Most of the weight you “lose” leaves your body as carbon dioxide when you breathe out, and the rest leaves as water in urine, sweat, and other fluids. The fat cells themselves mostly shrink rather than disappear, unless they are physically removed with procedures like liposuction.

The short version

When your body taps into stored fat for energy, the fat (mainly triglycerides) is chemically broken down into carbon dioxide and water inside your cells. You then:

  • Exhale the carbon dioxide through your lungs.
  • Excrete the water via urine, sweat, and a small amount in your breath and other fluids.

If you lose about 10 kg of fat, roughly 8.4 kg is breathed out as carbon dioxide and about 1.6 kg leaves as water.

What happens inside fat cells

Stored body fat is mostly triglycerides, made of carbon, hydrogen, and oxygen atoms. When you create a calorie deficit (eat fewer calories than you burn), your body:

  1. Releases triglycerides from fat cells into the bloodstream as fatty acids and glycerol.
  2. Sends them to tissues (like muscles) to be “burned” (oxidized) for energy.
  3. Produces carbon dioxide and water as byproducts of this oxidation.

The fat cells largely remain, but their contents go down, so the cells shrink and your body looks leaner.

Where the weight physically exits

Most of the lost mass leaves through your lungs as carbon dioxide. The rest leaves as water:

  • In urine via the kidneys.
  • In sweat through the skin.
  • In smaller amounts in breath, tears, and other fluids.

Very little actual fat mass is lost directly in poop; stool is mostly water, fiber, bacteria, and waste products, not big chunks of burned fat.

Why breathing and movement matter

Any activity that increases your energy use (from walking to intense workouts) makes your body pull more on those fat stores. As you burn more:

  • You breathe faster and deeper , so more carbon dioxide leaves.
  • You often sweat more , so more water (and thus mass) exits too.

This is why regular movement plus a sustainable calorie deficit is key for long-term weight loss, not just sweating a lot or trying to “flush” fat out.

Quick forum-style myth busts

“You just burn fat into energy and it disappears.”

Energy is part of the story, but the mass of the fat has to go somewhere: it becomes carbon dioxide and water that leave your body.

“You poop the fat out.”

Normal weight loss isn’t your body dumping intact fat into your stool; fat is mostly metabolized first, then leaves as gas and water.

“If I sweat more, I’m burning more fat.”

Extra sweat mainly means water loss, which quickly comes back when you rehydrate; real fat loss depends on your overall energy balance.

TL;DR: When you lose weight, fat is chemically broken down and most of its mass leaves your body via your breath as carbon dioxide, with the rest leaving as water in urine, sweat, and other fluids.