Why does my weight change from day to day?
Short answer
Day-to-day changes on the scale are usually water and the weight of food and drink, not fat. Your body stores carbohydrate as glycogen along with three to four parts water, so eating more or less carbohydrate can move the scale quickly. A 2020 study of 1,421 people found weight rose on weekends and fell on weekdays. The weekly trend tells you more than any single morning.
On this page
What makes your weight go up and down?
Water, stored carbohydrate and whatever is still in your digestive system. Fat changes slowly by comparison, so most of what you see from one day to the next is something else.
| Cause | What the research shows |
|---|---|
| Stored carbohydrate | Glycogen is stored with 3 to 4 parts water |
| Weekends | Weight rose on weekends and fell on weekdays, a weekly swing of about 0.35% |
| Holidays | Weight rose 1.35% over Christmas and wasn't fully lost in the months after |
| Menstrual cycle | Self-reported fluid retention peaked on the first day of the period |
| Body fat | Changes gradually: a lasting change in intake takes about a year to reach half its full effect |
Why does carbohydrate move the scale so much?
Because of the water stored with it. A 1992 paper on glycogen explains that the body stores it in the liver, muscles and fat cells together with three to four parts water. When you use up glycogen, that water goes with it, and when you refill it, the water comes back.
The authors pointed out two effects of this. It creates an illusion of easy weight loss early in a very low-calorie diet, and it causes an exaggerated regain if you then eat a lot of carbohydrate. Neither one is a change in body fat.
How much does weight swing in a normal week?
In a 2020 study that analyzed the weigh-ins of 1,421 people in a European weight-maintenance trial, weight rose over the weekend and fell during the week, a swing of about 0.35 percent. The same study found weight rose 1.35 percent over Christmas and wasn't fully lost in the following months.
For someone weighing 80 kg (176 lb), that's about 0.3 kg (0.6 lb) across a typical week and about 1.1 kg (2.4 lb) over the holidays. These are averages across many people, so your own swings may be larger or smaller.
The menstrual cycle can add another pattern. In a one-year study of 62 healthy women, self-reported fluid retention peaked on the first day of the period and was lowest in the mid-follicular phase, partway through the first half of the cycle.
Why doesn't the scale match what I ate yesterday?
Because real changes in body fat build up slowly. A 2011 analysis in The Lancet estimated that for an average adult with overweight, a lasting change in how much you eat takes about a year to produce half of its eventual change in weight. A single day of eating more or less than usual shows up on the scale mostly as water and food weight, not fat.
How should you weigh yourself?
Consistently, and then look at the trend. Weigh at the same time of day under the same conditions, such as after waking and before eating, and compare weekly averages instead of single days. A 2011 systematic review, which included studies of regular self-weighing, found self-monitoring consistently linked to weight loss.
Common questions
Should I weigh myself every day?
You can, as long as you look at averages rather than single readings. If daily weighing makes you anxious, or you have a history of disordered eating, weigh less often and talk to a clinician about what's right for you.
Why did I lose so much weight in my first week of dieting?
Mostly water. Cutting calories, especially carbohydrate, draws down glycogen, and the water stored with it leaves too, which the 1992 glycogen paper called an illusion of easy weight loss.
How long until the scale shows a real change?
Look at weeks, not days. Daily and weekly swings can hide real progress, and the Lancet analysis found that the full effect of a lasting change in intake builds up over months to years.
Sources
- 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;56(1 Suppl):292S–293S. pubmed.ncbi.nlm.nih.gov
- Turicchi J, O'Driscoll R, Horgan G, et al. Weekly, seasonal and holiday body weight fluctuation patterns among individuals engaged in a European multi-centre behavioural weight loss maintenance intervention. PLoS One. 2020;15(4):e0232152. pubmed.ncbi.nlm.nih.gov
- White CP, Hitchcock CL, Vigna YM, Prior JC. Fluid retention over the menstrual cycle: 1-year data from the prospective ovulation cohort. Obstetrics and Gynecology International. 2011;2011:138451. pubmed.ncbi.nlm.nih.gov
- Hall KD, Sacks G, Chandramohan D, et al. Quantification of the effect of energy imbalance on bodyweight. The Lancet. 2011;378(9793):826–837. pubmed.ncbi.nlm.nih.gov
- Burke LE, Wang J, Sevick MA. Self-monitoring in weight loss: a systematic review of the literature. Journal of the American Dietetic Association. 2011;111(1):92–102. pubmed.ncbi.nlm.nih.gov
Estimates, not medical advice. If you're pregnant, have a medical condition or have a history of disordered eating, talk to a clinician before changing how you eat.
Written by the SeeMeEat editorial team under a pen name.