If You Feel Sick After Going Keto, You’re Not Alone—Here’s How Long the Symptoms Last
According to new research, published in the journal Frontiers in Nutrition, those who start the keto diet can experience symptoms similar to the flu, such as brain fog, headache, decreased energy, gastrointestinal discomfort, feeling faint, and even perceived changes in heartbeat.
These symptoms should go away after the first few weeks, as your body adjusts to the diet.
With flu season in full swing and COVID-19 currently dominating the news, you’re hearing a lot about symptoms such as headaches, muscle fatigue, nausea, mental fog, and dizziness. But if you’ve made any changes to your diet recently—going keto, specifically—you can breathe a sigh of relief.
Why? New research, published in the journal Frontiers in Nutrition, points out that many people experience symptoms similar to the flu when they first begin to go keto—an eating style defined by very low amounts of carbohydrates and very high amounts of fat—and the adjustment period is often called the “keto flu.”
A quick refresher on how the keto diet works: Carbs make up five to 10 percent of your caloric intake, fat makes up 75 to 80 percent, and protein makes up the rest. This ratio puts your body into a state of ketosis, which when your body uses stored fat (ketones) for energy rather than the glucose that comes from carbs.
To determine how widespread the effect of the keto flu might be, researchers recorded the experiences of 101 people who had started a keto diet and described their symptoms across 43 online forums. The researchers found that when people experienced these flu-like effects, they tended to peak within the first week of a new keto diet and would eventually resolve after about a month.
In addition to experiencing symptoms like brain fog and headache, other common reactions include decreased energy, gastrointestinal discomfort, feeling faint, and even perceived changes in heartbeat. (And while your performance—specifically high-intensity efforts—might take a dip during the first week or two because of this, you should be able to get back on schedule after your body gets used to the diet.)
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So why does this happen? Lead study author Emmanuelle Bostock, Ph.D., a researcher at the Menzies Institute for Medical Research in Australia, told Bicycling that they are still unsure of the pathophysiological mechanisms here.
“However, this may possibly be associated with changes in the gut flora, as well an immune response triggered by the diet, and also by the caloric deficit—which is often present at the start of a diet,” she said.
Although further research is necessary to clarify the potential reasons behind the keto flu, it may be helpful in the meantime simply to be aware that this might happen in the first few days or weeks of starting the diet—or, as Bostock noted, any new diet since a change in caloric intake could cause your immune to react as a counterbalance.
But even if you are just starting on the keto path, symptoms like fever, cough, and shortness of breath are not related to dietary changes and may be the actual flu or potentially COVID-19. If you’re experiencing any of these symptoms, check in with healthcare provider.
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