Weight Loss Explained: Calorie Deficit, Rate of Loss, and Plateaus

Weight loss is simple in principle and difficult in practice, and the difficulty is not arithmetic. A sustained calorie deficit loses weight; the deficit must be large enough to matter and small enough to sustain; and the body’s expenditure falls as weight falls, so the deficit that worked in month one stops working in month four. This guide explains the deficit, the safe pace, the reasons the scale stalls, and the adjustments that restart loss without resorting to a crash diet. It is for adults. Children’s weight is a matter for their clinician, and nothing here applies to them.

The deficit: how loss is produced

Body weight is stored energy. To lose it, energy intake must fall below energy expenditure, and the shortfall is met from stored fat and, to a lesser extent, stored muscle. A kilogram of body fat represents roughly 7,700 kcal, so a daily deficit of 500 kcal, sustained for a week, is 3,500 kcal — about 0.45 kg in theory. In practice the loss is slower, because expenditure falls as the body gets smaller and lighter, and because the body defends its weight by increasing hunger and reducing spontaneous activity.

Finding the starting point means finding maintenance: the intake at which weight is stable. A calorie calculator estimates it from weight, height, age, sex, and activity, and two weeks of tracking intake and weight refines it. Subtract 300 to 500 kcal from maintenance, and that is the deficit target. Below that range, loss is too slow to notice and motivation fails. Above it, hunger, fatigue, and muscle loss rise, and adherence fails. The sustainable deficit is the one that can be held for months, not the one that looks best on paper for a week.

Protein and resistance training protect muscle during loss. An intake of about 1.6 to 2.2 grams of protein per kilogram of body weight, combined with two to four resistance sessions a week, preserves substantially more muscle than dieting alone, so a larger share of the loss is fat. Without either, a quarter or more of the weight lost can be lean tissue, which lowers expenditure further and makes the loss harder to maintain.

Try the method on your own figures with the Weight Loss Calculator.

The safe pace, and why faster is not better

A pace of 0.5 to 1% of body weight a week is the usual safe range for adults — 0.4 to 0.8 kg a week for an 80 kg person, less for a lighter one. At that pace, hunger is manageable, muscle is largely preserved with protein and training, and the habits that produce the loss are habits that can be kept. Faster loss is sometimes appropriate under medical supervision, for example before surgery, but it is not a do-it-yourself project: very low-calorie diets need medical oversight, and they carry risks that a general guide cannot manage.

Crash diets fail in a predictable pattern. A very large deficit loses weight fast for two or three weeks, much of it water and glycogen at first, then muscle as well as fat. Hunger rises sharply, energy and mood fall, and the diet is abandoned. Weight returns, often above the starting point, because expenditure has fallen with the lost muscle and the old eating pattern resumes at the old intake. The net result is a heavier person with less muscle than before — the opposite of the goal.

There are also people who should not pursue weight loss without medical advice: anyone with an eating disorder or a history of one, pregnant people, people with diabetes on medication that affects blood sugar, and anyone whose weight loss would be rapid or unexplained. This guide is general information for healthy adults, not a prescription.

Plateaus: why the scale stalls, and what to do

A plateau is a period of two to four weeks with no change in weight despite a continued deficit. The commonest cause is adaptation: as weight falls, the body needs fewer calories to maintain itself, and the original deficit shrinks to nothing. A deficit calculated at 90 kg is not a deficit at 80 kg. The fix is to recalculate maintenance at the new weight and reset the deficit, or to increase activity by a comparable amount.

The second cause is intake creep. Portions grow, snacks return, cooking oils and drinks are forgotten, and a 500-kcal deficit quietly becomes 200. The fix is a week of honest tracking — weighing food, counting drinks and oils — which usually finds the creep without any change in the plan. The third cause is activity creep in the other direction: as dieting fatigue grows, spontaneous movement falls — fewer steps, less fidgeting, more sitting — and expenditure drops by hundreds of calories a day without any conscious decision.

The fourth cause is water. Glycogen stores, salt intake, hormonal cycles, new training, and muscle soreness all hold water, and water can mask several weeks of fat loss on the scale. The signs are a stable waist measurement, clothes fitting better, and weight that drops suddenly after a stall — the pattern of water, not fat. Judge a plateau by the waist and the trend over a month, not by a single week’s weighings, and weigh under consistent conditions: same time of day, same clothing, after the same routine.

Keeping it off: the harder half

Losing weight and keeping it off are different skills. Maintenance needs a deliberate plan, not a return to old habits: a new maintenance intake at the new weight, continued resistance training, continued protein intake, and a monitoring habit — weekly weighing, or a waist measurement — that catches a regain of a few kilograms before it becomes a regain of all of them. The habits that lost the weight are the habits that keep it off, at a slightly higher intake.

Expect regain pressure. Hunger hormones rise after weight loss and stay elevated for a year or more, and expenditure stays slightly lower than predicted for the new weight. That is biology, not failure, and it is managed by structure: regular meals, protein at each meal, fibre-rich foods, sleep, and activity built into the day, rather than by willpower applied afresh every morning.

Setbacks are part of the process, not the end of it. A holiday, an illness, or a stressful month will interrupt the plan, and weight will rise. The useful response is to return to the plan at the next meal, not to restart on Monday with a larger deficit, and not to abandon the effort because a week went badly. Long-term studies of successful maintainers show the same pattern: they monitor, they correct early, and they treat lapses as data, not as verdicts.

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