Why your weight changes every day, and what to watch instead
You eat well all week. On Friday morning the scale says you are 0.8 kg heavier than on Thursday. Did you gain 0.8 kg of fat overnight?
No. By the usual rule of thumb, a kilo of body weight stands for about 7,700 kcal. To put on 0.8 kg that way, you would have to eat about 6,000 kcal more than you burned in one day. What changed is mostly water, and the food that is still inside you.
What moves the number
Your body is a bag that things pass through all day. John Walker, who wrote the well-known guide The Hacker’s Diet, puts numbers on it. His example dieter set out to lose 10 pounds over two months:
His daily weight loss during the diet was less than one fifth of a pound per day, yet each and every day almost 80 times that weight passed through his body!
That traffic is food, air and water, and Walker reckons about three quarters of it is water. The scale weighs whatever is in the bag at the moment you step on it. These are the usual causes of a jump:
- Salt. A salty meal makes you thirsty, and your body holds on to more water.
- Carbohydrate. Your body stores carbohydrate as glycogen, and each part of glycogen is stored with three to four parts of water. A day of bread and rice can add water weight. A low-carb day can take it off.
- Food still on its way through. A big dinner weighs something until it has gone.
- Time of day and clothes. You weigh more in the evening than in the morning, and more in jeans than without them.
None of these is fat. All of them can move the scale by more than a week of real fat loss.
How big the swings are
Walker kept daily records of his own weight for years. In one month his weight held almost still. He wrote:
While my true weight didn’t vary by more than a pound that month, the difference between the highest and lowest daily weights on the scale was six pounds.
So a single reading can be a few pounds away from your real weight in either direction. If you judge your diet by one morning’s number, you will often reach the wrong conclusion.
The trend line
The fix is to stop reading single days and to follow an average that moves slowly. Walker’s method is an exponentially smoothed moving average. The name is long but the sum is short. Each day, the trend moves one tenth of the way from yesterday’s trend to today’s weight:
new trend = yesterday’s trend + (today’s weight − yesterday’s trend) ÷ 10
Say your trend is 80.0 kg and you weigh 81.0 kg today. The difference is 1.0 kg. A tenth of that is 0.1 kg. Your new trend is 80.1 kg. One heavy day moves the trend a little. A real change, day after day, moves it a lot.
The dots in the chart are daily weigh-ins. They scatter up and down. The line runs through them and shows the direction that matters. Over a few weeks, the trend tells you whether you are losing, gaining or holding steady, and roughly how fast.
What to do with this
- Weigh yourself at the same time each day, for example in the morning after the toilet and before you eat or drink.
- Write the number down, or let an app keep it.
- Read the trend, not the day. Judge your plan by how the trend moves over two to four weeks.
- When the scale jumps after a salty dinner or a weekend away, expect most of the jump to go again once you eat as usual.
If the daily number makes you anxious, you can look only at the trend. The trend is the number that tells you how your body is changing. Our app, Downtrend, works out a trend line like this for you each time you weigh in, and puts it first on the screen, above the day’s reading.
Sources
- John Walker, The Hacker’s Diet, fourth edition, 2005. The quotations are from the chapter Signal and Noise. The trend sum is from Pencil and Paper.
- Kreitzman SN, Coxon AY, Szaz KF. Glycogen storage: illusions of easy weight loss, excessive weight regain, and distortions in estimates of body composition. American Journal of Clinical Nutrition, 1992. The source for glycogen being stored with three to four parts water.
- Wishnofsky M. Caloric equivalents of gained or lost weight. American Journal of Clinical Nutrition, 1958. The origin of the rule of about 3,500 kcal per pound, which is about 7,700 kcal per kilogram.
This article is general information, not medical advice. Talk to a doctor before a big change to what you eat or how you train, and especially if you are pregnant, under 18 or have a health condition.