Body weight measured on consecutive days can differ by a kilogram or more in either direction. Almost none of that movement is tissue gained or lost.
Water is the largest variable component
Water makes up the majority of body mass, and the amount held shifts with sodium intake, hormonal state, heat, altitude and hydration.
Sodium is retained with water to keep blood concentration stable, so a salty meal is followed by fluid retention lasting a day or two.
A single litre of water weighs a kilogram, which is the scale of change achievable without anything happening to body tissue at all.
Glycogen carries water with it
Carbohydrate is stored in muscle and liver as glycogen, and each gram is stored alongside several grams of water.
Depleting glycogen through low carbohydrate intake or hard exercise releases that associated water, producing a rapid drop.
Refilling the stores reverses it just as quickly, which explains the sharp early change when a diet reduces carbohydrate and the equally sharp return afterwards.
The digestive tract holds mass in transit
Food and drink consumed but not yet absorbed or eliminated still register on a scale.
Transit takes roughly a day or more, so the tract always contains material whose quantity varies with recent intake and fibre content.
None of this is body tissue, but it is indistinguishable from tissue to a device measuring total mass.
Hormonal cycles add a repeating pattern
In people who menstruate, fluid retention varies across the cycle, producing a fluctuation that repeats monthly.
Cortisol affects fluid balance as well, so periods of significant stress or poor sleep can shift weight without any change in intake.
Because these follow a rhythm, comparing measurements taken at similar points is more informative than comparing adjacent days.
The trend carries the signal
Fat tissue changes slowly, since it requires a sustained energy imbalance over an extended period, while water moves within hours.
Any single measurement is therefore mostly noise, and the underlying change becomes visible only across weeks of readings taken under consistent conditions.
Consistency is what makes the comparison meaningful: same time of day, same point in the routine, same clothing. Varying those adds movement that has nothing to do with the body.
This is why a rise after a single day is not information, and why people who weigh frequently are generally advised to look at an average rather than at the latest figure.
Rapid or unexplained weight change, particularly with swelling or breathlessness, is a medical matter rather than a measurement artefact and should be assessed promptly.