Keto flu is a collection of flu-like symptoms that typically appear two to seven days after starting a ketogenic diet. It’s not an actual infection. The most common complaints include headache, fatigue, brain fog, dizziness, nausea, constipation, and muscle cramps. Most people feel noticeably better within a week, though symptom severity varies widely from person to person.
The Most Common Symptoms
The symptoms that show up most frequently during the first week or so of a ketogenic diet fall into a few broad categories: energy-related, digestive, and cognitive. Gastrointestinal issues, particularly constipation, are the single most commonly reported symptom in both children and adults starting a ketogenic diet. Beyond constipation, diarrhea and nausea also occur. In adults, nausea affects roughly 8 to 16% of people, while vomiting is rare at around 1%.
Fatigue and general weakness are extremely common, often described as lethargy or feeling drained. Headache affects 8 to 25% of adults in the early days. Dizziness and lightheadedness show up in about 15 to 21% of adults. Brain fog, that hard-to-describe sense of mental sluggishness, has been reported by about 10% of healthy adults starting the diet. Other symptoms include mood changes, decreased exercise capacity, muscle cramps, and bad breath (a fruity or metallic taste that comes from your body producing ketones).
Despite the name “keto flu,” actual flu-like symptoms are uncommon. Fever, chills, coughing, runny nose, and sore throat are reported by fewer than 6% of people. If you develop a real fever or respiratory symptoms, that’s more likely an actual illness than a reaction to your diet change.
Why It Happens
Keto flu is driven primarily by a sharp drop in insulin and the water loss that follows. When you dramatically cut carbohydrates, your body’s 24-hour insulin output drops by more than 50%. Insulin doesn’t just manage blood sugar. It also tells your kidneys to hold onto sodium. When insulin falls, your kidneys start flushing sodium out at a much higher rate, a process called natriuresis. Potassium follows sodium out, and water follows both of them.
This is why people lose several pounds in the first few days of keto. That initial weight loss is almost entirely water. The rapid fluid and electrolyte shift is what causes many of the classic symptoms: headaches from dehydration, dizziness from low sodium, muscle cramps from lost potassium and magnesium, and fatigue from your body scrambling to adapt to burning fat instead of glucose for fuel. Your brain, which normally runs almost exclusively on glucose, needs time to become efficient at using ketones as an alternative energy source, which explains the brain fog.
How Long It Lasts
Symptoms generally start within two to seven days of cutting carbs significantly. For most people, the worst of it passes within the first week. Energy levels typically return to normal, and many people report feeling better than baseline once their body has adapted to burning fat. The full adaptation period can stretch longer for some, particularly for exercise performance, which may take a few weeks to fully recover. But the acute “flu” feeling is usually short-lived.
How to Reduce Symptoms
The single most effective strategy is staying on top of your electrolytes. Because your kidneys are dumping sodium at a much higher rate than usual, you need significantly more than a typical diet provides. Recommendations for a well-formulated ketogenic diet are 3,000 to 5,000 mg of sodium per day, 3,000 to 4,000 mg of potassium, and 300 to 500 mg of magnesium. For context, most people on a standard diet consume around 3,400 mg of sodium, so you may need to actively add salt to food, drink broth, or use electrolyte supplements to hit the higher end of that range.
Potassium-rich foods that fit a keto diet include avocados, spinach, and mushrooms. Magnesium is found in nuts, seeds, and dark leafy greens. If food sources aren’t enough, a magnesium supplement (look for magnesium glycinate or citrate, which are easier on the stomach) can help with cramps and sleep.
Drinking plenty of water matters, but water alone can actually worsen the problem if it further dilutes your electrolytes. Salted water or bone broth is a better choice than plain water during the first week. Making sure you’re eating enough total calories and getting adequate fat is also important. Some people inadvertently undereat when they first cut carbs, which compounds the fatigue.
Does Easing Into Keto Help?
One clinical trial tested whether gradually reducing carbs, rather than cutting them sharply, would prevent symptoms. Researchers randomized 77 adults into three groups eating 5%, 15%, or 25% of their calories from carbohydrates for three weeks. The group eating the fewest carbs did report slightly more symptoms overall, but the differences between groups were not statistically significant for most symptoms. Only bad breath and muscle weakness were meaningfully worse in the strictest group. So a gradual taper may take a small edge off, but it’s not a guaranteed way to avoid keto flu entirely.
When Symptoms Are Not Keto Flu
Keto flu symptoms overlap with genuine dehydration, electrolyte deficiency, and even low blood sugar (hypoglycemia), all of which can become serious if ignored. Persistent vomiting, heart palpitations, confusion, or fainting go beyond normal adaptation. These warrant medical attention, not just more salt. Kidney stones and skin rashes have also been reported in people on ketogenic diets, though these are less common and tend to develop later rather than in the first week. If your symptoms are getting worse after the first week instead of better, something beyond normal adaptation may be going on.