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Training decisions, run through the arithmetic

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The Five Things That Move the Scale Overnight, None of Which Are Fat

A kilogram appears, you have done nothing wrong, and the arithmetic says fat cannot move that fast in either direction.

By Tessa Lindqvist·Published ·285words

The arithmetic first, because it settles the question before the explanations start.

What 2 kg of fat would require

Energy per kg of fat mass: ~7,700 kcal

For 2 kg: ~15,400 kcal surplus

In one day, above maintenance: not physically plausible

Whatever the scale showed you, it was not fat. That is not reassurance; it is arithmetic.

The five actual causes

Sodium. A high-salt meal draws water into circulation and tissue. Half a kilogram to a kilogram, resolving in one to three days.

Carbohydrate. Each gram of stored glycogen carries roughly three grams of water with it. A large carbohydrate intake after a low period can add a kilogram or more of genuine, useful weight that is not fat.

Gut contents. Food and fluid in transit weigh what they weigh. A high-fibre day or a large meal volume shows up directly.

Training. An unfamiliar or unusually hard session produces fluid retention associated with repair. This one can persist for several days and is the most commonly misread, because it appears right after you did something virtuous.

Hormonal fluid shifts. Cyclical for many people and large enough to obscure a fortnight’s trend entirely.

Why it matters practically

Not because the scale is wrong — the scale is fine. Because of what people do next.

The standard sequence is a large downward adjustment to intake in response to a single reading, followed by hunger, followed by a rebound, followed by the conclusion that nothing works. The scale was behaving normally the whole time.

The rule

Interpret weekly averages. Never react to a single reading. If a week goes the wrong way and the fortnight is on track, nothing has happened.

Questions we get asked

Why did I gain 2 kg overnight?

Water, glycogen, gut contents, sodium and, for some people, hormonal fluid shifts. Fat mass cannot change by two kilograms in a day because that would require a surplus of roughly 15,000 calories. Whatever the scale showed, it was not fat, and it will resolve within a few days.

Does salt make you gain weight?

It makes you retain water, which the scale reports as weight. A high-sodium meal can add half a kilogram to a kilogram of retained fluid for a day or two. It is not fat gain and it resolves without intervention, which is why reacting to it with a calorie cut is the wrong response.

How long does water weight take to go away?

Usually one to three days for a sodium or carbohydrate load. Training-induced retention from an unfamiliar session can take longer, up to about a week, because it is associated with muscle repair. Either way it is shorter than the window you should be using to interpret your trend.

Tessa Lindqvist

Contributing writer, nutrition · Stronger by Math

Writes about intake, adherence and what the self-monitoring literature actually supports. Weighs her own food, which is why she is sceptical of estimates — including the good ones.

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