Why your weight fluctuates even when you are losing fat

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, and a big part of them comes from one thing: glycogen.

Short answer: One reason is glycogen, the carbohydrate your body stores in muscles and liver. For every gram of glycogen the body stores about 3 grams of water, 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 (in the form of fatty acids), and energy stored in muscles and liver as glycogen.

Glycogen is the form of carbohydrate your body keeps mainly to regulate blood sugar (glucose), and to fuel movement and higher-intensity work, where fatty acids alone cannot cover the acute demand.

An untrained person stores roughly 12 g of glycogen per kg of muscle. A trained athlete can store up to 33 g per kg, which is almost three times more. According to most sources, an average untrained person has about 500 g of glycogen in the whole body.

Your body responds to strength training by getting stronger, growing its glycogen stores and adapting the nervous system too. 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. Stronger muscles and longer, harder sessions mean higher energy use, mostly from carbs, so you can, and probably should, eat more calories, mainly from carbs. A well-trained athlete does harder and longer work and burns more energy per session than an untrained person. That is why he can and should eat more, and still does not gain much fat: more of the food energy goes into the work itself.

The fridge and the freezer

To make this clear without biochemistry, use a simple picture. 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). You fill the fridge first, until it is full. That means your body fills the glycogen stores in liver and muscles.
• The rest goes into the freezer, so it doesn't spoil. In the same way, your body puts the leftover energy into fat stores.
• 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: about 3 g of water is stored with every 1 g of glycogen.

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 about 3 g of water goes with each gram:

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 athletes with bigger stores, body weight can swing by several kilograms just from filling and emptying glycogen, no matter how much fat they have or whether they are losing weight right now.

A swing in weight alone says nothing 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. How to put this into a plan is in how to start losing weight.

I hope this makes it clearer. Next time the scale jumps overnight, you will know it is mostly the fridge and not the freezer.

Frequently asked questions

How much more glycogen does a trained person have in the muscles?

A trained athlete can store up to about three times more glycogen in the muscles than an untrained person. An average untrained person has about 500 g of glycogen in the body, according to most sources.

How much can weight swing because of glycogen alone?

In athletes, body weight can swing by several kilograms just from filling and emptying glycogen stores, no matter how much fat they have or whether they are losing weight. 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 nothing about fat. Even better, measure your body regularly with a tape and don't rely on a single weigh-in.

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 and uses more of the food energy for the work itself.

Sources, so you can check:

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

Martin Barna

Online nutrition and fitness coach. Since 2013 I have helped 600+ people lose fat and keep the result. Science, not crash diets.

Apply for coaching

This article is for information only. It is not medical advice. If you have a health condition, talk to your doctor.