto flu is a temporary condition that may occur in the first few days or weeks of a ketogenic (high-fat, low-carb) diet. Changes in electrolyte and energy levels may cause it.
1. Headache
You may experience headaches when first starting the keto diet as your body enters ketosis and adjusts to the shift in energy sources. When your body is in ketosis, it uses ketones instead of carbohydrates for energy. Your brain also shifts to using ketones as an energy source.
This shift may lead to lower blood sugar levels (hypoglycemia), which may cause headaches.
Headaches may also be caused by dehydration or electrolyte imbalances that may occur as your body adjusts to ketosis.
2. Fatigue
Fatigue is a hallmark symptom of the flu (influenza) and may also occur if you have keto flu.
In ketosis, your body uses fat for energy generation instead of carbohydrates. Glycogen, which is a stored form of carbohydrates that can be used for energy, also becomes depleted as you enter ketosis.
You may feel tired as your body adjusts to using fat as its primary source of energy and as your muscles deplete their glycogen stores.
3. Nausea
Nausea during the keto flu can be caused by a range of factors, including carbohydrate restriction, electrolyte imbalance, and high-fat intake.
When you restrict carbs too much, your body may enter ketosis, which is associated with nausea. On the other hand, consuming large amounts of fat can slow digestion and may cause nausea, as well as general stomach discomfort.
4. Constipation or Diarrhea
Changing your diet may cause constipation or diarrhea.
Dietary fiber is found in carbohydrates, which are severely restricted on the keto diet. A low-fiber diet has been linked to constipation because dietary fiber adds bulk to stools and increases bowel movements.
The keto flu may also cause diarrhea, although constipation is more common. High fat intake may cause diarrhea due to inflammation and imbalances in your gut microbiome.
5. Muscle Cramps
As you start to enter ketosis, your body may lose excess water and electrolytes. This may lead to muscle cramps.
Many people with keto flu report temporary muscle cramps. Research indicates that a reduced carbohydrate intake may lead to electrolyte loss and, in some cases, dehydration. This may result in muscle cramps, as well as reduced muscle function during exercise.
6. Brain Fog
Some people report confusion and difficulty concentrating (also known as brain fog) when first starting the keto diet.
Brain fog may be due to your body and brain transitioning from using glucose to ketones as a source of energy. The keto flu may cause an electrolyte imbalance, which may also contribute to brain fog.
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7. Bad Breath
Bad breath may be a temporary issue associated with the keto flu. Fortunately, this issue should go away as your body adjusts to the keto diet.
You may develop bad breath as the level of ketones in your body increases. Ketones, such as acetone, are considered volatile compounds and may cause bad breath. Keto flu breath is often described as having a fruity yet musty odor.
8. Irritability
You may experience anger or irritability as your body adjusts to using ketones as an alternative energy source.
One study found that 3-10% of children and adults on the keto diet report irritability when starting the keto diet. Although researchers are unsure why mood changes may occur, one possible explanation is the decreased availability of energy for the brain and muscles.
9. Food Cravings
As you reduce carbohydrate intake and adjust to the keto diet, you may find yourself craving the foods you used to eat.
Cravings for carbohydrates and other foods are common during the keto flu because your body is not used to the new diet. Low-carbohydrate diets, like the keto diet, have been linked to increased food cravings that may lead to binge eating and other disordered eating habits.
10. Increased Thirst
Increased thirst is a commonly reported symptom of the keto flu.
As your body depletes its glycogen stores and starts producing more ketones, you may lose excess water. Ketone production may increase the risk of dehydration. Increased thirst is an early sign of dehydration.
11. Insomnia
Some people experience difficulty sleeping when they have the keto flu. Sleep disturbances may include trouble falling asleep or staying asleep.
Although there isn’t solid evidence as to why insomnia may occur, some researchers think the issue may be due to hormone shifts that may occur during initial ketosis. However, sleep should improve as you continue the keto diet.