Short version: keto flu — the headache, low energy, and foggy-headed feeling some people get in the first week of cutting carbs — usually peaks around day three and eases within the first week for most people. It's uncomfortable, not dangerous, for most people who get it, and it responds mostly to salt, water, and eating enough fat. If it's severe or it's still going after a week, that's the point to call a doctor instead of pushing through.
Days 1–2: the calm before it. Nothing dramatic yet — your body is still running mostly on the carbohydrate stores it already had. This is also when you start shedding water fast, because low-carb eating flushes stored carbohydrate (glycogen) and the water bound to it. That quick early drop on the scale is water, not fat yet, and sodium goes with it — which is exactly why electrolyte advice always shows up before symptoms do, not after.
Day 3 (the peak for most people): this is "keto flu." The headache, flat energy, and foggy head that people mean when they say "keto flu" tend to cluster right around here. One of the more detailed sources on this — a 2020 Frontiers in Nutrition review that coded 300 first-hand reports across 54 symptoms — found flu-like symptoms were the most commonly mentioned (44.6% of reports), followed by headache (24.8%), fatigue (17.8%), nausea (15.8%), dizziness (14.9%), and brain fog (10.9%). Worth noting: that study coded what people who posted about it said, not a guaranteed rate for everyone who tries keto — so it tells you what symptoms look like if they show up, not that they will. Salt and water are the actual remedy here, not toughness; a lot of people add magnesium around this point too.
Days 4–5: the switch. Your body starts covering more of its energy needs by burning fat instead of carbohydrate. For a lot of people this is where the rough patch starts to fade — again, a range, not a date.
Days 6–7: past the worst of it. By the end of the first week, most of the symptoms that show up at all have usually passed. This is also, unhelpfully, one of the more common points where people quit — not because it stopped working, but because the first few days were the hardest and it's easy to conclude that's how it's always going to feel.
Salt and water, consistently, from day one — not just once symptoms show up. Eating enough fat matters too; under-eating tends to make everything on this list worse. None of this is a substitute for medical advice: if symptoms are severe, or they're still present past the first week, that's a "call your doctor" situation, not a "push through it" one.
The app walks through what's typically happening each day of week one, with the electrolyte guidance adjusted for anyone flagged for a blood pressure or heart condition at signup — so you're not guessing whether what you're feeling is normal.
Try Keto Coach — free, no account Get it on Google PlaySources: The Ketogenic Diet, StatPearls (NCBI Bookshelf); The role of β-hydroxybutyrate testing in ketogenic metabolic therapies, Frontiers in Nutrition, 2025; symptom-frequency figures from a 2020 Frontiers in Nutrition review of first-hand keto-adaptation reports (300 reports, 54 symptoms coded) — describes symptom patterns among people who reported them, not an incidence rate for everyone starting keto.
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