Why the scale lies day to day
Learn · Body composition · Explainer, Typed Supps
Day-to-day weight is mostly water, salt, carbs and what’s still in your gut. Fat moves far too slowly to explain the jumps.
Stage by stage
- The fast movers (Water, first and last): Glycogen carries ~3 g of water per gram and sodium shifts fluid overnight. These move kilograms; fat moves grams.
- Yesterday’s plate (Carbs and salt): A big carb day or a salty meal reads as morning weight gain that has nothing to do with energy balance.
- Cargo (1–3 days in transit): Food has mass and spends days aboard. More volume or more fibre means more of it at weigh-in time.
- The trend (Average, then compare): Daily weigh-ins averaged weekly, compared across 2–4 weeks — the cheapest accurate instrument you own.
What the evidence supports
- Glycogen is stored with roughly 3 g of water per gram — strong evidence. Long-established physiology, and the engine of every “first week” miracle.
- Sodium intake shifts body water and scale weight day to day — strong evidence. Well characterised through controlled sodium loading.
- Daily weight change reflects daily fat change — insufficient evidence. The assumption the misery runs on. The energy arithmetic rules it out.
- Averaged weekly trends track true body-composition change — moderate evidence. Weekly averages track energy balance well in practice; single days do not.
Timing
If you weigh in: same scale, same time, after the loo, before food or water. Consistency beats frequency — and skipping the ritual entirely is a valid choice if it taxes you.
Time to effect
- Overnight: How fast water, salt and cargo move the number.
- 2–4 wks: How long real fat trends take to surface from the noise.
Common myth
“I gained a kilo overnight — the diet isn’t working.” In reality: A kilo of fat needs about 7,700 surplus calories. Overnight jumps are glycogen water, sodium and gut contents, and they unwind on their own. Judge the weekly averages; the daily number is weather.
Sources