In the first few days of a ketogenic diet, you will likely experience some level of discomfort. You may start feeling tired, nauseated, spaced-out, achy, and generally unwell. But there’s no reason to worry. Many keto dieters experience these symptoms and refer to them as the keto flu.
The keto flu is temporary, lasting a week on average. The label “keto flu” is a bit of a misnomer because it’s not really a flu or even contagious. Calling it “carb withdrawal” would be a more accurate way of describing it. The reason? Well, the majority of keto flu symptoms are a result of a carb-addicted body craving its daily shot of carbs.
The keto flu can be unnerving, to say the least. It can also be downright scary for someone experiencing it for the first time and without prior knowledge. Knowing that the keto flu is both common and normal helps many first-time keto dieters follow through.
Learn more about the keto flu in the lines below, including what it is, why it happens, its symptoms, and ways to treat it. Also, check out our 3-minute overview of the keto flu in the video below.
What Is the Keto Flu?
In a nutshell, the keto flu is the range of symptoms people experience in the first 3-7 days of a keto diet.
These symptoms are a result of carbohydrate restriction, as well as the accompanying metabolic adaptations. Keto flu symptoms will go away as soon as you begin entering ketosis — the metabolic state where the body starts making and using ketones for energy .
But why do we get the keto flu? Well, after a lifetime of relying on carbohydrates for fuel, your body needs time to adjust to a low-carb, high-fat diet. In this adjustment period, it needs to use up glycogen stores; optimize the production of fat-digesting enzymes; and start favoring fat oxidation over fat synthesis [2,3].
In simpler terms, your body needs to undergo necessary metabolic changes in order to start burning for fat for fuel instead of glucose. Unfortunately, these changes don’t come easy and ignite a wide range of symptoms along the way.
As you can probably tell, your body fiercely resists going into ketosis. The reason being that we are hardwired to store fat rather than burn it; it’s a simple survival strategy. Another reason why your body doesn't like burning fat is because the process is expensive — requiring more oxygen and energy than burning carbs. But on the plus side, fat yields more energy than carbs once utilized.
Most (but not all) people experience keto flu symptoms every time they switch to a low-carb, high-fat, and adequate protein diet. They happen in the first days of a keto diet and can last from a week to a month. Most people, however, experience them for just one week. The symptom most commonly reported include the following:
- Muscle cramps
- Brain fog
- Mood swings
- Poor mental focus
- Food cravings
- Heart palpitation
You may experience all or just a few of these symptoms. Not experiencing them does not mean that you’re not on your way to ketosis. On the other hand, experiencing them does not necessarily mean that there’s something wrong with you.
However, severe or long-lasting keto flu could be a sign that your body is struggling. Metabolic, genetic, and lifestyle differences account for these differences (more on that later).
We already explained that carb restriction and metabolic adaptations lead to keto flu. However, the direct causes of keto flu symptoms have to do with your body being thrown off track. It experiences imbalances in blood sugar, hormones, and electrolytes, and this is the main reason why keto flu happens.
With that being said, the main causes of keto flu include these three things:
Low blood glucose
HPA axis hyperactivity
Declining blood glucose and insulin
When you eat fewer carbs than your body is used to, your blood sugar (glucose) levels drop. This decline leads to a wide range of symptoms, but most are centered around your brain and nervous system. You’ll experience this as concentration difficulties, brain fog, dizziness, clumsiness, and so on.
Low blood sugar has the strongest impact on the brain because this organ relies exclusively on glucose for energy — more so than any other organ. Studies show that the brain, which accounts for just 2% of your body weight, spends 20% of glucose-derived energy .
The above-mentioned symptoms are a result of low blood sugar, or hypoglycemia as medical experts call it. Hypoglycemia is sometimes a symptom of serious diseases like diabetes. But in healthy people, it is a normal reaction to fasting.
If you are relatively healthy, your blood sugar won’t drop to dangerously low levels. You’ll experience a short bout of hypoglycemia lasting a few minutes to a few hours. After that time has passed, your pancreas will releases a hormone called glucagon that signals the liver to release glycogen to normalize blood glucose levels.
HPA axis hyperactivity
Low blood glucose levels also trigger a rise in stress hormones like cortisol and adrenaline. This reaction is referred to as HPA axis hyperactivity . The HPA axis is the name for the set of interaction between three organ systems: the hypothalamus, the pituitary gland, and the adrenal glands. These glands control stress and regulate digestion, mood, and energy spending.
The HPA axis It is the reason why we have a fight or flight response as well as food cravings. Your body perceives a lack of food as a threat to your existence, so it causes a flood of stress hormones by over-stimulating the HPA axis. It does this to force you to eat because it has no way of knowing that you're just on a diet. Keep this in mind when you feel your heart beating fast and get panicky in the first few days of carb restriction.
You will feel hyperactive HPA axis as acute stress, nervousness, anxiety, and heart palpitations. But once your blood sugar levels stabilize and ketones start taking on the role of major fuel source, you’ll notice these uncomfortable symptoms wane.
Another thing that happens with low blood sugar levels is an imbalance of electrolytes. Electrolytes are minerals in your blood, urine, and tissue that have an electric charge. They normalize body fluids, blood acidity, heart rhythm, and muscle contractions. 
The most important electrolytes are calcium, magnesium, sodium, potassium, phosphorus, and chloride. We get most by eating a wide range of plant and animal foods. But we run the risk of lowering our electrolyte levels whenever we lose too much water, like when we sweat or urinate frequently. You’ll feel an imbalance of electrolytes as muscle cramps, irregular heartbeats, insomnia, constipation, and fatigue.
Low blood glucose causes a decline in insulin levels, which further signals your kidneys to produce more urine. A consequence of this is that much of your electrolytes get flushed out. Conversely, high insulin levels cause the opposite — water retention .
A loss of glycogen from your liver and muscles is another reason you lose fluids and electrolytes early in your keto diet. Glycogen molecules bind to a lot of water. Up to 65% of glycogen is water . Furthermore, for every part glycogen lost through a keto diet, you lose three to four parts water . With that all that water, you also lose electrolytes.
How to Treat the Keto Flu
Just like with the regular flu, you can't really treat the keto flu – but you can mitigate and even prevent it. When you do develop the keto flu, your goal is to address individual symptoms in order to feel better and stay motivated. Follow these tips for fast symptom relief:
Drink more water – Since you’ll be making frequent trips to the restroom, it’s a good idea to drink plenty of water to stay hydrated. It’s best to let thirst be your guide when it comes to your water intake. But as a rough guide, up to 12 cups a day is enough. Also, keep in mind that drinking too much water can dilute the sodium in your blood .
Replenish your electrolytes – You can replenish electrolytes by drinking adding a pinch of salt to your glass of water; making a daily bouillon cube drink; drinking sugar-free sports drinks, or even taking mineral supplements. Electrolyte loss accounts for the majority of your keto flu symptoms, so you may want to prioritize this part
Get plenty of rest– Your body is obviously going through major changes trying to adapt to a keto diet. Give it plenty of rest so it doesn't have to deal with excessive straining on top of carb restriction. If you are having trouble sleeping, take power-naps during the day and try to relax before bedtime.
Increase fat intake – Eating more fat can give you the energy you so desperately need in the first days of a ketogenic diet. Since your body is no longer getting its energy from carbs, you might as well up on the fat. Use our keto calculator to see how much fat exactly you should be taking.
Try digestive enzymes — Boosting your fat intake can create problems if you’re low in fat-digesting enzymes. This can not only make you feel tired but can cause diarrhea or constipation as well. Some keto dieters find that taking digestive enzymes helps prevent this.
Meditate— The keto flu can be stressful for your body, so it’s important to reduce your stress levels. Studies on meditation show that this practice is effective in downregulation the HPA axis and with that, improving sleep quality .
If you're just planning a keto diet and looking for ways to avoid or minimize the keto flu, consider the following:
Try light exercising— Your body may still not be ready for the high-intensity stuff you have in plan. But light exercise is a great way to prep for a keto diet. Studies show that a lack of exercise is the leading reason why some people become metabolically inflexible . Boosting your metabolic flexibility through light exercising will help you experience fewer and milder keto flu symptoms.
Use exogenous ketones— Many keto dieters use exogenous ketones solely to ease keto flu symptoms. Exogenous ketones work in this regard because they help get you into ketosis quicker. Our lemon lime-flavored Kiss My Keto Exogenous Ketones have electrolytes added for both easier absorption and electrolyte replenishment. Purchase them before going keto so you have them at hand once symptoms develop.
Take it slowly — If your keto flu symptoms are too much for you to handle, you may want to discontinue your keto diet for now. Try cutting out carbs gradually by 20 grams less per day until you reach the 30 grams per day mark. This method will reduce the severity of hypoglycemic crashes and other keto flu symptoms.
Why It Affects Everyone Differently
The severity and length of keto flu symptoms vary from person to person. The reason for this is something called "metabolic flexibility." Metabolic flexibility is a scientific term for your body's capacity to switch from glucose burning to fat burning .
Some people are more metabolically flexible than others, and this depends on a wide range of factors. Genetics, your lifestyle habits, and overall health most commonly determine metabolic flexibility. A lack of fat-digesting enzymes, poor dietary habit, inactivity, and metabolic disorders can all have a bad effect on metabolic flexibility.
For instance, if type II diabetes runs in your family or if you are insulin resistant, chances are your body will have a hard time switching to fat-burning mode, meaning you are not very metabolically flexible. But don't let this discourage you. A large body of research shows that ketogenic diets are effective in reversing and preventing type II diabetes .
People who are not metabolically flexible, tend to experience severe keto flu symptoms and for longer.
Similarly, being sedentary and eating high-carb meals on a daily could have jeopardized your metabolic flexibility over the years. On the other hand, the luckier metabolically flexible bunch usually report fewer keto flu symptoms that are short in duration.
If you belong to the not-so-lucky bunch, there are ways to boost your metabolic flexibility:
Exercise- Exercise — We already mentioned that not being very active is the leading cause of metabolic inflexibility . Not being physically active means you’ll have less muscle mass, and less muscle mass slows down metabolism . With all that said, adding an exercise routine to your weekly schedule before you go keto will help you avoid the keto flu and go into ketosis much easier.
Get enough sleep — Studies show that a lack of quality sleep slows down your metabolism and increases your risk of insulin resistance . Getting at least 6 and ideally 8 hours of restful sleep every day is crucial to keep your metabolism well and functioning.
Reduce your carbs — Sure, you will be reducing your carb intake once you go keto. But for now, try to reduce your intake gradually and eat only whole, unprocessed carbs. Your goal is to stabilize your blood glucose to help further boost your metabolic flexibility.
De-stress — Stress wreaks havoc on your mind and body. It makes you tired and it interferes with normal metabolism. De-stressing will help you sleep better and have more energy for workouts.
Most people get over the keto flu within a week. Some people have to deal with it all throughout the second week. In rare cases, people report experiencing the keto flu for up to five weeks. Your metabolic flexibility determines how long you’ll have to deal with the keto flu.
If you feel like your keto flu is taking too long to resolve and that it's affecting your functioning, then you may want to pause the keto diet for now. Consider reducing your carbohydrate intake gradually next time so your body can slowly adapt to a low-carb diet. This can be especially useful if you have diabetes or insulin resistance, which can make you prone to sugar crashes.
You may also consider taking digestive enzyme supplements if your keto flu is a result of poor fat absorption. You'll know you have this particular problem if you experiencing the following
Fat is difficult to digest simply because we’re not used to eating so much of it. Studies show that the most people’s diets consist of anywhere between 11 and 46% . On a keto diet, at least 75% of your daily calories need to come from fat. That’s a lot of fat to digest, and your body needs time and enzymes to make that happen.
If taking enzyme supplements is not for you, then consider using MCT oil such as our Kiss My Keto MCT OIL C8/C10 - Unflavored. MCT oil is made from medium-chain triglycerides (MCTs), which are types of fatty acids that don’t require digestive enzymes or bile to be used as energy. Instead, they go straight to the liver where they are used for energy or converted to ketones. They provide instant energy and are much easier to digest than other types of fat.
Your keto flu will start to subside once your body begins producing ketones. When your body starts fully utilizing ketones for energy, it has fully adapted to ketosis. At that stage, you’ll finally feel the long list of benefits you hear people talking about - from weight loss and increased energy to mental acuity and appetite suppression.
When it comes to conquering the keto flu, the more tips you can get the better. So, here are a few more useful tips to get your through your keto flu season:
Aspirin— It’s a great headache remedy, providing that you’re not allergic to it. Paracetamol is another pain reliever that works great for milder headaches and cramps. However, these medications are great for short-term pain relief. To keep headaches and muscle cramps at bay — drink plenty of electrolyte-enriched fluids.
Psyllium — Psyllium is a dietary fiber that helps with both constipation and diarrhea. Another benefit is that it stabilizes blood sugar levels , which is plus when you’re reducing carbs. Fiber combined with water also increases stool bulk and keeps you feeling full.
Fortified salt - Since you’ll be adding more salt to conquer the keto flu, why not used fortified versions. Many table salt brands add calcium and magnesium to their products. Fortified salt is a convenient way to boost your electrolytes intake.
Many keto dieters see the keto flu as an unavoidable and uncomfortable stepping stone to ketosis. But this doesn't necessarily have to be the case. Whether you'll develop the keto flu and to what extent depends on your genetics, your health, and your lifestyle habits. Luckily, it’s fairly and easy to treat. Just follow our tips and guidance and you'll be in ketosis in no time.
In any case, the keto flu is easy to remedy and even avoid. Drink electrolyte-enriched fluids to treat the most common of the keto flu symptoms, such as fatigue, leg cramps, and headaches. Also consider increasing your fat intake for more energy and fewer carb crashes.
Other tips to that help in remedying the keto flu include resting and taking exogenous ketones. Exercise daily before starting the diet is a great way to prep your body for the metabolic adaptation ahead. This is especially helpful if you’re diabetic. Cut out carbs gradually to avoid hypoglycemia if you’re at risk or if you’re sedentary and overindulgent with your carbs.
The keto flu, although uncomfortable, is a normal reaction to sudden carb restriction and higher-than-usual fat intake. Like with any major change, you can expect an initial struggle before the benefits kick in. If you want to learn more about what to expect on your keto journey, take our free learning course and become a keto expert in just 3 hours.