Distinguish fat loss from scale weight changes.
Scale weight is a mixture, and separating it changes what you should do next. The first kilogram or so of any diet is largely glycogen and the water bound to it — roughly three grams of water per gram of glycogen — which is why week one looks spectacular and week three looks broken. What remains is tissue, and the split between fat and lean mass is determined by three things: rate, protein and training. Losing more than about one percent of body weight a week pushes the body to break down muscle for fuel; taking at least 1.6 g of protein per kilogram and lifting twice a week or more does the opposite, and studies of dieting with resistance training routinely show eighty to ninety percent of the tissue lost coming from fat. Lean loss is not merely cosmetic: it lowers maintenance calories and makes the weight easier to regain.
Splitting scale weight into fat, lean and water
Rate = (loss / weeks) / body weight x 100; fat share starts at 70%, +10 if protein is at least 1.6 g/kg, +8 if training twice a week or more, -20 if the rate exceeds 1% a week; tissue lost = scale loss - water and glycogen; fat = tissue x fat share
An estimate from population-level research, not a body composition measurement or medical advice. Individual results vary widely with genetics, training history, medication and health status. Consult a doctor or registered dietitian before pursuing significant weight or body composition changes.
Mostly glycogen and its bound water, plus reduced food volume in the gut. Each gram of stored glycogen holds around three grams of water, so depleting it drops weight fast — and refilling it puts the number straight back, which is not fat regain.
Yes. Muscle is metabolically active, so losing it lowers your maintenance calories and makes the deficit harder to sustain. It also reduces strength and, over years, function — which is why protein and resistance training matter more than the rate of loss.
It is an evidence-informed estimate, not a measurement. Only DEXA, hydrostatic weighing or a careful multi-site skinfold protocol can tell you your real composition; use this to judge whether your approach is likely protecting muscle.