4 cups of anything?

At an average of say 17,280 normal breaths per day, about how many exhalations are needed to recycle the carbon-weight gained from each tablespoon of food? Stuff we eat (like meat, veggies & yogurt) has about the density of water, so a tablespoon of food has about 15 grams mass. If most of that mass (say 85% neglecting roughage) is carbon with between 4.5 to 9 kcal of energy per gram that is (eventually) turned into CO2, then we're talking about ~0.85×15/0.048 ≈ 265 normal breaths of exhalation per tablespoon of food.

Does this mean that about 17,280/265 ≈ 65 tablespoons (~ 4 cups or 2 lbs) of food provides all the carbon that you exhale with normal-activity in a day? More food than that, and you either need extra activity or you add carbon weight that will have to await exhalation some other day. Less than your limit, and you may find yourself losing carbon weight or taking steps to conserve energy instead. More (or less) than 4 cups a day might then add (or subtract) as desired. If nothing else, carbon-tracking might help put good-old available-work (i.e. Calorie) tracking into more concrete weight-for-weight terms...