What’s Keto Flu and How Do You Cure it?

Image of sick man with a black background

Has the keto diet got you feeling worse for wear? Don’t fall into the common trap of thinking it is not for you, as you may simply be suffering from keto flu. Thankfully, this is perfectly normal, manageable and transient. Keto flu is commonly experienced by many people switching to the ketogenic diet, with varying degrees of side effects. It is sometimes even described as ‘carbohydrate withdrawal’, much like withdrawal associated with addictive substances. In fact, recent studies have compared the effects of carbohydrates (particularly sugar) on the brain to that of addictive drugs like cocaine!1  

Keto flu is a common reason for stopping progress on a ketogenic diet. People report mood swings, irritability, fatigue, dizziness and nausea among other symptoms. Keto flu may last anywhere from a day to a few weeks. It usually begins within the first few days after carb elimination/drastic reduction. 

For those of you experiencing effects of the flu, don’t worry, as there are several ways to reduce, prevent or manage the symptoms. This article goes through some of the most common symptoms, the science behind them, and some solutions. 

Top 10 keto flu symptoms:

  1. Headaches 
  2. Dizziness
  3. Cramps and sore muscles
  4. Nausea
  5. Fatigue
  6. Mood swings, irritability and cravings
  7. Stomach pain and constipation 
  8. Brain fog and difficulty focussing
  9. Bad breath
  10. Insomnia

The science behind keto flu

By restricting carbohydrates, the body's preferred source of energy, the body must respond by making many changes to transition from glucose to fat metabolism.  This results in the keto flu as the body shifts gear. 

When you restrict dietary carbohydrate intake suddenly, a cascade of changes begins in the body. Firstly, blood sugar drops and causes hypoglycaemia. In response to this, changes occur to both the fuel being used for energy and changes in how neurons in the brain function. Secondly, changes occur in other systems in the body that alter electrolyte, water and hormone levels. Here is a step-by-step breakdown of what happens inside your body that causes keto flu when you start following the ketogenic diet:

  1. Your blood glucose falls
  2. Electrolyte imbalance occurs, leading to dehydration 
  3. Over time, hormone release is affected

1. Falling blood glucose

The physical consequences of sudden carb removal include hypoglycaemia (low blood sugar <55 mg/dL).2 To understand this we must first understand that our body generates energy using two main mechanisms: glycolysis and beta-oxidation.

Glycolysis: converting glucose into energy

Beta-oxidation: converting fat into energy

Complying with a low-carb ketogenic diet means switching from making energy using carbs and glycolysis to predominantly beta-oxidation of fats. After some time the body also begins to generate energy from the breakdown of ketones (‘ketolysis’) instead of glucose. The switch to ketone burning  occurs because the body starts to break down fatty acids into ketones so the brain can use them for fuel.3  

So, back to hypoglycaemia. this may occur as the body has not yet learned how to switch from burning glucose to burning fat and producing ketones. This results in a temporary energy deficit and thus low blood sugar. It is important to remember that this is a transient phase occurring only as the body is adapting. The ease with which people can switch to using fats and ketones depends on a mixture of genetics and habitual diet. Some individuals demonstrate greater metabolic flexibility than others as they switch from one metabolic state to the other. These lucky individuals may show far fewer symptoms or experience the flu for a shorter duration. Experiences are likely to be unique to the individual. 

Somewhat ominously, research has found that the same pathways of the reward system in the brain are activated by foods high in carbohydrates as are activated when consuming drugs like cocaine or heroin. Here, both cause a release of dopamine, a ‘feel good’ hormone. Over time, regular carb consumption modifies the gene expression and dopamine receptor availability in the reward system. This means there is a need for even more carbohydrates to activate the receptors than before as the brain becomes tolerant. Thus, sudden removal of carbohydrates can result in withdrawal symptoms. These can be both physical and psychological when the former high levels of the stimulus are not given. It explains why we crave certain foods or feel depressed or anxious without them. 

2. Electrolyte imbalance and dehydration

The blood levels of electrolytes (the minerals in the body that are derived from salts e.g. calcium, potassium, magnesium, chloride and sodium) are controlled by the kidneys (Pubmed Health2018). Electrolytes are crucial for maintaining bodily functions such as heartbeat regulation and muscle contraction.4  

Diagram illustrating the four basic electrolyte minerals

Why is does this go awry in the ketogenic diet?  Ketogenic diets restrict carbohydrates (which trigger the most insulin release). So with lower carb intake, insulin levels drop. Insulin not only signals cells in the body to absorb glucose in the bloodstream, but also signals the kidneys to store more water.5 Lower insulin levels (as a result of decreased carb intake) means the kidneys now store less water. This results in dehydration and the flushing out of electrolytes in the process.6 Moreover, glycogen (stored carbohydrate), which traps 3 grams of water per gram of glycogen, is also depleted on a ketogenic diet. This further contributes to the reduced amount of water and electrolytes in the body.

3. Hormonal stress response 

A ketogenic diet that is not well formulated (i.e. too low in calories or deficient in micronutrients) can trigger a starvation response in the body. If the body thinks it is starving, it raises levels of cortisol (stress hormone) (Waldman2017). Releasing cortisol is the body’s attempt to protect the brain by raising blood sugar, as the body tries to compensate for the now low blood sugar caused by carb reduction. If too much cortisol continues to be released, the stress response and blood sugar stability may be deregulated. Ensuring you are stress free particularly during the transition phase can be helpful! 

Another thing to consider is the thyroid hormones, which have several functions. This includes the maintenance and regulation of carbohydrate/energy metabolism. T3 or ‘euthyroid’ is the most biologically active form of the hormone and is linked to dietary carbohydrate consumption. T3 levels are reported to decrease in response to carbohydrate restriction below a certain threshold (which varies from person to person).7 This may result in a feeling of fatigue or difficulty focussing as the body adapts. Conversely, lowered T3, as long as thyroid function is normal, is also hypothesised to bring several benefits. This includes improving longevity and preserving muscle mass. What does this mean for you? Whilst discomfort may result during the transition, rest assured as lowered T3 does not appear to be indicative of hypothyroidism.8   

Solutions to common symptoms:

Lady with a headache


In ketosis, headaches can occur due to electrolyte imbalance and dehydration. With low insulin levels, the kidneys go into a diuretic state, so potassium, water and sodium are excreted. One positive, is that the decrease in stored water weight may contribute to short-term improvements in physical appearance! On the flipside, dehydration and electrolyte imbalance are the main culprits for several common keto flu symptoms. For this reason, salt and water intake should be monitored when switching to a keto diet and electrolyte supplementation should also be considered.

The role of electrolytes is often underestimated when it comes to low carb diets, and careful management is key to avoiding symptoms. Even if management of macronutrient intake (carbs, proteins and fats) is being managed correctly, maintaining the correct balance of electrolytes is often overlooked. A common cause for electrolyte imbalance is eating too few mineral rich fruits and vegetables when transitioning to a keto diet. In addition, removing several salt-laden processed foods means the body is now cut off from sodium, or electrolyte sources it once had. 

Sea salt is rich in both sodium and minerals as are bouillon cubes or homemade bone broth. This can be made simply bringing animal bones with some vegetables in water to a boil then leaving it for a low simmer for a few hours. Whilst people are often wary of increasing sodium intake due to the risk of raising blood pressure, it should be considered that removing processed foods from the diet and reducing carb intake already has a significant blood pressure lowering effect.9 Magnesium is another important mineral in the body. It contributes to muscle and nerve function, blood glucose control and protein synthesis.10 Potassium too is important for the proper functioning of the heart, digestion and muscle function.11 Foods rich in potassium and magnesium include tomatoes, avocados, salmon, nuts, leafy greens and animal proteins. One should consume these to minimise the risk of headaches. 


Cramping is the most common sign that electrolytes are out of balance. The common mistake of not drinking enough to compensate for loss of water occurring during the transition phase may result in low blood pressure and constipation, other than just cramps.6 Low potassium can contribute to muscular cramps so keto friendly dietary sources of potassium should be included. Animal protein is an excellent source, and the juices from cooking meats should be retained for this purpose. Low magnesium is another likely cause for muscular cramps as it is essential for keeping the brain, heart and muscles ‘relaxed’. Intense exercise, poor sleep as well as stress all contribute to depleting magnesium levels. Leafy greens are an excellent source and the darker the better!


Constipation can occur as the digestive system adapts to the transition to the keto diet. Any dramatic lifestyle change impacts the gut microbiome, inevitably altering bowel movements. This is worsened by dehydration caused by increased excretion of fluids by the kidneys. Cutting out certain higher carb fruits and vegetables also can mean a reduction in dietary fibre, further contributing to constipation. So, care should be taken to eat plenty of fibre rich fruits and vegetables, exercise and drink plenty of water each day. As a word of caution however, eating excessive fibre can also cause constipation, so a fine balance must be struck. How do you know? This is something that can only be determined from personal experience.12 MCT or medium-chain triglyceride oils are another solution. This may help to relieve constipation, as fat can help push bowel movements through. Finally, care should be taken to ensure calorie intake is adequate, as not eating enough calories can also contribute to constipation! 

Bad Breath

Several people may experience bad breath that has a fruity smell and is similar to nail polish remover. This is actually a good sign that your body is in ketosis, burning fats and converting them to ketones for energy. However, this is usually reported to go away within 1-2 weeks once the body adapts to the new metabolic state it is in. Maintaining good oral hygiene, increasing water intake and using gum or breath freshener can help mask or reduce the smell in the interim while the body is still adapting. 

Fatigue, Low Mood and Cravings 

Fatigue and weakness is also often reported, which can also cause a decrease in physical performance as the body adapts to ketosis. This fatigue can last anywhere from three days to two weeks as the body prepares new enzymes for the new diet it is entering. 

The tiredness may be also caused by thyroid hormone and cortisol changes. This occurs as the body tries to compensate for the lowered carbohydrate intake by releasing more cortisol (which in turn raises bloods sugar).

This may result in you feeling irritable and having reduced sleep quality. However, this should not be a major worry as the change is temporary. This is because cortisol levels are likely to reduce again once the body becomes keto-adapted. To lessen fatigue in the meantime, water and mineral intake should be increased and carefully monitored. B vitamins, particularly B5, are vital for helping with fatigue in lethargy. Again, its important to emphasise that it’s crucial to make sure you are still eating enough calories from fat to sustain yourself, as being under-fuelled is a key cause of fatigue. 

Taking out glucose from the diet can affect your mood and cause cravings. Replacing things you crave with lower carb alternatives or removing ‘triggers’ can help reduce psychological (and thus often physiological) symptoms of carb withdrawal. There are plenty of ‘low carb treat’ recipes out there on the internet. That said, many people who have successfully transitioned to the ketogenic diet say that just going ‘cold turkey’ on sweet tasting things helps to get rid of those nasty cravings sooner. 


To wrap up, keto flu symptoms are often transient and are likely to disappear completely once the body adapted to the new diet. Lifestyle also determines the length and severity keto flu for the individual. Withdrawal symptoms are likely to be greater for individuals who have a diet high in processed food and refined sugars. Even during the transition, the symptoms can be alleviated if the cause is treated smartly. A well-formulated low carb diet can progress without significant symptoms if the common mistakes of poor mineral intake, lack of fibre, electrolyte imbalance and dehydration are addressed. Avoid falling into the common trap of assuming your body is not suited to the low carb diet after just a few days, but instead consider careful monitoring of water and mineral intake particularly for the days/weeks it takes your body to adapt. Have a look online for some keto support groups if you have questions, and perhaps think about trying out exogenous ketones, like HVMN Ketone. Exogenous ketones can give you an energy boost as beta-hydroxybutyrate without needing to take in carbs. Good luck and give it a good shot! 

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  6. Artunc F, Schleicher E, Weigert C, Fritsche A, Stefan N, Häring HU. The impact of insulin resistance on the kidney and vasculature. Nature reviews. Nephrology. 2016; 12(12):721-737.

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  8. Fontana L, Klein S, Holloszy JO, Premachandra BN. Effect of long-term calorie restriction with adequate protein and micronutrients on thyroid hormones. The Journal of clinical endocrinology and metabolism. 2006; 91(8):3232-5.

  9. Chen L, Caballero B, Mitchell DC, et al. Reducing Consumption of Sugar-Sweetened Beverages Is Associated with Reduced Blood Pressure: A Prospective Study among U.S. Adults. Circulation. 2010;121(22):2398-2406. doi:10.1161/CIRCULATIONAHA.109.911164.

  10. Magnesium Fact Sheet for Health Professionals, NIH

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  12. Ho K-S, Tan CYM, Mohd Daud MA, Seow-Choen F. Stopping or reducing dietary fiber intake reduces constipation and its associated symptoms. World Journal of Gastroenterology : WJG. 2012;18(33):4593-4596. doi:10.3748/wjg.v18.i33.4593.

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