Two and a half kilos in seven days. Your tracking app drew a line straight down, and what the scale showed you matched the effort you were putting in. Then week two arrived with 0.4 kilos, and week three barely moved the decimal.
The math from week one became the standard. Every morning that failed to match it felt like proof the diet had stalled.
Why Your Weight Drops Fast the First Week of a Diet
Most of the weight you lose in week one comes from glycogen and water, not fat. Fat stores roughly five times more energy per kilogram, so burning it requires a much larger cumulative calorie deficit. The slower pace on the scale in weeks two and three is your body shifting from water loss to actual fat loss.
— Naude et al. 2022 · Cochrane Database of Systematic Reviews · 61 trials · n=6,925
Your body stores carbohydrates as glycogen, and every gram of glycogen holds onto roughly three grams of water. When you cut calories, those glycogen reserves empty within days. The water bound to them leaves too. A large-scale review of low-carb diet research puts the first-week water loss at 2 to 3 kilograms. The scale dropped fast because water is heavy and moves quickly.
Fat was burning alongside all of that, but at a pace the scale could never show with equal drama. Fat stores five times more energy per kilogram than the water-and-glycogen package that left in week one. Burning through a kilogram of actual body fat requires roughly five times the cumulative calorie deficit that shedding a kilogram of that early weight cost. The diet did not slow down. It graduated to a substance that takes far longer to budge.
5×
Fat stores five times more energy per kilogram than the water and glycogen that left in week one
Week one emptied a lightweight fuel reserve. From week two onward, fat becomes the primary component leaving your body. Slower on the scale, but exactly the outcome the deficit was built for.
One piece worth knowing: if carbohydrates come back up, the glycogen stores refill and the water returns with them. A 1 to 2 kilogram bounce after a high-carb day is the same water mechanism running in reverse, not fat reappearing overnight. Knowing the difference keeps one unexplained morning on the scale from undoing weeks of consistency.
The week that discouraged you was not a plateau. It was the shift from losing water to losing actual fat.
And if the scale confuses you again down the road, it helps to know that the number it shows is one measurement among several — and not always the most telling one.