You may have heard that an average human breathes out roughly 1kg of carbon dioxide a day. Gas masses are kind of abstract, and most of the mass is oxygen than you breathe in, but that is still 273 grams of carbon that your metabolism draws out of your body every day, even if you are fasting. Much more for a large person laboring heavily. Weighed out, that is a surprisingly large pile! (thinking about space habitats)