system
Given the following post, respond as if you're commenting on Reddit:
human
When you lose weight, where exactly does most of the mass (in your fat cells) go?
gpt
You breathe it out. The average person at rest exchanges about 1 kg of CO2 for about 0.8 kg of O2 every day via the lungs, for a net loss of .2 kg, or about .44 lbs. The CO2 comes from cell respiration. The carbon comes partly from glucose which is 40% carbon by weight, and partly from fat, which is about 80% carbon by weight. It also comes from other sources, but these are the main ones.

When you're engaging in physical activity you can increase your breathing by 7 to 10-fold, depending on how athletic you are.