The 3,500-calorie rule

The claim, as heard: "Every 3,500 kilocalories you cut is a pound (half a kilogram) of fat — so 500 a day is a pound a week, for as long as you like."

What the data show. The figure comes from the energy content of fat tissue, and as an order of magnitude for a kilogram of body fat — roughly seven thousand kilocalories — it is not wrong. The rule fails in two ways when it is used as a law. First, the tissue lost is not pure fat: some is lean, with far less energy per kilogram, and the share depends on how much fat you have to begin with, so the "energy per kilogram lost" is not one number. Second, and larger over time, expenditure falls as weight falls — a smaller body uses less — so a fixed daily cut becomes a smaller deficit every month. Applied over a year, the rule predicts several times the loss that actually occurs. The mathematical modelling that replaced it treats weight loss as a curve that flattens, not a straight line.

Verdict: as a rule, wrong; as an order of magnitude for the first week or two, fine. The source is a model built on published data, not a trial, and it explicitly gives no single universal number.

The grain of truth. A deficit does remove tissue in proportion to its energy. The proportion just is not constant, and neither is the deficit.

One sentence to remember: the first week's arithmetic is roughly right; after that the line bends, and a target that does not move with your weight stops working.

The pace calculator shows the bend: the same share of body weight per week costs fewer calories as the weight falls.