How much bodily mass do we give out through breathing? The chemical reaction for aerobic respiration is
C6H12O6 + 6O2 → 6CO2 + 6H2O
The amount of carbon dioxide we exhale is then
6 mol CO2 / mol glucose = 1.47 g CO2 / g glucose.
Processing of fatty acids yields a different amount of exhaled CO2. For instance, the oxidation of a saturated fatty acid is expressed as
CnH2nO2 + (3n/2 − 1) O2 → n CO2 + n H2O
44n/(14n + 32) g CO2 / g CnH2nO2.
This last expression also serves approximately for unsaturated fatty acids, since these are missing only a few lightweight hydrogen atoms with respect to the saturated acid with the same amount of carbon. Assuming most consumed fat is in the form of palmitate, we have a carbon dioxide yield of
2.75 g CO2 / g fat;
0.324 g CO2 / kcal.
For a daily energy consumption rate around 2,500 kcal, 810 grams of carbon dioxide are then expelled through breathing. There is a experimental study by the USDA on the subject that unfortunately I haven't been able to find, but some sources cite it as reporting a value beween 445 and 450 liters of CO2 (881-891 g) per day, which is in good agreement with our calculations.
The value calculated is not the answer to our original question about exhaled bodily mass: much of the oxygen in the CO2 produced comes from the air rather than catabolized substances, specially in the case of fat. We will assume that all the oxygen coming from the burned substance goes to CO2: under these conditions, the amount of bodily mass expelled through breathing is
0.933 g / g glucose,
0.875 g / g fat,
or, using the assumed ratios of carbohydrate and fat consumption,
0.138 g / kcal,
which yields 345 g for a typical daily energy consumption of 2,500 calories.