How much weight do you lose in the first 3 days of a water fast?

Commonly 3 to 6 pounds, but most of it is water, not fat. Stored glycogen binds water, so burning through it in the first 24 to 48 hours causes a fast scale drop. John logged 228.6 to 224.2 lbs, a 4.4 pound drop, in his first 24 hours. That's a typical early fast flush.

Why do you get a headache on day 2 of a water fast?

The usual culprits are sodium and fluid shifts, caffeine withdrawal, and low blood sugar during the metabolic transition. On supervised fasts, plain electrolytes in water prevent most of it. Persistent or severe symptoms are a reason to talk to your doctor, not push through.

Does hunger get worse every day of a fast?

No. Hunger comes in waves rather than climbing forever, and most long term fasters report it fades noticeably around days 3 to 5 as ketosis takes over. Day 2 is often the hardest day for hunger.

What Happens On Days 1 To 3 Of A Water Fast (Hour By Hour)

Q: When does ketosis start when fasting?

For most people the switch into ketosis begins around 12 to 24 hours after the last meal, as liver glycogen runs out, and deepens over days 2 and 3 as ketone production ramps up.

Search “water fast day 2” and you’ll find forum posts from 2014 and theory articles by people who have never skipped lunch. This one is different. As it publishes, John is on Day 2 of a doctor supervised 90 day water fast, and his numbers are on this site in real time. Here’s what the first 72 hours actually look like, the science timeline checked against a real 56 year old body.

Hours 0 to 12. Running on your last meal

Nothing dramatic. Your body digests your last meal and then leans on glycogen, the carbohydrate stored in your liver and muscles. Energy is normal. Hunger shows up on schedule because your body expects food at its usual times. This part is more psychological than physical.

Hours 12 to 24. The glycogen drain and the first big scale drop

Liver glycogen runs low. Here’s the part that surprises everyone. Every gram of glycogen is stored with about 3 grams of water. As glycogen burns off, that water leaves too. That’s why the scale falls fast in the first days of any fast or low carb diet.

John’s real number. 228.6 lbs on Day 1 morning, 224.2 lbs on Day 2 morning. That’s 4.4 pounds in 24 hours. Is that 4.4 pounds of fat? No, and any honest fasting site should tell you that. It’s mostly the glycogen water flush. It still feels great to see, but the honest math matters, because part of it also returns when the fast ends.

Hours 24 to 48, Day 2. The metabolic switch, and usually the hardest day

With glycogen mostly gone, the body flips its fuel source. Fat comes out of storage, the liver starts producing ketones, and you enter early ketosis. While the switch happens, many people feel the transition.

Hunger waves. Day 2 is commonly the hungriest day of a fast. The key word is waves. Hunger rises, peaks, and passes in 15 to 20 minutes. It does not climb forever.
Headaches or lightheadedness. Usually sodium and fluid shifts, plus caffeine withdrawal if you drink coffee. This is exactly what electrolytes in water are for.
Lower energy. The engine is switching fuels mid flight. Some sputter is normal.

John’s Day 2 report? Hunger present but familiar, “a low hum in the background,” no headaches, electrolytes on board. Prior long fast experience helps. His body has made this switch before.

Hours 48 to 72, Day 3. Ketosis deepens and the fog starts lifting

By Day 3, ketone production is ramping toward becoming the brain’s major fuel. Two things typically happen.

Hunger starts fading. Most experienced fasters report a noticeable drop in hunger somewhere around days 3 to 5, as ketones take over. It’s the most counterintuitive fact in fasting. It gets easier, not harder.
Autophagy is well underway. Cellular cleanup, meaning cells recycling damaged components, ramps up from roughly 16 to 24 hours of fasting and builds through 48 hours and beyond.

Day 3 is also roughly where John should feel his switch complete. That article, “When Does Ketosis Start When Fasting?”, publishes tomorrow, on the day he lives it. That’s the whole idea of this site. Every article publishes on the day of the fast it describes.

The first 72 hours checklist for supervised fasters

Water, steady through the day, not forced.
Electrolytes, meaning sodium, potassium, and magnesium, daily from Day 1.
Activity, light only. Walks yes, workouts no.
Warning signs. Severe dizziness, heart palpitations, vomiting, or confusion mean call your doctor. Pushing through is not toughness on a fast. It’s how people get hurt.
And the obvious one. A multi day fast belongs under medical supervision. John’s has monthly bloodwork and a physician with stop authority.

Watch the days happen

Day 1 and Day 2 notes are live, with the morning weights. The bloodwork dashboard tracks the numbers a scale can’t see.

Day 1 was 228.6 lbs. Day 2 was 224.2 lbs, down 4.4. Days 3 through 90 get documented as they happen.