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Why do skinny people gain weight at the gym?

Muscle gets hungry too
For anyone who’s curious
No. 86Brain & BodyPart 11 of 11Inspired by r/explainlikeimfiveFact-checked Oct 6, 2026
Video2:19 · English voice (AI) · English subtitles

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The full narration of the video.

You go to the gym almost every day and burn plenty of energy. So why does the number on the scale keep going up? A lot of skinny people notice this. Once they start training, they eat more, and finally gain weight. So what exactly makes us eat more? Here's the first thing. Your body is like a careful bookkeeper. It keeps track of every bit of energy you spend.

Spend more, and it makes you a little hungrier, nudging you to eat it back. That energy has a name: calories. But this bookkeeper takes its time. Right after a workout, many people actually feel less hungry. It's after a week or two of steady training that your appetite quietly grows. American scientists ran an experiment. Nearly two hundred people who rarely exercised trained for six months. The lighter-exercise group ate 91 extra calories a day. The heavier group ate 124 extra.

Here's the second thing. Muscle is like a little stove that never switches off. Even when you lie still, it keeps burning. A kilo of muscle burns about 13 calories a day. A kilo of fat burns only 4.5. When a skinny person trains, they build muscle, and the stove gets bigger. Scientists have found that the more of your body isn't fat, the bigger your meals tend to be. Muscle gets hungry too. The third thing: not all of that extra weight is muscle. Muscles store some backup sugar for the next workout. This backup sugar is called glycogen. Every gram of it brings along three grams of water.

So in your first few weeks of training, a pound or two of new weight is often just water. That's not getting fat. It's your muscles filling up their tanks. There's also a common myth: that working out makes you fat. In fact, exercise doesn't create fat out of nothing. You only gain fat when you eat more than you spend. And a higher number on the scale doesn't mean you got fatter. If you're a skinny person trying to bulk up, that's good news. Follow that hunger and eat well, so your muscles have something to build with. If you're trying to lose weight, remember: the more you train, the more the bookkeeper nags you to eat. Watch what you eat, or the workouts go to waste.

1

In one sentence

Train more eat more Scale goes up Bigger meals

Your body keeps score: whatever energy you spend, it nudges you to eat back. More muscle burns more even at rest, so you get hungrier. And most of the new weight is muscle and water.

2

What you think
vs. what happens

YOU THINK
The more you train, the more you burn, so you can only get thinner
As if burned energy just disappears and never gets made up
ACTUALLY
Your body nudges you to eat back part of what you burned
People who trained for six months ended up eating 91–124 extra calories a day

In plain words: training builds your appetite, not just your strength.

3

Three things that
make you eat more

① Your body keeps scoreIt tracks the energy you spend. Spend more and it makes you a bit hungrier to cover the gap
② Muscle is a little stoveIt burns even while you rest: about 13 calories per kilo a day, versus about 4.5 for fat
③ Backup sugar brings waterMuscles stock up on sugar for the next workout, called glycogen. Each gram holds about 3 grams of water

Number ③ is why the scale jumps: the pound or two you gain in the first few weeks is often mostly water.

4

Who burns more
while you rest?

Calories burned per kilo, per day:

Muscle13 cal
Fat4.5 cal

Scientists have also found that the more of your body isn’t fat, the bigger your meals and the more you eat in a day. When a skinny person trains and builds muscle, the stove gets bigger.

Muscle gets hungry too.

5

What really happens

Not hungry at firstRight after a workout, many people actually have less appetite
Hungrier in weeksAfter a week or two of training, appetite grows
Six months laterA US study: 91–124 extra calories a day
Hard work, big mealsAn Indian mill in the 1950s: the harder the job, the more workers ate

One odd finding at that mill: people who sat all day ate more, and weighed more, than those doing light work. When you move regularly, your appetite tracks what your body actually needs.

6

A common myth

“Working out
makes you fat”?
Exercise doesn’t create fat out of nothing. You only gain fat when you eat more than you spend. A higher number on the scale doesn’t mean you got fatter, so check your waist and the mirror too.
The more you train,
the more your body
asks for food.
Trying to bulk up? Follow that hunger and eat well, so your muscles have something to build with. Trying to lose weight? Remember your body will nag you to eat, so watch what you eat or the workouts go to waste.
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