What Is The Keto Flu—And How Can You Avoid It?

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by K. Aleisha Fetters, C.S.C.S.
Most of us are painfully familiar with the flu—the nausea, headaches, brain fog, and distinct urge to hide under the covers are not anyone’s idea of a good time. And while flu season is thankfully over, the threat of these symptoms still looms for people who are jumping on the keto bandwagon.
The ‘keto flu,’ which has nothing to do with actual influenza, has become a rite of passage for all who take part in the ketogenic diet, which involves slashing carbs and loading up on healthy fats in order to transition the body from burning sugar to burning fat.
Toying with the keto lifestyle? Do not fear the ‘keto flu’! It doesn’t have to be as awful as it sounds.

First, The Basics​

To understand the keto flu, you first have to understand ketosis. Ketosis, the holy grail of a keto diet, is the state in which the body converts both dietary and stored fat into fatty acids and compounds called ketones, which the body can use to produce energy instead of relying on carbs and sugar. Simply put, this is the body’s ‘fat-burning state,’ and it’s obviously an attractive concept for anyone interested in dropping fat, says dietitian Jaime Mass, R.D.
Achieving the fat-burning glory of ketosis is no joke, though. To get there, you have to cut carbs significantly lower than the average low-carb diet (we’re talking just about 20 grams of net carbs per day) and drastically increase fats to upwards of 70 percent of your total calories. Think of it like your backup generator; it won’t switch on unless your primary power source shuts down.

Enter ‘Keto Flu’​

It takes most people at least three weeks of eating a keto diet to actually shift into ketosis. In that time, your blood sugar (glucose), glycogen, and insulin, all plummet, explains Kelly Pritchett, Ph.D., R.D., C.S.S.D., assistant professor of nutrition and exercise science at Central Washington University and spokesperson for the Academy of Nutrition and Dietetics. In order to begrudgingly shift over to burning fat, you have to completely empty your body of all its available sugar sources.
While that happens, most people feel pretty terrible. “In that shift of going from glucose to ketones, there is a period where the body is essentially adjusting to the fuel you are providing—and in a large way,” says Mass. Essentially, your cells are caught in limbo: They’re not getting carbs they’re used to having for optimal function, but they’re not yet efficient at running on fat. As a result, your energy plummets and you may experience fun symptoms like lightheadedness, dizziness, fatigue, and even headaches.
Keto flu symptoms tend to spike once you’ve completely emptied your glucose tank, and subside as your body shifts into full-force ketosis. Even then, though, some may continue to feel lethargic and find they can’t push their bodies to the same intensities they could during their sugar-burning days, says Pritchett. That’s because the process of converting ketones into energy is pretty complicated, time-consuming, and inefficient compared to the process of using glucose—so while endurance athletes often thrive in ketosis, weightlifters and HIIT-lovers may struggle.

Surviving (And Minimizing) ‘Keto Flu’​

Transitioning into ketosis will never be a 100 percent seamless process; you’re pretty much bound to run into some sort of keto flu-like issue along the way. However, there are a few tricks to save yourself some major suffering.
For starters, decrease your carb consumption down to keto-friendly levels gradually instead of going cold turkey. “If you ate a lot of carbohydrates—especially processed and sugar-dense foods—regularly for years, I would suggest first cutting out the highly-processed sweets for a week,” says Mass. “Then, the next week, cut all processed carbs.” By easing your way into the eating style, you limit the severity of any sugar withdrawals.
Related: Want To Try Keto? Here’s What A Healthy Day Of Eating Fat Looks Like
From there, up your fluid intake to make sure that you’re properly hydrated as your body depletes itself of that blood glucose and stored glycogen. “For every gram of glycogen we store, we store three grams of water. So when you start keto and break down that glycogen for energy, you release that stored water,” explains Mass. Translation: The water weight you quickly lose on keto can leave you unknowingly dehydrated if you’re not careful. Research published in the British Journal of Nutrition shows mild dehydration (losing more than one percent of your body weight in water) decreases cognitive function and memory—so that water loss can definitely contribute to the brain fog many experience during the keto flu.
Pritchett also recommends taking it easy on exercise as your body transitions into ketosis those first few weeks; using what little energy you do have on exercise can just exacerbate keto flu symptoms. As your body and energy levels adjust, you can slowly increase your exercise frequency and intensity back up to those of your normal routine.