Start your fasting journey!
Keto diet
Fasting timer small image Time to read: 9min

The Keto Flu: Symptoms & How to Avoid It?

"Andrea is a copywriter and editor specializing in holistic nutrition, health, and wellness. She holds a Bachelor of Arts in Communication Studies from McMaster University, is a certified personal trainer with the National Academy of Sports Medicine (NASM), and Certified Nutritional Practitioner (CNP) with the Institute of Holistic Nutrition."

There’s no question that the keto diet boasts an impressive list of benefits. Whether it’s weight loss, more energy, better sleep, radiant skin, improved glucose control, better digestion, or any of the other advantages you see from cutting out carbs, there’s no shortage of reasons why someone would want to start on a ketogenic diet.

One of the main reasons why people begin following a ketogenic diet and stop without having experienced any of the true benefits is because of something called the ‘keto flu.’ A flu?! Sounds terrible, but we assure you that it’s not quite what you’re imagining.

The keto flu is actually a collection of flu-like symptoms that occur during the transition period into ketosis when your body is learning how to use fat for fuel. Unlike the normal flu, these symptoms aren’t caused by bacteria or a virus.

Think of it this way. Your car (i.e. your body) has been running on low-quality fuel for its entire life and it’s chugging along so-so. One day you decide that you want your car to drive a bit faster, handle a bit better, and stop being sluggish, so you decide it’s time to upgrade—we’re going to premium fuel. It’s so accustomed to low-quality food that it has a bit of an adjustment period until it learns to use the premium stuff.

That’s pretty much how your body feels when you take away refined sugars and start giving it premium fuel—fat. Your body has a natural adjustment period that is responsible for keto flu symptoms which are typically caused by two things: carbohydrate withdrawal and electrolyte imbalances.

Carbohydrate withdrawal + electrolyte imbalances

The human body is accustomed to using glucose as fuel; it’s the most abundant source and the primary energy substrate for the brain and all organs. However, excessive consumption of carbohydrates – like we often see in the standard American diet – can cause drastic spikes in blood glucose and insulin and in turn can prevent your body from burning fat for fuel.

So, when you’re consistently eating high-carb meals, your body is in cruise-control. It readily has access to a consistent supply of energy to power it through its activities. However, when you switch to a low carb diet and your carbohydrate intake decreases dramatically, your body gets confused; it needs to make a big adjustment when it’s not being supplied with the fuel it is used to using.

As a result, you may start to feel fatigued. Your body has access to fuel (fat), but is not used to using it.

When your body no longer has access to carbohydrates for energy, insulin levels will drop and your body can tap into fat stores to produce ketones, which are then preferentially taken up by the brain in place of glucose (1).

And when insulin levels drop, the body responds by excreting sodium and water, which is why trips to the bathroom become more frequent in the first few weeks in ketosis. This change is also the exact reason why we experience drastic (and very welcomed) weight loss when starting keto, but it’s also the same reason why we can feel pretty crappy for the first few days.

When does keto flu start?

There’s no set time when symptoms may appear. For some people, it may start a few days after they drop their carb intake, while others may never experience these symptoms. The extent to which the keto flu happens is dictated by a few factors:

  1. Diet (Pre-Keto)
  2. Hydration Status
  3. Mineral Balance

The severity and duration of keto flu symptoms can vary greatly per individual. Typically those who were consuming a higher-carbohydrate diet pre-keto, especially those with diets high in refined sugars or processed carbohydrates, tend to have more severe symptoms. On the other hand, people who were already consuming a relatively healthy and moderate carbohydrate diet may barely notice symptoms throughout the duration of this transition.

With that said, those who do experience keto flu symptoms will typically notice an onset around the 24 to 48 hour mark when glycogen stores are depleted and glucose intake is minimal. The symptoms are due to increased urinary sodium, potassium, and water loss during the first 4 days following a ketogenic diet or fasting (fasting gets you into ketosis faster) in response to lowered insulin levels and a short-term reduction in circulating glucose to the brain (2).

At this point, the body hasn’t fully adjusted to fat as its primary fuel source, hence the possibility of all of the nasty symptoms of carbohydrate withdrawal and electrolyte imbalances.

You can read more about what happens during the first 96 hours of reduced carbs here to help you understand the shift your body is going through.

What are the symptoms of keto flu?

We’ve all been hit by the flu and it can leave you feeling pretty beat and rundown. Signs of the keto flu can look pretty similar. So, what does it feel like? To be honest, not that enjoyable!

Some of the most common keto flu symptoms include (2):

  • Headache
  • Nausea
  • Muscle cramps
  • Irritability
  • Dizziness
  • Lethargy and fatigue
  • Brain fog
  • Stomach pain
  • Sleep disturbances
  • Sugar cravings
  • Constipation

The type and duration of symptoms can vary greatly per individual.

How long does keto flu last?

Biochemical individuality will dictate how long the keto flu sticks around. For some people it may be a “one-day-and-done'', while for others it may last for several weeks.

Whether you experience symptoms, and the extent to which you experience them, may be dependent on how metabolically flexible you are; metabolic flexibility refers to your body’s ability to use different substrates for fuel. Your health status and lifestyle prior to starting a ketogenic diet may also be indicative of the extent and duration of symptoms you’ll experience.

Keto flu cure

The reality of the keto flu is that it can make you feel miserable. And for most people, it becomes the deciding factor for whether they’re going to stick to a ketogenic diet or toss in the towel.

When it comes to the ideal remedy, it’s not as simple as just popping a magic pill and everything is gold—although, wouldn’t that be nice. However, what you can do is take preventative measures to reduce the symptoms.

By taking the transition from a higher carbohydrate to a lower carbohydrate diet slowly, you can allow your body time to adjust to this new way of functioning and reduce symptoms that often come with a more dramatic shift in dietary habits. 

How to avoid keto flu?

Completely avoiding the keto flu may be hard to do, but there are a few points to consider that may help reduce your risk and limit the symptoms you may experience. 

1.   Ease into it

The more extreme your transition is into the ketogenic diet, the more likely you are to experience the keto flu side effects. Since carb withdrawal and mineral imbalances are largely behind this ‘sickness’, slowly tapering your carbohydrate intake can help to mitigate some of the keto flu symptoms without sacrificing the many benefits that a ketogenic diet offers.

Starting with a reduced carb diet can help to ease your body into relying on fewer carbs to function. You can try it out by reducing your carbohydrate intake by 10-20g each day and continue eating keto-friendly foods until you reach your desired carb threshold (20-30g).

2.   Stay hydrated with salt + water

Because your body is eliminating water and salt (sodium) as it burns glucose, it can lead to a state of dehydration and electrolyte imbalances. Increasing your sodium and water intake can help you stay hydrated and mitigate the nasty side effects that come with the transition into ketosis (3).

To start, you could try adding ½ teaspoon of salt to 1 L of water in addition to your regular hydration regimen. You could also try consuming a sugar free sports or electrolyte-replacement beverage to support your sodium and water balance. 

Another option is to incorporate bone broth into your diet. Bone broth contains loads of vitamins and minerals, including sodium and potassium, and can help to restore electrolyte levels and mitigate symptoms. You can even add more fat and flavor, by incorporating one tablespoon of butter or tallow/chicken fat and a 1/2 teaspoon of sea salt into your broth.

3.   Bump up your fat intake

If you’ve increased water and salt intake and you are still experiencing symptoms, try increasing your fat intake. Drastically cutting your carbs without increasing your fat intake may cause your body to think it’s starving. Ensure you are hitting your macro goals by consuming loads of healthy fats like avocado, coconut, nuts and seeds, olives, and healthy oils.

It’s important to note that you should increase your fat intake at the beginning of your keto transition period — not just when the keto flu hits.

4. Don’t go overboard with the exercise

You can expect your physical performance to take a bit of a hit during your keto transition period and it may not return to normal for several weeks until your body has adjusted to its new source of fuel. Instead of pushing your body to extremes, take it easy and give yourself time to adjust. To stay active, you could try some gentle stretching yoga, walking, and other mind-body exercises to help ease the transition and support both the mental and physical aspects of this change.

The takeaway

A well-balanced ketogenic diet can provide a host of benefits for those seeking changes.

If you are eager to reap the benefits of the ketogenic diet, remember that while a quick transition to keto can help you achieve results quicker, you may experience unpleasant side effects due to the dramatic change in energy substrates. 

To avoid the keto flu, be sure to take a look at your existing dietary habits and evaluate if a slower transition into a low-carb diet might be best for you. Being smart about how you approach the keto diet can help ease your transition while keeping you on track to achieve your goals!

successful fasting
Your personal guide to a healthy fasting lifestyle
Fasting window