The “keto diet” (or, the “ketogenic diet” in full) was first introduced in the 1920s as a treatment for epilepsy. Now, dieters have re-discovered this fat-focused diet. It’s based on the idea that by limiting your carb consumption, your body will transition into a state called ketosis, which is where healthy fats are used as your primary fuel source. But this can lead to some unpleasant side effects that the keto community has named the “keto flu.”
What is the Keto Diet?
On this diet, you’ll get about 75 percent of your total calories from fat, 20 percent from protein, and five percent from carbs. On a 2,000 calories per day, that’s only 25 grams of carbs, or about the amount of carbs in one medium apple.
“Your body’s favorite energy source is carbs,” says Denis Faye, M.S. and Openfit’s executive director of nutrition. “The keto diet tries to put you in a state of ketosis, where you’re not getting enough carbs to be used as energy.” So instead, your body kicks into a different energy system where it burns fat for fuel.
But the transition is stressful, so our bodies can experience what has come to be called the “keto flu”—a condition that, you might guess, resembles the flu.
What Is the Keto Flu?
Dieters who transition to ketosis can become “ill” with the keto flu. This condition is like the flu, but it isn’t from a virus. It occurs as your body works to correct to the new balance of macronutrients and increased fat intake.
“Many of my clients feel like they actually have the flu, but it’s just the body adjusting to its new fuel source.” says Alicia Galvin, R.D. “So, there is no risk of ‘catching’ the keto flu. It is not contagious.”
What Are the Keto Flu Symptoms?
Common keto flu symptoms include constipation, headache, halitosis (bad breath), muscle cramps, diarrhea, rash, and general weakness.
“When people are eliminating the primary and preferred fuel source, headaches, muscle cramps, weakness, poor concentration, and the other symptoms of the keto flu occur because the body is not getting what it needs,” says Shelley Rael, M.S., RDN. “In my interpretation, it’s the body screaming and rebelling against the fact that it is not getting what it needs.”
When Does the Keto Flu Start?
The keto flu begins when your body starts dealing with the effects of the lack of carbs it’s receiving. This usually starts between one day and one week after you start the keto diet—but it depends on what your diet was before the keto diet.
How Long Does the Keto Flu Last?
Every body is different, and so is the amounts of time that it can take for the keto flu to run its course. It generally takes between a few days and one week before the body can enter ketosis—and the keto flu can be experienced for some or all of that time, Rael explains.
How Do You Avoid the Keto Flu?
The best advice for avoiding the keto flu and alleviating the symptoms is the advice for dealing many other illnesses: drink more water.
“I recommend my clients drink at least 75 percent of their body weight—in ounces—during this time to keep well hydrated,” Galvin says.
However, the easiest solution is to simply incorporate more carbs into your diet. “[The keto flu] would go away faster is someone would have some carbs, like having an apple, some berries or any fruit or vegetable,” Rael adds.
In short, if you want to avoid the keto flu, avoid doing the keto diet from the start.