The equation and its hidden variables
Energy balance states that weight change follows the difference between energy consumed and energy expended. Over time this thermodynamic relationship holds, but it is not a simple subtraction of two fixed numbers. Both sides shift in response to each other: eating less can lower the energy a body spends, and eating more can raise it.
On the intake side, the calories a body actually absorbs differ from the calories on a label, because fiber, food matrix, and gut processing change availability. On the expenditure side, resting metabolism, the thermic effect of food, exercise, and spontaneous movement (NEAT) all vary day to day.
Why the body fights large deficits
When a deficit is sustained, adaptive responses tend to narrow the gap: appetite hormones rise, spontaneous activity often falls, and resting expenditure can dip somewhat beyond what the loss of tissue alone predicts. This is why progress frequently slows even when intake is unchanged, and why the equation describes a moving target rather than a static budget.
This is general educational information, not medical or dietary advice. Energy needs are individual, and anyone managing weight for a health reason should consult a qualified professional.