The first week of any diet is water. The second is the truth

The arithmetic doesn't work

A kilogram of body fat stores roughly 7,700 kcal. A 500 kcal daily deficit therefore removes about 0.45 kg of fat in a week. Yet a typical first week on a new diet drops 1.5 to 2.5 kg on the scale. The deficit cannot explain that number. Around three quarters of it was never fat. It was water, and it left first.

The scale weighs everything at once — fat, muscle, water, glycogen, the contents of your gut, this morning's coffee. Fat is the slowest-moving item on that list and, in week one, the smallest. What a single weigh-in cannot tell you is which compartment moved. The arithmetic above is your only way to guess.

Where the water goes

You store 400 to 500 g of glycogen in your muscles and liver, and each gram binds roughly 3 g of water. Start a deficit — especially a low-carb one — and glycogen drains within days, taking 1 to 1.5 kg of water with it. At the same time, lower insulin tells your kidneys to excrete sodium, and water follows the salt out. Less food in transit also means a lighter gut. None of this is fat loss. All of it registers on the scale.

Week two is the truth

Once glycogen and sodium settle — usually by day 7 to 10 — the slope becomes what the deficit actually buys: roughly 0.4 to 0.7 kg per week on a moderate deficit. Next to week one it looks like failure. It is the opposite. It is the first honest week. Week two matches the arithmetic; week one never did.

This is also where diets get abandoned. The steep line flattens, the dieter reads flat as broken, and quits during the only stretch that was actually working. Studies of regular self-weighing consistently find the people who keep logging through the slow weeks are the ones still lighter six months later.

The jump that isn't fat

The mechanism runs in reverse, too. One high-carb dinner refills glycogen, and glycogen brings its water back. The scale can jump 1 to 1.5 kg overnight. Gaining that much fat would take a surplus of more than 7,000 kcal — not achievable at one dinner, however it felt. A single weigh-in cannot distinguish a rebound from a relapse. Seven of them in a row can.

How to read the scale

Weigh daily under the same conditions and let a 7-day rolling average do the judging. Daily readings can swing 1 to 2 kg on water, salt and gut contents alone; the average cancels the noise and leaves the slope. Menstrual cycles, salty dinners, alcohol, travel and a cold all move water too — more reasons to judge the line, not the day. One weigh-in is weather. The rolling line is climate. And if week two's line falls more slowly than week one's, good. The slow part is the part that was ever fat.