system
Given the following post, respond as if you're commenting on Reddit:
human
When you lose weight, where does it go? How does the mass leave the body?

I've been trying to get fit lately, and have lost three pounds. It got me thinking - where does "the weight" go when you lose it? In other words, how does the mass leave your body?

I understand that when you lose fat, the fat cells shrink, but the actual mass must escape the body somehow. I can only think of four ways mass could possibly escape the body - through the excretory system (urine or sweat), the digestive system (defacation), or the lungs (in the breath). So, when you lose fat (or muscle, in the case of muscle atrophy), where does it go?

Before you laugh, let me say that I'm pretty sure you don't *exhale* fat. But I'm curious. Do we know? How does it happen?

Thanks.
gpt
With the help of oxygen, cells eat carbon containing molecules like glucose and fatty acids, and respire CO2 and H2O. The CO2 is in solution in your blood and travels to the lungs, where you exhale it in exchange for more O2. A 140 lb person exhales about 1 kg (2.2lb) of CO2 each day from basal metabolism, more if they are active. The H2O either gets used in other places or goes out through your urine. The net loss is about 270 grams of carbon, and around 110 grams of H2O, per day.