Why the scale jumps 2 kg overnight
"Yesterday 84.1, today 85.6. How?" Not through fat, and the arithmetic shows it.
Fat does not move that fast
A kilogram of body fat holds on the order of seven thousand kilocalories — the exact figure depends on how much of the tissue is fat and how much is water and protein, which is why the classic "3,500 per pound" rule is only an order of magnitude and not a law. To gain a kilogram and a half of fat in a day you would have to eat roughly ten thousand kilocalories above what you use. A big dinner does not come close. Whatever the scale added overnight, almost none of it is fat — and the same is true of what it took away.
What it is instead
- Water bound to salt. A salty meal holds water for a day or two; the scale returns when the salt is excreted.
- Water bound to glycogen. Carbohydrate is stored in muscle and liver as glycogen, and glycogen is stored with water — about three grams of water per gram of glycogen in the measurement studies, which is why a high-carbohydrate day after a low one can add a kilogram that has nothing to do with fat, and why the first week of any low-carbohydrate diet "loses" so much.
- Gut contents. A large or fibrous meal is simply still in you.
- The menstrual cycle, which shifts water by a kilogram or more for many women.
- Training. A hard session causes muscle inflammation and water retention for a day or two — the scale goes up after the workout that was supposed to bring it down.
- Some medications.
None of these are fat and none of them last.
How to read a scale
Weigh at the same time, in the same state — typically morning, after the bathroom, before breakfast — and look at the trend over a week or two, not at any single reading. A weekly average removes most of the noise; a smoothed trend removes more. The app keeps a trend weight for exactly this reason and plans from it, not from this morning's number. If a single reading can change your mood, stop reading single readings.
When a jump is a signal
When it is not a jump but a slope: a trend that has moved up over two or three weeks, with no obvious water reason. That is worth acting on — the plateau page has the checklist. One morning is never worth acting on.
In the app
The progress page shows the trend, not only the points, so a salty dinner cannot look like a failed week. The daily target is recomputed from the trend weight for the same reason.