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The Keto Flu: Your A-Z Guide to Why It Happens & How To Fix It

You’re a week, or maybe two weeks into the ketogenic diet.

You’re feeling great, losing weight and making huge progress… And then you’re hit with experience flu-like symptoms like fatigue, headaches, and brain fog.

One Keto 30 Challenge member described this in the Facebook group as feeling “drained”. Others complain of…

This feeling is called the “keto flu”, and it’s one of the biggest reasons people fail on the ketogenic diet.

They begin to second-guess themselves, wonder if keto is even right for them, and begin to start thinking about those bagels in the freezer. This is a shame, because they’re just about to turn the keto korner, and giving up now means you are throwing away all of that great momentum and progress you’ve built up, just before the best part.

The keto flu is common – we estimate that one in 12 people who start the keto diet show signs or symptoms of the keto flu – but don’t despair, it gets better.

What Is the Keto Flu?

Also known as carb flu or low carb flu, keto flu is a popular term that describes flu-like symptoms that some people experience in the first few days of starting the ketogenic diet.

Keto flu is not actually a flu. It’s not contagious, and it’s not dangerous. But it can definitely be unpleasant.

The discomfort that you’re feeling is a natural reaction your body goes through when it transitions from burning glucose to burning fatty acids (ketones) for fuel.

You see, fat is your body’s secondary or backup fuel source, behind sugar or carbohydrates (in the form of glucose)[*].  You will naturally burn fat when there’s not enough glucose in your diet. Once your body has switched over to burning fat, you have officially entered ketosis.

Keto flu usually sets in 3 to 5 days into your diet and can last up to 2 weeks (or longer, in rare cases.)2

So what causes keto flu?

What Are the Symptoms of the Keto Flu?

There’s no way to predict whether you’ll experience keto flu symptoms until you start the diet.

If you already eat a healthy diet that’s relatively low in processed sugars and starches, you may notice only mild symptoms, or none at all. But if you’re switching from a highly processed, high sugar diet, you may be more likely to experience “withdrawal” symptoms.

Some of the more frequently mentioned symptoms of carb flu include:

If you’ve been following the typical Standard American Diet (SAD), you’re likely used to eating 250 – 350g of carbohydrates, including sugar, every day.3 When you adopt a ketogenic diet, you’re slashing your carb intake to under 25g per day. That’s quite an adjustment.

If that’s you, don’t worry. You’re not alone. The carb problem has gotten more serious over time.

In 1700, the average American was eating about five pounds of sugar per year. But by 1900, our sweet tooth was in full swing and we were consuming upwards of 70 pounds every year. Today, this number has ballooned to more than 100 pounds of sugar annually — that’s more than 40 teaspoons a day. This toxic substance comes by many names — from barley malt to fruit juice — but its effects are clear.

Most Americans now consume so much refined sugar and processed carbs that they are “metabolically rewired” to burn sugar for energy and store the excess as fat. Breaking this cycle can throw your body out of balance and cause temporary fatigue and flu-like systems.

Your symptoms aren’t all in your mind. Lots of science has been done to try and figure this out.

The Science Behind Keto Flu

There is no scientific consensus on where the keto flu comes from, but there are many theories.

Some researchers and doctors believe these side effects of keto can be attributed to one of the following:

#1 Keto Flu Cause: Fat Adaptation

There’s a period of time between when you have stopped eating your regular carb load, and when your body reaches ketosis. This period is called the fat adaption stage.

During fat adaption, your body is adapting to using fat as energy. It doesn’t have enough carbohydrates to use as fuel (albeit cheap fuel), but it also hasn’t gotten the hang of switching to using the quality fats you’re consuming, either. In the interim, it must still get you around. So your body uses something called “muscle glycogen”, which are your glucose stores.

Some research (like this study) suggests that reduced muscle glycogen (because your body is using it for energy) can cause lethargy and mood swings. 

#2 Keto Flu Cause: Electrolyte Deficiency

This is the most widely accepted keto flu culprit, because almost all symptom-sufferers see significant improvement in their keto flu-like symptoms when they begin to supplement electrolytes (usually with beta-hydroxybutyrate or BHB).

That’s because when your body is using glycogen to fuel itself, it uses water for this metabolic process and flushes water out of your system. With that water goes the electrolytes your body needs. Electrolytes such as magnesium, calcium, sodium, and potassium are crucial for your body to operate efficiently.

When you have an electrolyte imbalance, you experience flu-like symptoms. If you’ve ever felt sick, sluggish or depleted after a long run in the heat or a day at the beach, or if you’ve ever experienced a hangover, you’ll know what we mean.

There are a few other common theories about the cause of the keto flu, but the third and most relevant is insulin levels.

#3 Keto Flu Cause: Reduced Insulin Levels

Insulin is a hormone that helps your body absorb glucose from your bloodstream. When you have less glucose in your system, you don’t need as much insulin to process it (i.e. to lower your blood sugar levels).

That’s why when you switch from a high-carb diet to a diet that’s super low in carbs, your insulin levels usually drop drastically.

That’s a good thing. It’s one of the main reasons the ketogenic diet is so beneficial. People often notice improved brain function, greater energy, stabilized blood sugar, fewer seizures, and much more.

That’s because when glucose is no longer available as an energy source and your insulin levels are very low, your liver starts converting fat into ketones. Most of your cells can use these ketones in place of glucose for energy.

Once your body is mainly using ketones and fat for energy, you’ve entered ketosis.

But it takes time for your brain and some of your other organs to adjust to using this new fuel source.

When your insulin levels drop, your body reacts by ridding itself of excess water and sodium (which bind themselves to the glycogen or stored carbohydrates in your system). This is why you may urinate more often in the first week after starting a keto eating plan.5

This also explains the weight loss that often occurs in the early stages of the diet. You’re mostly losing water weight due to lower sodium levels.

Losing sodium and water-weight can actually bring on the unpleasant symptoms of keto flu.

How to Treat Keto Flu Symptoms 

Even though your body is undergoing some significant changes, there’s no need to strain or suffer. The symptoms of the “keto flu” are almost all completely and easily treatable. So what should you do if you’re feeling uncomfortable or even downright miserable?

1. Replenish What You’re Losing

In a word – hydrate.

Remember that your body is ridding itself of stored water. So you’ll need to drink plenty of clean water to replace what you’re losing.

Staying hydrated can relieve both fatigue and muscle cramping.

Replacing your fluids is particularly important if you experience diarrhea, which can dehydrate you quickly.

2. Replenish Electrolytes & Micronutrients 

You lose essential micronutrients and minerals when your body sheds excess water, and along with that water often goes electrolytes.

Most new keto-ers are electrolyte deficient and need to supplement electrolytes. This can be done quite easily through beta-hydroxybutyrate (aka BHB or exogenous ketones), which are ketone salts that contain high levels of potassium, magnesium, sodium, and calcium (the 4 cornerstone electrolytes your body needs to function properly). 

We use a ready-to-use ketone body that is clinically shown to provide a boost to your energy, performance, and focus in KetoLogic BHB Exogenous Ketones, and 1-2 scoops of Keto BHB or KetoEnergy per day should have you covered for electrolytes. You can add some coconut or MCT oil to support ketosis. 

You can also replenish some electrolytes and other essential micronutrients through food (just make sure to eat a lot of them!):

You can also try coconut water. Just be aware that some brands of coconut water add sugar, so check the macros.

3. Eat More (Yes, Really)

Whatever you do, don’t go hungry!

Many people inadvertently eat too little because they feel nauseous. Or they don’t fully replace the calories from the carbs they’ve removed from their diets.

Instead, consume as many keto-permitted foods as you want, and keep healthy snacks like hard-boiled eggs close by to encourage grazing to help increase your daily caloric intake.

Add as many of these fat sources to your meals as you can:

Or arm yourself with our Master Keto Food List. It has 100+ keto food mapped for you!


4. Ease Out of Your Sugar Addiction

If you’re having a tough time adapting to the ketogenic diet, you may have to eliminate carbs gradually instead of all at once.

Instead of immediately cutting back to 25g of carbs per day, try easing into a more moderate low carb diet of 20 to 50g carbs per day, while increasing your protein and fat intake.6

Choose healthier, “clean” carbs and not the usual junk found in processed foods. Good carbs and essential micronutrients from fruits and vegetables can help relieve your symptoms as your body adjusts.

5. Take It Easy

If you don’t feel like you have the energy for a full workout, don’t push it. Allow yourself time to ease into your new lifestyle.

Choose less demanding, moderate exercise, especially in the beginning — activities like walking, gentle stretching, or yoga might be a good start.

Remember that your body is already under stress adapting to a new fuel system, so don’t place an additional burden on it by forcing a strenuous workout. Listen to your body and gradually increase your exercise intensity over time.

6. Get Sufficient Rest

Lack of sleep can add to your body’s stress, so make sure you’re getting enough of it. Slow down and relax, especially at night.

If you’re having trouble falling or staying asleep, you may experience relief with:

7. Take Exogenous Ketones

Finally, you can manage your flu-like symptoms with KetoLogic BHB — a great tasting and convenient fuel source for those on, or transitioning to, a low carb or ketogenic diet. Grab some KetoLogic BHB during afternoon energy “slumps” or during exercise, to help fuel your mental and physical performance.


Kick Your Keto Flu Symptoms to the Curb 

Even though the symptoms of Keto Flu can be uncomfortable, you’ll find that they’re both short-lived and worthwhile once you push through them.

Not only will you most likely lose weight, you’ll also have more energy, feel less bloated, and experience fewer digestive issues, among other benefits.

Keto works, but only if you stick to it.

Don’t make your transition more difficult than it needs to be. Listen to your body. If you need to ease into a low carb diet or take supplements, then do it.

And don’t despair if you backslide. Simply take the long view and start again the next day. There’s no room for guilt in your personal journey to wellness.

Once the temporary discomfort of the keto flu has passed, you’ll most likely feel better than ever… and you’ll be glad you stuck to it!

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