The Arithmetic and Its Limits
A kilogram of body fat stores about 7,700 calories; a pound about 3,500. A deficit of 500 calories a day therefore predicts roughly 0.45 kg a week. This is the standard model, and it is right in direction and wrong in detail.
The error is that it treats maintenance as fixed. A lighter body burns fewer calories, so a constant deficit shrinks as you lose weight. Someone losing 15 kg finds their maintenance has fallen by roughly 200–300 calories a day, which is why the last few kilograms take disproportionately longer than the first.
Calorie Deficit Chart
| Rate of loss | Daily deficit | Weekly deficit | Suits |
|---|---|---|---|
| 0.25 kg a week | 275 | 1,925 | Lean people, muscle preservation |
| 0.5 kg a week | 550 | 3,850 | Most people, sustainable |
| 0.75 kg a week | 825 | 5,775 | Higher starting weight |
| 1 kg a week | 1,100 | 7,700 | Short-term only, with supervision |
A useful guideline is a deficit no larger than 20–25% of maintenance, or roughly 1% of body weight a week. Beyond that, the share of loss coming from muscle rather than fat rises sharply.
Floors That Should Not Be Crossed
Intake below about 1,500 calories for men and 1,200 for women makes it difficult to meet micronutrient needs and increases muscle loss. If the target deficit would push intake below those figures, the answer is to raise activity rather than cut further — or to accept a slower rate.
Adaptive Thermogenesis
Beyond the arithmetic of a smaller body, metabolism adapts: non-exercise movement falls, hormonal changes reduce energy expenditure slightly, and the body becomes marginally more efficient. The effect is real but frequently exaggerated — typically 5–15% below what body size alone predicts, not the "starvation mode" that stops weight loss entirely.
What usually stalls weight loss is not metabolic adaptation but unrecorded intake. Studies comparing self-reported food records with doubly labelled water consistently find under-reporting of 20–40%.
Protein and Training Change What You Lose
In a deficit without resistance training, roughly a quarter of the weight lost is lean tissue. With adequate protein — 1.8–2.4 g per kg — and resistance training two to four times a week, that share falls to close to nothing. The scale moves more slowly and the outcome is far better.
Frequently Asked Questions
Why has my weight stopped moving?
Usually intake creep or a smaller maintenance figure than assumed. Water retention also masks fat loss for weeks at a time — particularly around menstrual cycles, high-sodium meals and new training. Judge progress on a four-week average, not day to day.
Should I take diet breaks?
Periods at maintenance every 8–12 weeks help adherence and may reduce adaptation slightly. The main benefit is psychological, and that is enough of a reason.
Is a very low calorie diet faster?
Faster on the scale, worse in composition, and much harder to sustain. Very low calorie diets belong under medical supervision, not as a self-directed strategy.