Boxi Learn › Biology › Stage 3 · Structure and Evidence
Where does fat go when you lose weight?
BIO-3 Matter cycles, energy flowsX-1 Guess, then check
What many people think
When you lose weight, the fat turns into energy or heat, or leaves through the toilet.
The idea
Most of the mass of burned fat leaves the body as carbon dioxide you breathe out; the rest leaves as water.
Explain it
A friend says lost fat “turns into energy”. Using the 10 kg example, explain where its atoms actually go, and why the energy cannot account for the missing mass.
Try at home
Breathe onto a cold mirror: the mist is water leaving you. Then count your breaths for one quiet minute and use 0.04 g of CO₂ per breath to estimate how long it takes to breathe out 1 kg of CO₂.