Keto Flu: An In-depth Guide to Beating It

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Picture of a Young Woman Feeling Fatigued Holding Her Head.When starting a ketogenic diet, some people experience initial side effects from carbohydrate restriction known as ‘keto flu.’

These symptoms can have some mild and potentially severe effects on the body.

While the condition is popularly known as keto flu, people also commonly refer to it as induction flu, low carb flu, and Atkins flu.

This article will explain what it is, why it happens, and the best strategies for avoiding or beating it.

What is the Keto Flu?

Firstly, it is not the real flu.

It just shares the name because it has several of the same symptoms.

Coming from a high carbohydrate diet, the body is well-adapted to using glucose for fuel.

However, when restricting carbohydrate, the supply of glucose falls before the body has adapted to burning fat for fuel.

In other words, your body is in ketosis but not fully keto-adapted.

If you are curious about this, you can find out your level of ketosis by using ketone strips.

The liver and gall-bladder need time to upregulate the number of fat-burning enzymes to burn larger amounts of fat efficiently.

Severely restricting carbohydrate is a massive change to the way the body works and your body needs time to adjust to the metabolic changes.

When Does it Start?

There is no exact timeframe, but symptoms may appear as quickly as 10-12 hours after starting to restrict carbohydrate.

For some people, it might be slightly earlier or later.

Of course, there are also people who won’t experience the dreaded keto flu at all.

How Long Does it Last?

Based on anecdotes, this induction flu lasts somewhere between two days and about two weeks.

The worst symptoms appear in the first few days and then taper off.

Regarding the intensity of the symptoms, this likely depends on the previous diet, hormonal state, and prior carbohydrate intake.

For people who are following extremely low carbohydrate diets (especially zero carb diets), these symptoms may be more intense.

Fortunately, there are several ways to reduce the side effects or even completely remove them.

Key Point: Keto flu is a condition that some people experience when drastically reducing carbohydrate intake. It lasts somewhere between a few days and two weeks.

A Young Lady Feeling Dizzy and Holding Her Head.Symptoms

So, what kind of things should you be looking out for?

Some symptoms may be mild, and in extreme cases, others can be more serious.

As previously mentioned, the intensity of the effects will depend on the hormonal state and metabolism of each person.

Mild Symptoms

  • Difficulty sleeping: Some people have problems getting to sleep and waking up in the middle of the night is not uncommon.
  • Digestive issues/upset stomach: Mild stomach discomfort can be a problem for some.
  • Fatigue: As the supply of glucose falls with nothing (initially) to replace it, a lack of energy is a common symptom.
  • Headaches: A headache is a frequent side effect of going low carb in the first day or two.
  • Irritability: Making large-scale dietary changes affects hormones and may make people irritable.
  • Lack of focus/lethargy: As energy levels drop, a lack of focus and lethargy is a natural result of cutting the carbs.
  • Mental Fog: A lack of mental clarity, cloudy memory, and a spacey feeling can accompany a sudden reduction in carbohydrate.
  • Nausea: Occasionally, people report nausea during the first several days.
  • Sleepiness: Again, this feeling is due to a lack of energy from your body continually trying to burn (a falling supply of) glucose.
  • Sugar and carbohydrate cravings: refined carbs and especially sugar can feel very addictive, and giving them up can lead to withdrawal symptoms.
Key Point: Keto flu brings a broad range of mild symptoms with it. These can be frustrating, but they generally disappear within a few days.

Serious Symptoms

Picture of a Man Who Looks Very Dizzy.

  • Arrhythmia: Otherwise known as heart palpitations, some people experience these in the first few days of a very low carb diet. This symptom tends to be due to mineral deficiencies.
  • Constipation: Changes in bowel movements are a common complaint. In this case, they are usually caused by dehydration from the body flushing stored water as glycogen drops.
  • Cramps: Also a side effect of dehydration and mineral deficiencies, some people experience cramps — particularly in the legs and feet. These events may occur more frequently during the night.
  • Dizziness: This is only in more extreme cases, but dizziness can occur. It happens as a result of low blood sugar and sodium deficiency.
  • Drowsiness: While similar to a lack of energy, feeling drowsy can be dangerous if an individual is driving or in public.
  • High (or low) blood pressure: In severe cases, high or low blood pressure may be a symptom.

Before we look at how to solve these problems, it’s important to know why they occur and we’ll examine this in the next section.

Key Point: The induction stage of very low carb diets can, in rare cases, cause serious side effects. Knowing how to avoid these symptoms is crucial.

Why Do These Keto Flu Symptoms Happen?

One simple reason why these side effects occur is that the body is trying to burn glucose, but being fed fat.

Keto-adaptation takes time.

Over a potential lifelong diet of high-carbohydrate meals, the body has developed a wealth of enzymes to burn carbohydrate, but it is ill-prepared for burning significant amounts of fat.

As an analogy, it is like trying to fuel a diesel car with gasoline/petrol.

Once someone switches to a very low-carb, high-fat diet, the body will begin to upregulate the production of fat-burning enzymes.

However, this may occur over a period of a few days or it may take a few weeks, and this adaptation phase can be a painful process.

All the ‘keto flu’ symptoms disappear once the body starts burning fat (ketones) instead of carbs (glucose).

How Can We Make the Adaptation Phase Easier?

Nobody wants to feel like they have the flu for days on end, and all it takes is a bit of research to minimize these side effects.

For me, there are five important considerations during keto-adaptation;

  • Hydration (water consumption)
  • Sodium
  • Potassium
  • Magnesium
  • Fat consumption

If we carefully consider each of these points, then we can seriously reduce (or completely remove) the symptoms of keto flu.

Key Point: The side effects from going very low-carb are due to sudden and large changes in diet. However, it is possible to reduce these side effects vastly.

1. Water: Stay Hydrated

Girl In Exercise Clothes Opening a Bottle of Water.

The first thing to remember is that when we are consuming a high carbohydrate diet, our body stores lots of glycogen.

Glycogen is a form of sugar that is easy to use for the body, and it is stored in our muscles and liver.

For every gram of glycogen, the body stores approximately 3 grams of water (1, 2, 3).

Upon starting a ketogenic diet, these glycogen stores progressively deplete and at the same time flush the no-longer-needed stored water. As we lose this water, we also lose electrolytes such as sodium, potassium, and magnesium.

Additionally, we also lose water due to a reduction in insulin levels.

As carbohydrate intake reduces, our blood glucose levels drop significantly. This fall has the knock-on effect of reducing our circulating insulin levels.

Higher insulin levels encourage our kidneys to retain sodium, so when our insulin levels drop, this stored sodium is excreted.

In other words, we lose yet more water as we excrete this sodium through urination.


As we are losing a significant amount of water during the first few days of carbohydrate restriction, it’s important to stay hydrated.

Dehydration can easily occur and helps explain many of the symptoms such as headaches and cramps.

Thirst is a great cue which we should listen to. While I don’t believe in an arbitrary water recommendation that fits everyone, it’s worth considerably upping water intake during the keto-adaptation phase.

If you experience headaches or any cramping, then there’s a good chance you’re just not drinking enough.

Key Point: Adopting a ketogenic diet results in significant water loss over the first few days, so it’s important to consume enough fluids.

2. Sodium

Sodium is one of the most important nutrients to human health.

As previously mentioned, our body flushes large amounts of this electrolyte away during the adaptation stages of keto.

Quickly losing large amounts of sodium in this way can lead to many deficiency symptoms such as headaches, thirst, brain fog, and lethargy.

This explains a significant part of what keto flu is — symptoms of electrolyte deficiencies and inadequate hydration.

On the negative side, critically low sodium levels may also lead to hypotension (low blood pressure). This condition results in feelings of dizziness, lightheadedness, and in severe cases, even passing out.

It is, therefore, critical to get extra sodium when starting on a ketogenic diet.

How Much Sodium is Enough?

Starting on a ketogenic diet without upping water and salt intake is likely going to cause keto flu symptoms.

Phinney and Volek, respected low-carb research scientists, recommend 3-5 grams of sodium per day (4).

This amount works out to around 1.5 to 2 teaspoons of salt.

Here are some ways to ensure sufficient sodium intake;

  • Focusing on naturally sodium-rich foods like eggs, meat, and fish
  • Making bone broth and salty soups
  • Including sea vegetables such as kelp and kombu
  • Liberally salting each meal

And don’t worry too much about high salt consumption.

In fact, studies show that—for most people—salt consumption plays a minor role in hypertension (high blood pressure) compared to sugar and refined carbohydrate intake (5).

Also, in the presence of lower insulin levels, the body does not store salt in the same way.

Key Point: A smooth keto-adaptation requires higher intake of dietary sodium. The lower sodium levels are, the worse the keto flu symptoms might be.

3. Potassium

The Chemical Element Symbol For Potassium "K".

Sodium and potassium need a delicate balance in the body, and this ratio is essential for controlling the fluid balance in every cell.

As sodium levels fall when starting a ketogenic diet, so too does potassium as the body excretes it through urine.

Not surprisingly, symptoms of potassium deficiency also mimic those of the keto induction flu (6);

  • Weakness
  • Cramping
  • Constipation
  • Heart palpitations
  • Fatigue


There are several ways to keep potassium levels high;

  • Make yourself aware of low carb foods that are high in the mineral.
  • Due to the link between sodium and potassium, keeping sodium intake high helps to preserve potassium levels.
  • Emphasize leafy greens — spinach and seaweed are especially high in the mineral.
  • All meat and fish contain decent amounts of potassium, but these foods are generally part of a ketogenic diet anyway.
Key Point: Keeping a sufficient potassium intake helps to avoid symptoms such as cramping and muscular weakness.

4. Magnesium

Despite magnesium playing a vital role in every cell of our body, deficiency is an increasing problem (7, 8).

In my opinion, magnesium is the single most important mineral.

The fact that our magnesium levels drop when starting a keto diet is an important consideration.

For instance, sufficient magnesium intake also helps regulate potassium and sodium levels (9).

Short-term magnesium deficiency is associated with;

  • An increase in food cravings (10).
  • Symptoms such as nausea, fatigue, and cramps (11).


Here are some strategies to maintain sufficient magnesium levels;

  • Many delicious low carb foods are high in magnesium, so try to emphasize them.
  • If you’re not getting enough dietary magnesium, it’s a good idea to consider using supplements. Magnesium citrate is one of the best, or you can use a magnesium complex like the above.

Ensuring a proper dietary intake of magnesium, potassium, and sodium will assist in providing sufficient levels of electrolytes.

Through this and drinking enough fluids, you can usually neutralize or dramatically reduce most of the keto flu symptoms.

Key Point: Magnesium is one of the most important health considerations for everyone, but particularly those starting a ketogenic diet.

5. Fat Consumption – Get Enough

Picture of Steak, Half an Avocado and Cherry Tomatoes.

When riding a bike for the first time, how do you learn? By riding more!

Likewise, eating a sufficient amount of dietary fat helps your body to acclimatize to burning fat for fuel quickly.

Fearing fat is one of the biggest mistakes that so many people make. For instance, limiting dietary carbohydrate, but then eating low-fat chicken breasts and trimming the fat from red meat.

If carbohydrate is low, then fat needs to be high. Without a sufficient supply of energy, we will feel terrible.


Prioritize sources of fat in the diet, from foods such as;

  • Fatty meats (pork belly, bacon, ribeye steak, roasted chicken with skin).
  • Oily fish (salmon, mackerel, trout, herring, sardines).
  • Healthy fruit sources of fat such as olives and avocado.
  • Eat a handful of nuts each day – macadamia nuts are one of the tastiest!
  • Use healthy sources of fat liberally – butter, ghee, coconut oil, olive oil, avocado oil, and animal fats all make good choices.

Temporarily eating more fat can help hasten the keto-adaptation phase, and then you can cut down once fully adapted.

Key Point: Starting a very low carb diet requires a high fat intake. Don’t fear natural sources of dietary fat.

Final Thoughts

Ketogenic diets can be healthy and they have a lot of benefits if you eat right, including weight loss and better health markers.

However, it is important to fully research them before adopting the diet for the first time.

Some of these possible symptoms can be scary, and all it takes is a bit of knowledge to avoid them.

If you do experience these symptoms, then do be prepared and make sure you get enough water and electrolytes.

Lastly; remember that although these keto flu symptoms can be a pain, they don’t last forever.

For more on keto diets, see this guide to ketogenic diets and sports performance.

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I actually didn’t have any problem with Keto Flu when cutting out carbs. In fact I had a lot more consistent energy. When I eat carbs my energy levels are up and down and not consistent. Also I’ve never eaten much sugar or sweet foods so that may have something to do with it.


Hi there, I started the keto about two weeks ago but, a week prior to that I started the slow carb diet and then I switched over to Keto… I’ve lost about 6 pounds. I finally got the strips today and tested in the 80 ketosis.. I’ve had some of the symptoms you’re talking about but they only happen periodically, I will get kind of dizzy and then I will get a little nauseous with a little bit of cramping but then I feel great with a lot of energy and then two hours later I feel exhausted. My questions… Read more »


Thanks for this


Hello, I am hypoglycemic and borderline diabetic. How would this affect that? When starting is there a chance that my blood sugars might drop to low?


I want to really try this, i have heard nothing but good things about this diet , but i do have a few concerns. I have a few medical issues that are controled with Coumadin and diuretics daily, after reading about the sodium , magnesium , and the potassium , i am now wondering if this will work for me. I am all ready to start this diet ,but now I’m wondering with the meds i take that all deal with my sodium ,mag , and potassium, will interfere with what needs to happen,


There is a lot of good information here. Very helpful. Thanks for sharing!


Hello, This is my second attempt at the keto diet. First time -1 month- and like Marissa not a single ounce lost and never got into Ketosis. The 3rd week I started vomiting violently. It truly felt like the flu. After a month I had to stop. So I took 2 weeks off, on it now 6 days and have lost 1 lb. I have an extremely slow metabolism. Fatigue, brain fog, sleepy…etc. What am I doing wrong? Now next week I’m starting a Zero to Fit running program and I’m nervous on top of the Keto Flu I won’t… Read more »


Hello. Just wanted to say thanks for the info you’ve provided. I’m 49 yr old white male,2 nd time trying the keto diet. 3 rd day and at 8.0 on the test strip and feeling the flu. Today weighed in at 217 this morning. My goal is 190s. Thanks again for everything.


Thanks for the info. I’m new to this diet and still on experiment process. I try to collect more information to make it works for me. I’m 68 yrs old and have a maintenance med for cholesterol and high blood pressure. My blood sugar levels is alarming so need to reduce carbs intake. Weight 180lbs with a fat belly “sigh”. Also bothered by Sciatica

Libby Beaty
Libby Beaty

I have had this keto flu for a week. The worst day was the 7th day where I woke up with diarrhea and then felt like I was going to pass out. I’m doing the broth to help with lost sodium and trying to increase my liquids but I really just feel terrible. Wondering if this is the diet for me as I am 62 ys. old!!


I have been trying Keto diet and bought Keto strips. I am doing intermittent fast (do not eat from 8pm-2pm next day. Bulletproof coffee and then eat at 2pm 4pm 6pm and a snack before 8pm. High fat, reasonable protein, low, low, low carbs. No ketones showing and no weight loss. How long should I see results and what am I doing wrong.

lesia Ledford
lesia Ledford

Thanks for all your information. You have really been a great Help