Why the heck does it happen? Normally, the body uses glucose—or sugar—as its main source of energy. When you cut your carbs below 50 grams per day (the typical threshold for a ketogenic diet), you enter a metabolic state called ketosis: Instead of burning carbs for fuel, your body begins burning ketones—an alternative fuel source that the liver makes by breaking down fat. In turn, this can make you feel run down.
That sounds awful. How long does the keto flu last?
Ketosis is pretty different from what your body is used to, and this onslaught of symptoms is its way of dealing with carb withdrawal—and adapting, explains The Ketogenic Diet. “It takes a while, usually a week or two, for the metabolic machinery to shift to fat-burning mode. And you’ll feel pretty crummy while the transition is happening,” she says., RD, author of