Key Takeaways

  1. The “keto diet” is short for “ketogenic diet,” which is the drastic reduction of carbohydrate intake to make your body switch to using fat as it’s major fuel source.
  2. Keto flu is the result of your body being forced to use fat as the main source of energy instead of glucose (aka carbs).
  3. There’s no guaranteed way to avoid keto flu aside from not following a ketogenic diet, but there are other arguably more effective ways to lose weight that don’t cause keto flu (keep reading to learn what they are!)

Unless you’ve been living under a rock the past few years, you’ve no doubt heard of the ketogenic diet and the supreme weight loss claims surrounding it. 

The point of the ketogenic diet is to keep the body in a state of ketosis. 

The traditional keto diet was used for treating epilepsy and had prescribed 4 grams of fat for every gram of carb. The biggest problem with this diet for the general public was the allotment for protein was also very low.

The form of the keto diet that’s most popular today is a modified version that has moderate protein (10 to 20% of calories), carbs under 50 grams, and lots and lots of fat.

When first starting the keto diet, you may find yourself feeling drained, sick and moody. 

Many people refer to this as “keto flu,” and it’s particularly common among people who switch from a high-carb diet to a keto diet.

If you haven’t done much research you may believe that the keto diet is basically Atkins to the extreme: very high fat and next to no carbs (sugar). 

While this is correct in theory, it’s not as simple as cutting out all carbs, eating all the avocados, and watching the pounds melt away while experiencing mental clarity and increased energy. The process of getting into ketosis is a lot more nuanced and requires a strict dietary protocol that can leave you feeling miserable—at least to start. 

Before we dive into the aspects of the keto flu, let’s talk about the ketogenic diet as a whole first.


What Is the Keto Diet?

The “keto diet” is short for “ketogenic diet,” which is the drastic reduction of carbohydrate intake to make your body switch to using fat as it’s major fuel source. 

This forces your body into a state of ketosis, which you’ll learn about in a moment.

The keto diet was developed for the treatment of seizures in those with epilepsy and can also be effective for those with type 2 diabetes, metabolic syndrome, insulin resistance, cancer or neurological disease. It can also work as well as any calorie-restricted diet for those who are overweight and sedentary.

Although the ketogenic diet has become popular as a weight loss method, most research shows it works simply by helping people eat less, and has no inherent weight loss effects. (What’s more, it’s also highly restrictive and not as easy to stick to as a moderate carb diet for most people). 

If all of this talk of ketosis and ketones is foreign to you, check out the resources below to deep dive into all things keto:

The Definitive Guide to the Ketogenic Diet 

Everything You Need to Know About the Ketogenic Diet

The Complete Guide to High-Fat Diets [According to Science]

Eric Helms on the Ketogenic Diet for Building Muscle

Research Review: Are Ketogenic Diets Best for Fat Loss?

Summary: The Keto diet was originally developed as a treatment for epilepsy, but recently it’s been used as a weight loss diet because of its elimination of carbs.

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What Is Ketosis?

keto flu

Ketosis refers to a metabolic state wherein your body primarily uses ketones instead of glucose for fuel.

Ketones, also called ketone bodies, are the products of fat metabolism that are used for energy. 

Ketones are simple molecules made of carbon and oxygen, and the three most common kinds of ketones are acetone, acetoacetic acid, and beta-hydroxybutyric acid. 

As glucose is the body’s main source of energy, you might be wondering why anyone would want to force it to use ketones instead. Proponents of the keto diet claim that once in a state of ketosis, they experience increased weight loss, improved focus and mental functioning, reduced appetite, reduced blood sugar levels, and other benefits. 

They can also marshal scientific evidence to prove their point, like a 2013 meta-analysis of 13 studies on Very Low Carb Ketogenic Diets (VLCKD) compared to Low Fat Diets (LFD). In this case, the researchers found that people assigned to the VLCKD achieved greater fat loss than the groups assigned LFDs, thus supporting the increased weight loss claims. 

Of course, what these people fail to mention is that when you dig into the details on these individual studies and others, you see two things: 

  1. Most of the additional weight loss caused by ketogenic diets is due to water weight loss, not fat loss.
  2. In studies where people following a ketogenic diet lost more fat than people following a moderate or high-carb diet, invariably the people following the ketogenic diet also ate more protein. It’s well established that higher protein diets lead to more fat loss than lower protein diets, so it’s not an apples to apples comparison.

A good example of this is a study conducted by scientists at Arizona State University, which demonstrated that people following a ketogenic diet lost the same amount of weight as people following a high-carb diet with an equal amount of protein. 

In other words, if total weight loss is your only goal, how much you eat is more important than what you eat. 

We don’t recommend keto as a preferred diet over anything else. It generally doesn’t improve weight loss more than other dietary methods, and it’s more restrictive. 

If you still want to give keto a try, though, the general recommendations are to keep protein at 25% of your daily calories, carbohydrates around 5%, and fat at around 75% of your total daily calorie intake.

If you think this all sounds good and are ready to cut out the bread and stock up on avocados, you will want to read ahead so that you’re not caught off guard when you start the diet and end up feeling horrible. 

Or, maybe you’ve started and can’t figure out why you’ve suddenly started feeling like you’ve come down with the flu. 

Summary: Ketosis refers to a metabolic state where your body primarily uses ketones instead of glucose for fuel, and this transition from glucose to ketones is responsible for keto flu. 

What Is Keto Flu?

Keto flu is the result of your body being forced to use fat as the main source of energy instead of glucose (aka carbs). 

When you begin the ketogenic diet, your body goes through an adjustment phase.

This can cause withdrawal-like symptoms that may feel similar to the flu.

The cause of this rough transition has to do with how your body turns food into usable energy.

The body primarily makes energy in two ways: glycolysis and beta-oxidation. Glycolysis is how your body uses sugars, specifically glucose, for energy. This is your body’s preferred way to make energy and it can use a variety of sugars including fructose, mannose, and galactose for fuel. 

Beta-oxidation is how your body uses fat to make energy. The fatty acids that you get from your diet and the ones that you store in body fat are used by your cells to make energy. This method takes more work for your body and is the result of getting your body into ketosis.

Now, at this point you may be wondering: if the body normally relies on glucose for energy, and gets keto flu from relying on fat, why don’t you get keto flu when training fasted

After all, if you don’t have any glucose in your system, then you must be burning fat and ketones, right? 

Not necessarily.

Your body can also derive energy from a form of stored carbohydrate called glycogen, which is stored in your muscles and liver. 

Like plants, humans make a form of starch called glycogen that can be used for energy. Where plants make starch in the simple forms of amylose and amylopectin, humans make the complex form of glucose called glycogen.

All of these forms are ways to store glucose in the body for later use. Glycogen is important because it’s a quick fuel source for sudden needs especially when fasted. Between fat stores and glycogen, your body can get the energy it needs when it needs it.

Your body will rely on fat metabolism and glycogen stores when training fasted. Because this fasted state is temporary and you generally have enough glycogen stored, you won’t experience the symptoms associated with keto flu.

It takes a few days for the body to adapt to using fat as an energy source and for some, this transition can be particularly unpleasant. 

Signs can start within the first few days of beginning the diet and symptoms can range from none at all to very severe depending on the person and how easily their body switches from using carbs to using fats for energy.

Since fat isn’t the preferred energy source for the brain, you have lower energy levels for the first few days that your brain is getting adjusted. This energy imbalance is where most of your flu-like symptoms come from.

Summary: The keto flu is an inevitable set of flu-like symptoms caused by the energy imbalance that happens when your body begins to transition from using carbs to fat as its primary energy source.

What Are the Symptoms of Keto Flu?

While some are lucky enough to enter ketosis without any negative side effects, others may experience any or all of the following:

  • Constipation/diarrhea
  • Headache
  • Nausea/vomiting
  • Irritability/mood swings
  • Weakness
  • Dizziness
  • Poor concentration/brain fog
  • Muscle cramps/muscle soreness
  • Difficulty sleeping/insomnia
  • Sugar cravings

Can You Prevent or Avoid Keto Flu?

Avoid Keto Flu

The short answer to this is no.

If you want to experiment with this diet, here are some things you should consider when it comes to keto flu.

1. There’s nothing you can do to totally prevent keto flu.

The only way to avoid keto flu completely is to not follow the keto diet

The ease with which you transition into ketosis will vary depending on a number of genetic and environmental factors, and there’s nothing you can do to guarantee you won’t run into symptoms of keto flu. In many cases it just depends on the luck of your genetic draw.

Or, more specifically, your metabolic flexibility.

2. Metabolic flexibility is key.

Metabolic flexibility is the ability of the body to adapt to changes in energy demand and environmental conditions. 

For example, people who are able to easily switch between burning fat and carbohydrate have high metabolic flexibility, whereas people who aren’t have low metabolic flexibility.

Those with greater metabolic flexibility will generally experience fewer and less severe symptoms of keto flu. 

Food restriction and exercise are two physiological conditions that require metabolic flexibility. With regard to the keto diet, when your body can quickly adapt to the changing source of energy, you’ll experience fewer symptoms of keto flu.

One method that may improve your metabolic flexibility is intermittent fasting, although there’s still no research on how this might affect your risk of developing keto flu. 

Check out the article below to learn more about intermittent fasting: 

The Definitive Guide to Intermittent Fasting

3. Slow carb reduction can help, but it’s not guaranteed.

Because keto flu mostly comes from the energy imbalance that happens when you switch from carbs to fat, you may be able to ease the keto flu slightly by slowly reducing carbs. However, this isn’t guaranteed to work.

4. Avoid reducing your calories too much when starting keto.

Many people spontaneously eat less when they start following the ketogenic diet (in the short-term), and so they often accidentally find themselves in a large daily calorie deficit. 

Of course, this can also cause severe fatigue, lethargy, and so forth—symptoms similar to keto flu.

If you aren’t careful, this calorie deficit combined with the effects of reducing your carb intake can produce a compounded negative effect that leaves you feeling significantly worse. 

So, to be safe, try to eat enough calories to maintain your weight when you first start following a keto diet. 

Remember, the only way to avoid the keto flu is to avoid the keto diet. Luckily, the keto diet is not the only way to lose weight.

It’s honestly not even the best way. There are many other ways of achieving your weight loss goals and getting the body you want without struggling through the keto flu so don’t feel like this is the end all be all.

If you want to learn about a more sustainable (and arguably, more effective) way to lose weight that doesn’t involve keto flu, check out this article: 

The Complete Guide to Safely and Healthily Losing Weight Fast

Summary: The only way to avoid keto flu is to avoid a keto diet, but if you want to try reducing some of the symptoms, gradually reduce your carb intake, eat enough calories to maintain your current weight, and work on improving your metabolic flexibility.

A Better Way to Lose Weight

Lose Weight (1)

If the idea of enduring flu-like symptoms and saying goodbye to your favorite fruits, sweet potatoes, cereal, and muffins sounds like a living hell, I have good news. There are alternative science-based ways to help you lose that stubborn weight that don’t involve ketosis. They are:

1. Ensure you’re in a large enough calorie deficit.

Eat 75% of your TDEE daily (in other words, maintain a 25% daily calorie deficit). This is enough to produce rapid weight loss without flirting with the side effects of crash dieting.

(And if you’d like even more specific advice about how many calories, how much of each macronutrient, and which foods you should eat to reach your health and fitness goals, take the Legion Diet Quiz.)

If you aren’t sure how to calculate your TDEE or measure your calorie intake, read this article: 

This Is the Best TDEE Calculator on the Web (2019)

2. Eat a high protein, high carb diet.

Eat around 0.8 to 1.2 grams of protein per pound of body weight per day, around 20 to 30% of your calories from fat, and the rest from carbs.

Read these three articles to learn why and how to do this: 

How to Know Exactly How Many Carbs You Should Eat

How Much Protein Do I Need? The Definitive (and Science-Based) Answer

How Many Grams of Fat Should You Eat Per Day?

3. Do a lot of heavy compound weight lifting.

By “heavy,” I mean you should work primarily with weights in the range of 75 to 85% of your one-rep max (1RM), which includes weights that you can do 6 to 10 reps with before failing. Compound exercises train multiple large muscle groups at once, and some of the best compound exercises are the squat, deadlift, and bench and military press.

Read these articles to learn more about why and how you should be doing these exercises: 

How to Squat: The Definitive Guide (Plus 12 Proven Ways to Improve Your Squat!)

This Is the Definitive Guide to Proper Deadlift Form

The Definitive Guide on How to Bench Press (and the 8 Best Variations!)

The Ultimate Guide to the Military Press: The Key to Great Shoulders

4. Keep cardio to a minimum.

Too much cardio can interfere with your weight loss and muscle-building goals in a variety of ways, but that doesn’t mean you should avoid it altogether. 

Instead, check out this article to learn how much cardio you should do to lose weight and what kind of cardio you should be doing: 

The Beginner’s Guide to How Much Cardio You Should Do

5. Take the right fat loss supplements.

You don’t need to take supplements to build a body you can be proud of. That said, the right supplements can help you lose fat faster.

Read this article to learn which fat loss supplements are worth taking and which aren’t: 

The 3 Absolute Best (and Worst) Fat Loss Supplements

The Bottom Line on Keto Flu

Keto flu describes the feelings of lethargy, irritability, and weakness that many people experience when they start a keto diet. 

The number one thing you need to understand about the keto flu is the only way to avoid it is to avoid the keto diet.

Why some people experience keto flu and others don’t isn’t fully known. Those who consume a lot of carbs before transitioning to the keto diet may have a harder time adjusting.

What is known is that there are better ways to lose weight and get the body you want without forcing your body to go through ketosis. 

What started as a way to treat a debilitating disease has turned into a fad diet that people are putting themselves through unnecessarily. 

Weight loss takes time. If you’re getting serious about your body and health, why put more stress on yourself when there are better options?

If this is an option that you’re dead set on trying, remember that Keto flu typically lasts for about a week. These symptoms gradually decrease as your body gets accustomed to using ketones for fuel. If these symptoms don’t seem to be falling over a week or so, it might be best to contact your doctor to rule out other causes.

Oh, and if reading this article has got you thinking the keto diet isn’t right for your circumstances and goals, then take the Legion Diet Quiz! In less than a minute, you’ll know exactly what diet is right for you. Click here to check it out.

What’s your take on keto flu? Have anything else to share? Let me know in the comments below!

+ Scientific References