Keto Flu – Side Effect of Keto Diet

Keto Flu

You’ve been trying hard to stay true to your low carb diet. All that effort only to stumble upon a bunch of unpleasant symptoms? If that sounds familiar, chances are, you’ve been hit by a keto side effect – the notorious keto flu.

It’s something that almost all people have encountered at least to some extent when starting their keto journey. For some the symptoms are tough and take a long time to pass, while the others walk away with either minor or no discomfort at all.

In this article I will try to explain everything there is to know about keto flu. What causes it, what symptoms you can expect, and finally – how to prevent it. Let’s go!

What Is Keto Flu?

It’s an umbrella term for a range of symptoms that occur when a person starts a low-carb or a ketogenic diet. The symptoms are very similar to the ones that people get when they get the common cold or the flu.

The symptoms include lack of energy, sore muscles, headaches, nausea, stomach problems, reduced cognitive performance and so on. In turn, it’s not uncommon for people to feel unmotivated and be irritable. This can be frustrating for both the person on a diet and their friends and family.

However, the keto flu is temporary and ends as soon as the body finishes adapting to ketones and normalizes ketone production. In addition, the negative side effects can be greatly reduced by following a couple simple rules.

What Causes Keto Flu?

Keto Diet Foods

To put it simply, your body uses glucose for energy. If you’ve been eating without any significant dietary restrictions, chances are, your body got used to this high-energy fuel. Afterall, it’s unsurprising as sugary and starchy foods are readily available and they don’t cost a lot.

Once you completely cut out glucose from your system, your body starts relying on ketones as the primary source of energy. While your body has no problem producing ketones, it comes like a bit of a shock for organs that constantly require energy.

The end result is a situation with withdrawal-like symptoms that can last anywhere from a couple of days to a couple of weeks. In the end, your body adapts to the change and starts producing the necessary amount of ketones, reducing the unwanted symptoms.

What exactly causes keto flu varies from person to person. Food choices and lifestyle can greatly influence the effects. For most people, the cause is dehydration and lack of electrolytes. Both of these can be resolved relatively easily.

The other part is psychological. Many people have their comfort foods which, most of the time, are something like pizza or burgers. Both dishes is something that’s not suitable for a ketogenic diet (at least not in the form that brings them comfort).

So, if the person on a diet has some underlying psychological difficulties, it can be very difficult both emotionally and physically to be restricted in that way. However, it’s important to have good moral support and keep the big picture in mind. Always remind yourself that the feeling is temporary!

Keto Flu Symptoms

There’s a big list of side effects that you might encounter. However, it seems worse than it actually is. It’s very unlikely for a person to experience every single problem listed below. Most of the time, people just feel one or a couple of side effects.

Upset Stomach

Stomach Discomfort

While an upset stomach is a common low carb diet side effect, it usually manifests itself as nausea and stomach pain. In rarer cases, it can lead to constipation. Even rarer are diarrhea and vomiting. Increase your water intake to reduce the symptoms.

Fatigue and Lack of Concentration

Keto Flu Causes Fatigue

Low carb flu often results in reduced concentration and fatigue. In early days of keto flu you may feel weakness and dizziness, especially after increased physical activity.

Tiredness from keto flu can also affect your mental performance. You may feel like your thoughts are a bit slower. This can have a negative effect when you’re studying or working a job that requires attention.



Headaches are a bit less common but they can appear if you’re prone to migraine or headaches in general. The good news is that this side effect usually passes along with keto flu – normally in a week or so.

Muscle Soreness

Keto Flu Causes Muscle Soreness

Keto flu is called a flu not without a reason. Your muscles may feel sore to the touch or exercise. Sometimes the soreness can grow and result in muscle cramps. This is usually a sign that you’re low on electrolytes.

Cravings For Sugar

Sugar Cravings on Keto

One of the keto nice things about keto is that your cravings for sugar and starchy foods reduces. However, this is not immediate. In early days, when you’re feeling the keto flu, you will feel increased cravings for sugary foods and drinks.

However, always remember that this is temporary. Just push through for at least a week and you will see the cravings disappear. Stay away from sweeteners, zero-carb processed foods and drinks that taste sweet. These tend to give the unwanted cravings a boost.

Reduced Athletic Performance

Reduced Physical Performance

If you’re regularly exercising, you will suffer from this more than most people. Many things come in play that result in worsened physical performance but the most prominent one the fact that your body is adjusting to a new fuel.

This often means that you just have less energy to spare. In addition, if you’re suffering from other side effects like upset stomach and headaches, it can make working out an unpleasant experience.

Like before – good news is that this is temporary. Once the low carb flu is gone, you will feel an improvement in your performance!

How To Avoid Keto Flu?

The symptoms can sound very intimidating, especially if you’re a beginner with keto. Thankfully, most of the keto flu symptoms are caused by the same group of problems that can be (at least partially) addressed.

Every single person is different, so the solutions for keto flu I’ve listed below can be effective for some people but others can feel no change in their wellbeing. Whatever the case might be, the rules are really simple and anyone should be able to try these keto flu remedies.

Enter Keto Diet Gradually

Easing Into Keto

Some people don’t want to enter the diet gradually. Mostly because they want to see the results as soon as possible and want to quickly reach ketosis for maximum results.

However, it is important to understand that the low carb flu is caused by your body making the shift from glucose to ketones. If you completely cut out the carbs, your body needs time to adjust to the new source and keto flu is the result.

If you enter the diet gradually, the fuel shift will happen over time and you will either not feel the side effects at all or feel them at a lower scale.

You can enter the diet gradually by reducing your carbohydrates from what you normally eat to 5% over a course of 7-10 days. This should be enough time for your body to adjust. However, if you feel like you need more time, feel free to extend it to whatever seems comfortable for you.

In addition, make sure you’re consuming the correct amount of fat as you’re making the switch . Once carbs are reduced, fat should take up a larger part than protein. If you’re unsure about the ratio or the amounts, you can use our keto macros calculator.

Drink Lots of Water

Drinking Plenty of Water Can Reduce Keto Flu

Once you enter ketosis, your body’s ability to retain water will diminish. This happens because water in your body is stored together with glycogen. The closer you get to ketosis, the lower are the levels of glycogen in your body (source).

This means that your body needs to expel the water that would otherwise be stored with glycogen. This is what causes people on keto to make such frequent trips to the bathroom to pee.

If you don’t counteract this by drinking plenty of water, your body will eventually start lacking it. People don’t always realize it and this results in side-effects like headache, fatigue, poor concentration, muscle cramps, stomach upsetness and other negative side-effects associated with low carb flu.

So, the lesson is: you must stay hydrated at all times if you wish to reduce or avoid keto flu. It might be a good idea to always carry a water bottle with you, so you always have access to some water.

Replenish Your Electrolytes

Sodium Can Reduce Keto Flu Symptoms

Another thing that happens in your body after you enter ketosis has something to do with salt. When your body runs low on carbs, your insulin levels drop. Low insulin levels cause your kidneys to expel excess sodium (source).

On top of that, keto diet restricts you to a very specific range of foods. Most of them naturally don’t have a lot of sodium in them. Unfortunately, this just accelerates this unwanted process even further.

Having enough sodium is essential for normal muscle and nerve function. Sodium is also responsible for keeping the fluids in a normal balance (source). Having too little sodium can result in numerous side effects, most prominent of which I’ve described in the previous section.

In order to fix this, you can increase the consumption of keto-friendly foods that have salt in them. This includes leafy green veggies, fatty fish, avocados and some dairy products. However, probably the easiest way is to simply use a little more salt when seasoning your food.

If you don’t feel hungry, you can replenish your electrolytes by adding a half teaspoon of salt into a glass of water. Drinking a mixture like that once or twice a day should be sufficient to keep your electrolytes in check.

Consider Taking Supplements

Keto Flu Supplements

There are numerous supplements available both online and in grocery stores that can help alleviate the symptoms by resolving the issue that causes it. The most popular supplements for keto dieters are exogenous ketones and MCT oils.

To put it simply, exogenous ketones are regular ketones that are exactly the same as the ones produced by your body. However, you can consume exogenous ketones as needed. They work as a keto-friendly fuel that you can use reduce numerous keto flu symptoms. They’re especially good at improving your mental (source) and physical performance (source).

MCT Oil is another supplement that can help you keep your macros in check, by increasing the amount of healthy fats in your diet. MCT stands for medium-chain triglycerides. Making sure your body has enough fat to use as energy is essential. MCT oil is arguably one of the best sources for that. Aside from keto flu reduction, it has numerous other health benefits.

While supplements are optional and it’s definitely possible to make it through without them, some people feel much more comfortable knowing that they have at least some control over the symptoms.

Low Carb Flu – The Bottom Line

All in all, keto flu is something that most people who aim to lose weight through ketosis will face. The good news is that the feeling is temporary and soon after, you can get to enjoy the benefits that the ketogenic diet brings.

Meanwhile, you can reduce the symptoms by keeping yourself hydrated and making sure you’re consuming the right amount of electrolytes. If you wish to enhance your dieting even further, you can consider taking keto diet supplements.

However, always make sure to stick to your common sense. If the symptoms seem to hard to bear or they last a long time, immediately contact a healthcare professional and seek for help. Normally, keto flu and the keto diet itself shouldn’t cause anything more than just mild discomfort.

Frequently Asked Questions

Why am I exhausted on the keto diet?

Exhaustion is a very common side effect for people on a ketogenic diet. It’s one of the symptoms of keto flu – a sign that your body is transitioning to a new energy source as ketones replace glucose.

The symptom can be reduced by keeping yourself hydrated and making sure you eat sodium-rich keto-friendly foods.

Why do people get the low carb flu?

Low carb flu is the result of a person’s body adapting to ketones. On low-carb and ketogenic diets, glucose becomes scarce and the body starts breaking down fat in order to compensate for the energy losses.

While this results in weight loss, it also comes with keto flu, which is an unpleasant feeling that shows up in a form of nausea, upset stomach, lack of energy, reduced physical performance and so on.

However, the side effects are temporary and usually fade away relatively quickly. This normally happens within a week, but can be less or more, depending on the diet and person’s metabolism.

When does keto flu start?

It depends on what your strategy is towards ketogenic diet. If you’re transitioning towards it by gradually reducing carbohydrates each day, you will feel it later than those who outright cut carbs. However, gradual reduction reduces the symptoms.

Whatever the case might be, keto flu usually starts when ketosis kicks in and the side effects reach their peak around the day 3 in ketosis. The effects then last anywhere from a couple days to couple of weeks.

How long does keto flu last?

It varies from person to person. It’s not uncommon to not feel any major side effects at all. However, for those who do, it usually lasts around a week, give or take.

If the side effects get stronger or last more than a couple of weeks, you should consult your physician for further advice on how to proceed.

Leave a Reply

Your email address will not be published.