Plenty of people start keto, feel grim by day three, and conclude the diet does not agree with them. Most of the time what they are feeling is the so-called keto flu, a short, avoidable patch of feeling off that has very little to do with the diet failing and almost everything to do with salt and water.
What it actually is
When you cut carbohydrate hard, your body stops holding on to as much water, and a lot of sodium leaves with it. That flush is why people drop several pounds in the first few days, almost all of it water. The downside is that losing sodium and other minerals quickly can leave you headachy, tired, foggy, irritable and a bit crampy. It feels like the onset of a cold without the cold. It is not your body rejecting fat; it is a salt and electrolyte dip while you adapt.
The symptoms to expect
Headache and fatigue are the common ones. Some people get muscle cramps, especially in the legs at night, lightheadedness when they stand up, poor sleep, or a short temper. It usually arrives a day or two in and, handled properly, fades within a few days to a week.
How to head it off
The fix is mostly minerals and fluid. Add salt deliberately rather than avoiding it: salt your food well, and a cup of warm broth once or twice a day during the first week makes a real difference. Keep drinking water, but do not drown yourself, since flushing more water without replacing salt makes things worse. Top up potassium and magnesium through food where you can, and many people take a magnesium supplement in the evening, which also helps the cramps and sleep. There is more detail in the electrolytes on keto piece.
Easing in
You do not have to slam carbs to zero on day one. Cutting them down over a week, rather than overnight, gives your body time to adjust and softens the dip. Eating enough is just as important: the early days are not the moment for a steep calorie cut on top of everything else, so eat to satisfaction while your body learns to run on fat.
When it is not the keto flu
If you feel genuinely unwell rather than just flat, or the symptoms drag on well past a week despite salt and fluids, that is worth taking seriously rather than pushing through. The keto flu is mild and short. Anything beyond that deserves a proper look.
This is general information about the ketogenic diet, not medical advice. Keto does not suit everyone; if you are pregnant, on medication, or managing a condition such as diabetes, speak to a doctor or dietitian first.