It's like when you eat, you do not turn your food in pure gas, do you? And then the excreted/remaining mass will be buried etc (and will partly be turned into earthworms, nematodes, bacteria, moss, fungi, etc).
A person breaths out about 2.3 lbs of CO2 each day, which amounts to 0.6 lbs of carbon, all of which must have come from your food.
Meanwhile, a person defecates about 1 lb per day, about 75% of which is water. Of the residue, about 25-50% of it is bacteria, and the remainder the undigested portion of your food. Bacteria is typically accounted 53% carbon; the undigested fats, carbohydrates and proteins will vary a lot in percentage, but is 40% carbon for carbohydrates, 50% for proteins, and "more" for fats.
Which leaves a lot of variance, but something like 0.15 lbs of carbon exiting at as a solid seems like a reasonable estimate.
So the answer would be: you turn about 80% of the carbon in your food into CO2 gas.
But after reading several times your calculation, it seems legit. Thanks.
Then, as a conclusion, when you eat you delay 20% of your carbon footprint, and a fraction of these 20% may well be stored for a long duration (in sediments, or back into the food chain, etc).
Then again, similarly, the fallen leaves of trees may store a fraction of their carbon for a long duration.
Here:
> While this process naturally releases CO2 through the respiration (or breathing) of microbes that break down dead organisms, some fraction of plant carbon can remain underground for decades or even centuries. Together, land plants and soils hold about 2,500 gigatonnes of carbon—about three times more than is held in the atmosphere.
This was the point I wanted to make, probably worded awkwardly.