Mass exhalation

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

C_nH_2nO₂ + (3n/2 − 1) O₂ → n CO₂ + n H₂O

yielding

44n/(14n + 32) g CO₂ / g C_nH_2nO₂.

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;

Carbohydrates produce 4 kcal per gram, whereas fatty acid processing yields 9 kcal per gram of fat. While at rest, the human body takes 30% of the energy from carbohydrates and 70% from fat; so, this results in a combined production of carbon dioxide of

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.