You cut back on bread, pasta and sugar, and a few days later you feel flat, headachy or foggy. People call this the "keto flu". It isn't an infection, and it isn't a sign the diet is "working" or "failing". It is a short adjustment period that some people notice and others don't. Here is what the research says happens, and when.
Your body keeps a small store of carbohydrate, called glycogen, in your muscles and liver. When you eat far fewer carbs, you start using that store up. Glycogen is held with water, and in the first days you also lose salt (sodium) and potassium in your urine. A 2025 review of the research found that this loss peaks around days 1 to 4 and settles by about day 14.
That is why the scale often drops quickly at the start: it is mostly water, not body fat. It also comes back fast when you eat carbs again. Treat it as a change in fluid, not a result.
Less clear than the internet suggests. The 2025 review that pulled the studies together found that most are small (often 10 to 30 people), rely on self-reporting and have no comparison group. Rates vary widely, and the review notes that no single study has directly linked the fluid and salt losses to the symptoms. The salt-and-water explanation is plausible, not proven.
Brief tiredness or a headache in the first week is common enough to expect, and usually passes. It is not a reason to cut calories further.
Eating enough, drinking to thirst, keeping fibre up and going easy on hard workouts for a couple of weeks are low-risk. Some people salt their food or drink broth, because sodium is being lost. The evidence for electrolyte products, ketone salts and similar is thin and mixed, so we don't recommend any. If you have high blood pressure, kidney disease or take medicines for either, ask your doctor before changing how much salt you eat.
Low-carb does not mean no carbs. EFSA, the European food safety authority, sets a reference range of 45 to 60% of energy from carbohydrate for adults and did not set a minimum intake. A moderate cut is still a low-carb change, and it is up to you how far to go.
Talk to a doctor or dietitian before starting if you have diabetes (especially if you take insulin or an SGLT2 inhibitor tablet, where regulators list restricted food intake and dehydration as risk factors for a serious condition called ketoacidosis), kidney, liver or heart disease or high blood pressure, are pregnant or breastfeeding, take medicine for blood pressure, or have or have had an eating disorder. Keep your intake above about 1200 kcal (women) or 1500 kcal (men) a day unless a professional tells you otherwise.
This guide is general information, not medical advice. Weight is one health goal among several, and a diet that leaves you drained isn't a better one for being strict.