Gaining a kilo overnight is not fat — so what is it?

Yesterday the scale said 70.2. This morning it says 71.3, and you ate the same as always. This is the single most common reason people stop weighing themselves — and it is a situation where the number is not lying. It just is not answering the question you thought you asked.

Can overnight weight be fat?

Not in practice. A kilogram of body fat stores about 7,700 calories. To actually add a kilogram of fat you would have to eat 7,700 calories more than you burned, which is not a quantity a single dinner delivers — not even the heaviest dinner of your month.

If the number moves a kilo overnight, what changed is water, not tissue.

What can move that much water?

Four things explain almost all of the daily swing.

• Sodium — a saltier meal than usual makes the body hold water to keep the salt concentration in the blood steady. One Thai meal can carry half a day's sodium on its own.
• Carbohydrate — glycogen stored in muscle and liver brings water with it, roughly 3 grams of water per gram of glycogen. Eat one heavy carbohydrate meal after a stretch of restraint and the scale can rise by a kilo without a single gram of fat being added.
• Hormones — in women, body water rises and falls by hundreds of grams across the cycle, most noticeably in the days before a period.
• What has not been passed yet — food and drink still in the digestive tract weigh something, and they are not you.

So what should you watch instead?

The 7-day rolling average, not today's number. The average swallows the daily noise and leaves only the direction, which is the only thing you can actually make a decision on.

A reading that works: if this week's 7-day average is below last week's, you are losing — whatever today's figure decided to do.

When should you weigh yourself so the numbers are comparable?

In the morning, after the toilet, before eating or drinking, wearing about the same amount. The point is not that this moment is the truest one; it is that it is repeatable. A number measured under the same conditions every day can be compared with itself, and one measured at random cannot.

Every day, or once a week?

Every day, and then read the average. That sounds like more work but it is the opposite: weighing once a week gives you a single reading that carries all of the daily noise and none of the smoothing, so a bloated Tuesday becomes your whole week.

The one condition is that a daily weight has to be treated as raw material for the average, not as a verdict on the day. If watching it move makes the day worse, weigh less often — an accurate number you dread is worth less than a rougher one you can live with.