Water weight: how much it hides and for how long

Body fat changes slowly and in one direction. Body water changes hourly and in both. That mismatch causes more abandoned diets than any nutritional error.

The mechanisms, with magnitudes

Glycogen. Stored in muscle and liver with roughly three grams of water per gram. A full glycogen load carries one to two kilograms of associated water. Deplete it and that water leaves; refill it and the water returns.

Sodium. A saltier day holds more fluid until the kidneys catch up. Worth 0.3 to 1 kg, resolving in one to two days.

Gut contents. Food in transit weighs what it weighed. A high-fibre day or a missed bowel movement is worth 0.2 to 0.8 kg.

Training. Unfamiliar or heavy work causes short-term retention in the muscles worked, part of the repair process. Worth 0.3 to 1 kg for a few days.

Cortisol. Poor sleep and psychological stress both promote retention. Worth 0.5 to 1.5 kg in some people, and it can persist for as long as the stress does.

The menstrual cycle. Retention commonly rises in the late luteal phase and falls once menstruation begins, often by a kilogram or more, on a predictable monthly pattern.

The whoosh

Here is the phenomenon that confuses people most.

Someone in a genuine deficit sees nothing for three weeks, then loses 1.5 kg in four days. They did not lose 1.5 kg of fat in four days — they had been losing fat the whole time while retaining fluid that concealed it, and the retention released.

This is extremely common and it is the reason a three-week stall is worth investigating rather than panicking about, per what a weight loss plateau usually turns out to be.

The first week of any diet

Almost every diet produces fast early weight loss, and almost none of it is fat.

Reducing intake reduces carbohydrate intake, which depletes glycogen, which releases water. Two to three kilograms in the first fortnight is ordinary.

Low-carbohydrate diets produce the most dramatic version, which is a large part of their reputation. The same mechanism works in reverse when carbohydrate returns, which is why people who resume normal eating see several kilograms appear in days and conclude they undid months of work. They did not. See carbs: how many you need and when it matters.

Judge from week three onwards.

What to do about it

Nothing, and this is a genuine recommendation rather than a shrug.

Do not take diuretics. They shift water temporarily and carry real risks. There is no cosmetic benefit worth that.

Do not cut salt severely. Sodium is required, exercise increases losses, and severe restriction causes fatigue and poor performance.

Do not dehydrate deliberately. Drinking less does not reduce retention; if anything the body holds more.

Do keep weighing daily and reading the average. Seven consecutive days cancels most of the noise, which is the entire method in daily weight lies, the trend does not.

A second measurement helps

Because water masks fat loss on the scale, a measurement it does not affect is useful.

A tape measure at the waist, taken weekly at the same point, tracks fat loss with less interference. So does how a specific pair of trousers fits. So does a training log showing strength holding or rising while weight falls, which is the reading described in why food and training belong in the same log.

When it is worth a doctor

Persistent swelling in the ankles or legs, sudden weight gain of several kilograms over a few days with no dietary explanation, or shortness of breath alongside it are not the phenomena described here and are worth medical attention.

Ordinary fluctuation is the kilogram that appears after a restaurant meal and is gone by Thursday — the everyday version covered in why the scale went up overnight when you did nothing.