Why does weight loss slow down even when you keep eating the same?

Because the target moves. A calorie deficit is the difference between what you eat and what you burn, and the amount you burn falls as you lose weight — so the same intake produces a progressively smaller deficit.

Several things drive this:

A smaller body costs less to run. Basal metabolic rate scales with body mass. Lose ten kilograms and you need meaningfully fewer calories at rest, simply because there is less tissue to maintain.

Movement costs less. Every step, every stair, every activity burns fewer calories when you are carrying less weight. This is a substantial effect over a day.

Adaptive thermogenesis. Beyond what body size predicts, energy expenditure drops a little further during sustained dieting. Thyroid output shifts, and the body becomes modestly more efficient. The size of this effect is debated, but it is real and it is not imaginary willpower failure.

Non-exercise activity falls unconsciously. People in a deficit fidget less, walk more slowly, and take the lift more often without deciding to. This is often the largest single component, and it is invisible because it is not a choice.

Intake creeps up. Portion estimates drift over months, and small untracked additions accumulate. This is very common and worth checking honestly before concluding metabolism is the cause.

What to do about it: recalculate intake periodically rather than setting it once. Accept a slower rate as you get leaner — that is normal, not failure. Diet breaks at maintenance can help adherence and reduce some adaptation. Resistance training and adequate protein preserve muscle, which protects your metabolic rate.

A plateau lasting two or three weeks is normal fluctuation; one lasting two months is a genuine signal to reassess.