Why does my weight change every day?

If you weigh yourself every morning, you'll have seen it: a good week, then a jump of 1 or 2 lbs overnight that seems to undo it. That jump is almost never fat. Fat changes slowly, by a fraction of a pound a day at most. What moves your weight from one day to the next is mostly water and whatever's still on its way through you.

What moves the number

• Water. Your body holds more water after a salty meal and lets it go again over the following days.
• Carbohydrate. Your muscles store carbohydrate as glycogen, and each gram is stored with at least 3 grams of water (Fernández-Elías and colleagues, 2015). A carb-heavy day can show on the scales the next morning, and cutting carbs can make the first week's loss look bigger than it is.
• Food and drink still in you. A big dinner weighs something until it's gone.
• The week itself. In one study of 80 adults weighing in daily, weight rose over the weekend and fell back through the week, and the people who lost weight were the ones whose weekdays made up for their weekends (Orsama and colleagues, 2014).
• Hormones, hard exercise and poor sleep can all hold water for a few days, too.

None of this means your plan isn't working. It means one weigh-in is a noisy reading, and the signal is in the trend.

How to weigh in so the trend is clear

1. Same time, same conditions. First thing in the morning, after the loo and before you eat or drink, in the same clothes or none.
2. Same scales, on a hard floor. Moving bathroom scales between a tile floor and a carpet can change the reading.
3. Weigh in often. More weigh-ins make the ups and downs easier to see past. A review of 17 studies found regular self-weighing went with more weight loss, and not with worse mood or anxiety (Zheng and colleagues, 2015). If daily weigh-ins upset you, weekly is fine: just keep the day and time the same.
4. Judge the weeks, not the days. Compare this week with the week before, or this month with the last.

Seeing the trend in Weight Loss Tracking Buddy

Weight Loss Tracking Buddy is built to show the trend rather than the wobble:

• The Overview's Trend card draws your last week, month or more as a line, with the change over the period beneath it.
• Charts plots every weigh-in, and the Weight card can draw a trend line from the first weigh-in on screen to the latest (turn on Show Trend Lines in Settings, under Preferences). Hold a finger on the chart to read any day.
• The Goal chart carries your trend forward, at your rate over the week, month or longer, to the day it would reach your goal. A bad week shows against the months around it, so you can see whether it's a blip or a change of direction.

A week of the weight chart and the goal projection are free. WLTB Pro lets you scroll back through every weigh-in.

When to look more closely

If your trend has been flat or rising for three or four weeks while you've stuck to your plan, that's worth acting on: check your portions, or recalculate your budget in Settings, under Goal, as your needs fall a little as you lose weight. And if your weight changes quickly without a reason, or with swelling or breathlessness, talk to a doctor.

Sources

• Fernández-Elías VE, Ortega JF, Nelson RK, Mora-Rodriguez R. Relationship between muscle water and glycogen recovery after prolonged exercise in the heat in humans. European Journal of Applied Physiology, 2015; 115(9): 1919 to 1926.
• Orsama AL, Mattila E, Ermes M, et al. Weight rhythms: weight increases during weekends and decreases during weekdays. Obesity Facts, 2014; 7(1): 36 to 47.
• Zheng Y, Klem ML, Sereika SM, et al. Self-weighing in weight management: a systematic literature review. Obesity, 2015; 23(2): 256 to 265.