Water weight
Water weight is the short-term change in body mass caused by shifts in body water, glycogen, sodium and gut contents, which can move the scale by one to three kilograms without any change in body fat.
Body fat changes slowly. Body water changes hourly. That mismatch is why a scale can read two kilos heavier the morning after a meal out, and why almost nobody should take a single weigh-in seriously.
What moves it
Carbohydrate. Glycogen is stored with water — roughly three grams of water per gram of glycogen. A weekend of larger carbohydrate intake refills glycogen stores and brings water with it. This is most of the famous first-week "weight loss" on low-carbohydrate diets, in reverse.
Sodium. A saltier day holds more fluid until the kidneys catch up, usually within a day or two.
Gut contents. Food in transit weighs something. A high-fibre day or a missed bowel movement can be worth several hundred grams.
Training and stress. Unfamiliar or heavy training causes short-term fluid retention in the muscles worked. Poor sleep and elevated cortisol do something similar.
The menstrual cycle. Fluid retention commonly rises in the late luteal phase and falls once menstruation begins, often by a kilogram or more.
The practical response
Nothing. Water weight resolves on its own. Weigh under the same conditions each morning, watch the seven-day average rather than any single number, and let the trend answer the question the daily reading cannot.
Sources
- 1.Weight management — NIDDK, National Institutes of Health
- 2.Dietary Reference Values — European Food Safety Authority
Related reading
- Daily weight lies. The trend does not.
Weigh every morning and read only the seven-day average. Why the daily number is mostly noise, and how to build a trend that tells you what to do next.
- Why the scale went up overnight when you did nothing
Sodium, carbohydrate, gut contents, training and the menstrual cycle all move body water by a kilogram or more. What each one does and how long it lasts.
- Water weight: how much it hides and for how long
Body water moves by one to three kilograms on carbohydrate, sodium, training and hormones. What each does, how long it lasts, and why none of it needs fixing.
- What a weight loss plateau usually turns out to be
Four causes account for nearly every stall: the deficit shrank, the log drifted, movement fell, or water is masking real loss. How to tell which is yours.