You are always breathing, but you breath about 3x more while you're working out cardio style, but you're not significantly changing your average breath rate for the day unless you're working out for hours daily. 24 "rest-hours of breath" on a lazy day, 26 "rest-hours of breath" on a day when you spend a full hour doing cardio/running/etc, so spending an hour doing intense workouts only increased your carbon exhaled by 8.3%. Also, somewhere around 2/3 (if not 3/4+) of the weight you lose per breath is simply water vapor restored when you have your next drink, not carbon atoms being attached to oxygen molecules in the air. Also, working out makes you hungry, rightfully so (in a world that isn't essentially full of infinite tasty food for those of us with a little money). The only way to lose significant weight is to feel hungry, and to stop eating before you "feel" full. This is de facto psychologically insurmountable (or nigh unsustainable) to everyone who is overweight and wants to lose it but just can't bear to be hungry enough for long enough. Spoken as someone slightly overweight who just needs to feel a little hungry for a couple months to get to a much better weight. And of course, once you've made a habit of eating over your ideal calories, even after you've lost the weight, your stomach is still bigger and it's effortless to go back to eating your usual large-self amounts even after you've lost all the weight, and go right back to large-self weight.
