You eat well, you train, and still the scale says +1.5 kg on Tuesday. You did nothing wrong. Day-to-day swings on the scale have very little to do with fat. Water, salt, gut contents and glycogen move the number much faster.
Short answer: One reason is glycogen, the carbohydrate your body stores in muscles and liver. Research suggests each gram of glycogen is stored with at least 3 g of water (1), so filling and emptying these stores moves the scale regardless of fat. Weigh yourself every morning in the same conditions, compare weekly averages, and measure your body too.
How your body stores energy
Your body has more than one energy store. Two matter here: energy stored as fat (triglycerides, which are broken down into fatty acids when you need fuel), and energy stored in muscles and liver as glycogen.
Liver glycogen helps keep your blood sugar (glucose) steady. Muscle glycogen stays in the muscle and fuels movement and harder work, where fat alone can't keep up.
Trained people can store considerably more glycogen in their muscles than untrained people. As a rough estimate, an untrained adult has about 500 g of glycogen in the whole body.
Your body responds to strength training by getting stronger and adapting the nervous system. Regular training, together with enough carbs, also tends to increase glycogen stores. The stores grow mainly because heavier and longer sessions need more energy, and high intensity cannot be covered from fat stores alone. When you work hard in the gym, glycogen goes down, and then it is refilled from food and by processes inside the body.
The longer someone trains, the bigger the stores tend to be. Stronger muscles and longer, harder sessions mean higher energy use, mostly from carbs, so your maintenance calories go up, and a good part of that extra energy usually comes from carbs. A well-trained athlete does harder and longer work and burns more energy per session than an untrained person. Their daily energy expenditure is higher, so their maintenance calories are higher too. Energy balance still decides whether they gain fat.
The fridge and the freezer
To make this clear without biochemistry, use a simple picture. It is a simplification, but it helps. Glycogen in your muscles and liver is your fridge. Fat stores are your freezer.
- You go shopping and buy a lot of food (you eat a lot). Part of it goes into the fridge: your body tops up the glycogen stores in liver and muscles.
- Part of it goes into the freezer: your body also stores some energy as fat after a meal, even if the fridge is not full. Between meals it draws on both. Whether the freezer gets fuller over weeks depends on your overall energy balance, not on whether the fridge is full.
- A full fridge weighs more than an empty one. In the same way, full glycogen stores make your body noticeably heavier.
- And there is the water: research suggests at least 3 g of water is stored with every 1 g of glycogen in muscle.
Why does the body keep glycogen and not only fat? Imagine you work at home at night and you get hungry. You go to the fridge, grab food and eat fast at the computer, because you have to keep working. Your body does the same: for acute, intense effort (boxing, a hard workout) it uses carbs first. Fat stores are like the freezer: you would have to take the food out and wait for it to thaw, and you can't work while you wait.
Put the numbers together
An average untrained person holds around 500 g of glycogen, and at least about 3 g of water goes with each gram (a lower estimate, measured in muscle):
500 g glycogen + 3 x 500 g water = 500 g + 1,500 g = about 2 kg.
So a full tank of glycogen and its water is worth about 2 kg on the scale. You rarely go from completely full to completely empty, but even a part of it shows. In trained people with bigger stores the possible swing is larger than in untrained people, so a noticeable jump on the scale without any change in fat is nothing unusual.
A swing in weight alone says very little about whether you are gaining or losing fat. On top of that, water, gut contents and, for women, the phase of the cycle also show on the scale.
What to do about it
This is why I measure body measurements with clients and don't rely on a single weigh-in. If you use only the scale, at least weigh yourself every morning in the same conditions (for example after the toilet and before eating) and compare weekly averages, not single numbers from different days. Add a tape measure at the waist once a week. If daily weighing makes you anxious, weigh less often and rely on measurements. If food or weight takes over your thoughts, talk to your doctor. How to put this into a plan is in how to start losing weight.
Next time the scale jumps overnight, remember: the fridge, water and gut contents move much faster than the freezer.
Frequently asked questions
How much more glycogen does a trained person have in the muscles?
A trained person can store considerably more glycogen in the muscles than an untrained person. As a rough estimate, an untrained adult has about 500 g of glycogen in the whole body.
How much can weight swing because of glycogen alone?
In trained people with bigger stores the possible swing is larger than in untrained people, so a noticeable jump on the scale without any change in fat is nothing unusual. For an untrained person a full tank of glycogen with its water is worth about 2 kg.
How should I weigh myself when the scale keeps jumping?
Weigh yourself every morning in the same conditions and compare weekly averages. One number from one morning tells you very little about fat. Even better, measure your body regularly with a tape and don't rely on a single weigh-in. If daily weighing makes you anxious, weigh less often and rely on measurements.
Why can a trained person eat more and not gain weight?
Stronger muscles and longer, harder sessions use more energy, mostly carbs. A well-trained athlete burns more per session, so their daily energy expenditure and maintenance calories are higher too. Energy balance still decides whether they gain fat.
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1) Glycogen and water: Fernandez-Elias VE, Ortega JF, Nelson RK, Mora-Rodriguez R. Relationship between muscle water and glycogen recovery after prolonged exercise in the heat in humans. Eur J Appl Physiol. 2015;115(9):1919-1926. DOI: 10.1007/s00421-015-3175-z | PMID: 25911631. Small study: 9 trained cyclists, muscle biopsies after exercise in the heat.
Martin Barna
Online nutrition and fitness coach. Since 2013 I have coached 600+ clients. Evidence-based coaching with plans you can keep up.
Apply for coachingThis article is for information only. It is not medical advice. If you have a health condition, talk to your doctor.