Weight change slows over time on an unchanged routine, a pattern often described as a plateau. The underlying arithmetic is that energy expenditure is itself a function of body size, so the target moves as the body changes.
Maintenance costs scale with tissue
Resting energy expenditure accounts for the largest share of daily energy use and reflects the ongoing cost of maintaining organs, ion gradients and protein turnover.
Losing weight means losing both fat mass and some lean tissue, and each contributes to that resting cost, though organs and muscle contribute far more per unit than fat does.
A smaller body therefore requires less energy at rest, and that reduction happens automatically without any change in behavior.
Movement becomes cheaper
The energy required to walk a given distance is roughly proportional to the mass being moved, so the same walk costs less as body weight falls.
Activities that involve lifting the body against gravity, including stairs and hills, show this most clearly.
Exercise performed at the same duration and perceived effort therefore accounts for fewer calories than it did earlier in the process.
Digestion accounts for a proportional share
The thermic effect of food, the energy spent digesting and processing what is eaten, is roughly proportional to intake.
When intake is reduced, this component falls with it, removing part of daily expenditure in step with the reduction.
It is a modest share of the total, but it moves in the same direction as the other components rather than offsetting them.
Adaptive changes add to the mechanical ones
Measured expenditure after weight loss is often lower than predicted from the new body size, a difference researchers describe as adaptive thermogenesis.
Proposed contributors include changes in thyroid and leptin signaling and improved efficiency of muscular work, though the magnitude and persistence are actively debated.
Unconscious movement, including fidgeting and general activity, also tends to decline, and this component varies widely between people.
What the mechanism implies about measurement
Because expenditure is a moving quantity, any fixed estimate produced at the start becomes progressively less accurate over months.
Device and equipment estimates of calories burned are population approximations and inherit the same problem, which is why they diverge from observed change over time.
Weight management involving medications, existing metabolic conditions, or a history of disordered eating should be handled with a physician or registered dietitian rather than by recalculating targets alone.