Let's Talk About the Keto Flu

If you're trying the keto diet for the first time, you might be surprised by some of the initial side effects. We asked the experts what to watch out for.

by Tula Karras Health Writer

Any time you change up the way you eat, your body notices. Sometimes, that’s a good thing (more energy!). Sometimes, not so good (another trip to the bathroom?). With the keto diet, many people experience what’s known as the “keto flu” when they begin following the plan. While symptoms usually subside after a week or two, they can be disruptive in the early stages of the diet. Let’s take a closer look.

Keto Flu

Our Pro Panel

We went to some of the nation's top experts in the keto diet to bring you the most up-to-date information possible.

William S. Yancy, Jr., M.D.

William S. Yancy, Jr., M.D.

Director; Associate Professor of Medicine

Duke University Diet and Fitness Center; Duke University School of Medicine

Durham, NC

Elisabetta Politi, R.D.

Elisabetta Politi, R.D.

Nutrition Director

Duke University Diet and Fitness Center

Durham, NC

Lisa Sasson, R.D.

Lisa Sasson, R.D.

Associate Dean for Global Affairs and Experiential Learning; Clinical Professor of Nutrition and Food Studies

New York University

New York, NY

Keto Flu
Frequently Asked Questions
What’s the most common symptom of the keto diet?

Constipation! It can be hard to keep things moving with such limited amounts of carbs. Adding keto-friendly fibrous produce to your meals, like cauliflower, chard, and berries, can help.

How serious is the keto flu?

As far as medical risks go, not very. In fact, it’s not really a flu at all: You won’t have a fever and it’s not caused by an infection. But you may feel sluggish and have GI issues.

Does the keto flu cause insomnia?

As first, it might. Your body uses carbs to produce a hormone known as melatonin, which sends “feeling sleepy” signals to your brain. Without enough carbs to kickstart melatonin production, you may have trouble falling asleep.

Why do I feel tired on keto?

A diet low in carbs makes it hard for your body to hold onto water, and therefore electrolytes. That can result in dehydration (which makes you tired) or an electrolyte imbalance, which can affect blood pressure and muscle strength.

Recap: What’s the Keto Diet?

A diet that burns fat while letting you eat fat? Sounds too good to be true. But that’s essentially the premise of the keto diet. OK, it’s a bit more detailed than that: Keto is short for “ketogenic,” which describes a very-low-carb/high-fat diet that shifts a person’s body into a state of “ketosis,” or ketone-burning. Ketones are chemicals produced by your liver after breaking down fat.

Of all the foods your body can use for energy, carbohydrates are the easiest for you to break down; your body turns them into glucose, or blood sugar. But if you cut your carb intake to very low levels, your body is forced to look to fat for fuel. Fat doesn’t burn as easily and requires your body to expend slightly more energy converting it into ketones first, which is one reason why some experts say you lose weight on this diet. Other potential reasons for weight loss: Ketones may suppress your appetite; in addition, the keto diet can lower insulin levels, which helps with weight loss since insulin tells the body to hold onto fat rather than burn it.

The actual breakdown of the keto diet is as follows:

  • 70% to 75% fat

  • 20% protein

  • 5% to 10% carbs (measured in grams, no more than 50 grams/day; some people may need to drop to 20 grams to achieve ketosis)

Like many restrictive diets, the keto plan can have side effects, although most are manageable and disappear on their own within the first week or two of following the diet. The most commonly reported side effects are a cluster of symptoms known as the “keto flu.”

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What Is the Keto Flu?

Within several days of embarking on the keto plan, many dieters report a number of mild side effects, including:

  • fatigue

  • headache

  • foggy thinking

  • moodiness

  • sleep problems

  • diarrhea

Together, these symptoms are frequently referred to as the “keto flu.” Dieters may also experience constipation due to the lack of fiber.

What Causes These Symptoms?

Despite its name, this “flu” doesn’t involve fever or infection—it feels more like a hangover after a night of one too many. No one is certain what causes it, though a loss of fluids and electrolytes could be at play: Because carbs help your body hold onto water—along with electrolytes like sodium, potassium, and magnesium—when you ditch the bread and pasta, you may also lose H20 and electrolytes and feel dehydrated.

Keto eating plans also lower your body’s insulin levels, and insulin is what tells your kidneys to hold onto water and sodium. This is helpful in one way (it can lower your blood pressure), but may also lead to fatigue, lightheadedness, and headaches.

In addition, the diet discourages some of the foods that typically deliver the electrolyte potassium to your system, such as fruit and milk. Potassium plays an essential role in helping your body regulate blood pressure, as well as sending nerve impulses that tells muscles, including your heart, to contract. A sudden drop in potassium or other electrolytes in your diet, therefore, could trigger symptoms like fatigue and headaches.

Some researchers also speculate that keto flu symptoms may also have to do with a change in the gut’s microbiome—the billions of microbes and bacteria that hang out in your GI tract—which is altered as a result of the high-fat, low-carb diet; researchers are still figuring out exactly how those shifts affect your health.

The symptoms usually resolve within a week or two, and can be minimized by adequate fluid intake and adding electrolytes such as sodium to your diet (for example, in the form of broth). Some people report feeling more energized and clear-headed after the flu phase passes. Let’s take a closer look at some of the individual symptoms you might initially encounter.

Keto and Stomach Distress

GI issues might be the #1 complaint of people on the keto diet, with constipation leading the way, thanks to a lack of fiber-rich whole grains and fruit on the plan, which help keep traffic moving. To help, experts say to drink a lot of water and include plenty of fibrous, keto-friendly vegetables—brussels sprouts, cauliflower, chard—as well as avocados and nuts. And berries, which are allowed on the diet in moderation, are a great source of fiber.

Alternately, you might run into a bout of diarrhea, especially if you’re replacing sugary drinks and sugar-filled products with artificial sweeteners. You can ease some of this stomach distress by choosing natural sweeteners like stevia.

Also, fat itself might be responsible for a case of the runs, since your body is most likely not use to having to process such copious amounts of it. Because you can lose a lot of water with diarrhea, be sure to gulp more H2

  • You can also try adding sugar-free psyllium fiber supplements, which can bulk up loose stool.

  • Keto and Brain Fog

    Dazed and confused? The mental fatigue some people feel at first on the keto diet is probably due to the lack of glucose, which your body converts from carbs. The brain loves glucose (it takes up a substantial amount of the available glucose energy in your body), using it to power everything from cognitive reasoning to commandeering your muscles and organs. During the shift from carb-burning to fat-burning, your noggin has to adapt, which may slow the wheels a bit.

    Once your ketones build up, however, your brain starts gobbling them up and fogginess should clear. Some research—albeit in lab animals—finds that a keto diet improves thinking and spatial skills (a.k.a. animals were better able to find their way through a maze).

    Another possible source of a hazy mind: Low electrolyte levels, leading to an electrolyte imbalance (see above). If you’re on the keto plan, follow experts’ advice and make sure you are getting enough of the electrolyte sodium, since a deficit might make you feel foggy. Yes, high amounts of sodium—a.k.a. salt—can lead to various health issues, so no need to overdo it, either. Aim to keep your intake at the FDA guidelines of 2,300 mg per day.

    Keto and Fatigue

    The shift into ketosis can cause physical fatigue for the same reasons your brain is taxed: Your body is transitioning out of glucose-burning and into ketone-burning and that takes a toll. But the body is a master of adaptation, and after several weeks, it usually learns to torch fatty acids as proficiently as glucose.

    Your muscles also have to adapt, and you may notice that weight-lifting or other strength workouts feel tougher. That’s because your muscles may have less glycogen—stored glucose— and aren’t as quick to respond.

    If you do a lot of strength workouts or HIIT, you might consider cycling in some non-keto days into your weekly eating plan to deliver enough carbs for glycogen, which your muscles call on to perform quick movements, like sprinting or lifting. Also, be sure to include potassium-containing foods—like avocados and leafy greens—and magnesium-rich fare such as nuts, avocados, and spinach, since your muscles rely on both nutrients for max performance.

    Keto and Mood Swings

    The food-mood connection is real: We’ve all been guilty of hangry behavior when we’ve skipped a meal. Similarly, with the keto diet, a lack of carbs can initially result in irritability. It’s not a true hunger (in fact, many people report feeling less hungry on keto), but your brain has been cut off from its usual source of get-up-and-go fuel, and during the several days you’re figuring out how to use ketones for energy, things can feel off not just physically but emotionally, too.

    Not a lot is known about how the keto diet affects depression. But because the diet can alter the brain’s chemistry, if you suffer from a mood disorder, consult your doctor to make sure it’s safe for your condition and won’t interfere with any medications you’re on.

    Keto and Dizziness, Nausea, and Headaches

    A sudden drop in blood sugar—the result of extreme carb-cutting—can bring on a feeling of wooziness, and even nausea and headaches. Like with other keto-flu symptoms, dehydration and electrolyte imbalance can add to the equation.

    Staying hydrated and not exerting yourself too much during the first few days can help; now is not the time to start training for your first marathon. Usually, lightheadedness, queasiness, and headaches resolve once your body gets the hang of ketone combustion.

    Keto and Sleep Issues

    For more than a few people, a drastic change in diet and the symptoms that go along with it can disrupt sleep, making it more challenging to nod off to dreamland and stay there.

    Some experts think that the lack of carbs might trigger sleeplessness, at least in the beginning, because carbs increase your body’s levels of L-tryptophan—an essential amino acid which gets converted to the neurotransmitter serotonin and then melatonin—a hormone that helps you us feel drowsy.

    But there are also those who maintain that the keto diet can be dreamy and improve sleep! That’s because the diet is associated with an increase in adenosine, a brain chemical that builds up through the day and promotes relaxation and drowsiness. In a nutshell: Any dietary change has the potential to affect your shut-eye—initially and over time—and each person will respond differently.

    Dealing With the Keto Flu

    Just because it’s not a “real” flu, doesn’t mean life will be any easier when you’re going through these symptoms in the early days of the keto diet. Starting any food-restricted plan is tough (if weight-loss was easy, everyone would be slim, right?). Having a clear understanding of why you’re starting this journey in the first place can help you stick with it on especially trying days.

    Before you begin the keto diet, write down your top three or four reasons for giving it a go. Maybe your doctor told you that your BMI put you in the obese category and you’re at serious risk for health complications. Maybe you’ve been diagnosed with type 2 diabetes and are looking for a natural way to help improve your insulin sensitivity or management. Maybe you’re just curious what the hype is about.

    Whatever the reason, make your list and refer to it when keto flu symptoms crop up. Remember, they likely won’t last forever (a few days or a week, more likely). And of course, if you experience any symptoms that feel extreme or unusual, don’t hesitate to call your doctor.

    Tula Karras
    Meet Our Writer
    Tula Karras

    Tula Karras is an award-winning journalist, editor, and content creator who specializes in health, nutrition, and profile writing. She has written for numerous print and online publications, including Self, Parents, Real Simple, National Geographic, Prevention, Cosmo, Consumer Reports, Health, Good Housekeeping and Yoga Journal. She has ghost-written two books and is based in Austin, Texas.