What is a calorie deficit?

Your body needs a certain amount of energy each day to run itself and do whatever you ask of it. Food supplies that energy. When the supply falls short of the demand, the difference comes out of storage — mostly body fat, and some muscle.

This is why the various diets that work all share one mechanism underneath. Low-carb works by making it harder to overeat carbohydrate. High-protein works partly because protein is filling. Intermittent fasting works by shrinking the window in which eating happens. None of them suspends the arithmetic; they are different routes to the same shortfall.

The planning convention is that roughly 7,700 kcal is equivalent to a kilogram of body mass — about 3,500 kcal per pound. A deficit of 550 kcal a day, held for a week, therefore predicts about half a kilogram lost.

That figure is a useful rule of thumb and a poor prophecy. It comes from the energy density of body fat, but real weight loss includes water and glycogen, and your expenditure drifts downward as you get smaller and as your body adapts to a sustained shortfall. Expect the arithmetic to be roughly right over months and frequently wrong over days.

You can eat less, move more, or both. Eating less is the more powerful lever for most people, simply because it is easier to not eat 500 kcal than to burn 500 kcal — the latter is roughly an hour of hard running, every day.

Movement still matters, but its main contribution is not the calories burned during exercise. Resistance training protects muscle while you are in a deficit, which changes what you lose rather than how much. And everyday movement — walking, standing, taking stairs — adds up to more over a week than most training sessions do, precisely because it is unremarkable enough to sustain.

Roughly 15–25% below maintenance suits most people: often 300–700 kcal a day, or about 0.25–0.7 kg (0.5–1.5 lb) a week. A larger deficit does lose weight faster, but a greater share of it comes from muscle, and the hunger, fatigue, and irritability that come with it are what make plans collapse.

The useful test is whether you could hold the plan for a month without your sleep, training, and mood falling apart. A slower plan you finish beats a faster one you abandon in week three — and if you have a lot to lose, you can sustain a larger absolute deficit early on, then shrink it as you get leaner.

There is a floor worth respecting: around 1,200 kcal a day for women and 1,500 for men. Below it, meeting your protein and micronutrient needs from food gets genuinely difficult, and that is a supervised-by-a-professional situation rather than a do-it-yourself one.

You can be in a real deficit all week and weigh more on Sunday than you did on Monday. Body weight includes water, and water moves for reasons that have nothing to do with fat: salt intake, carbohydrate intake, hormones, hard training, and simply how much food is currently in your digestive tract.

A kilogram of day-to-day fluctuation is normal and means nothing. What means something is the direction of the weekly average over three or four weeks. Weigh under consistent conditions if you weigh at all — same time, same circumstances — and read the trend, not the reading.

The most common reason a deficit stops working is that it stopped being a deficit. Untracked oils, drinks, sauces, and bites accumulate quickly, and portions tend to creep upward as vigilance fades. Weight loss also lowers your expenditure, so the target that worked in month one may be maintenance by month three.

The fix is unglamorous: tighten the logging for a couple of weeks to see what is really going in, recalculate your target for your current weight, and pick a pace you can hold. A stall is information about the plan, not a verdict on you.