A scale reading can move by a noticeable amount between one morning and the next without any corresponding change in body tissue. The measurement is of everything the body contains, and most of that turns over rapidly.
Water accounts for most short-term movement
Water makes up the majority of body mass, and total body water shifts continuously with fluid intake, sweat, urine output and hormonal signalling.
Sodium intake has a direct effect, since the body retains additional water to keep the concentration of sodium in the blood within a narrow range.
A salty meal can therefore produce a measurable rise the following morning that reflects retained fluid and nothing else.
Carbohydrate is stored with water attached
Glycogen is the stored form of carbohydrate held in muscle and liver, and each gram is held alongside several grams of water.
Filling those stores after a period of low intake or hard training adds weight quickly, and depleting them removes it just as quickly.
This is the main reason a low-carbohydrate approach shows a sharp early drop and why the reading rebounds when carbohydrate returns.
Gut contents are weighed along with everything else
Food takes a day or more to pass through the digestive tract, so the mass present at any moment reflects several previous meals rather than the current state.
Fibre intake affects this directly by increasing the water held in the intestinal contents and altering transit time.
Constipation and its resolution can move the reading by an amount comparable to a week of deliberate change in energy intake.
Hormonal cycles and inflammation add further variation
Fluid retention varies across the menstrual cycle in a reasonably predictable pattern, which can obscure any underlying trend within a single month.
Unaccustomed exercise causes muscle tissue to retain fluid as part of the repair process, so a new training programme frequently coincides with a temporary rise.
Illness, poor sleep and travel all affect fluid balance through hormonal routes, which is why readings during these periods are particularly noisy.
Why the trend matters and the reading does not
Because the daily variation is often larger than a week of genuine change, any two readings compared directly carry very little information.
Averaging over a week or more allows the fluctuations to cancel, which is why research studies and clinics work from trends rather than single measurements.
Rapid or unexplained weight change, in either direction, is a reason for clinical assessment rather than an occasion to adjust a routine.