I know this is a weird question, and I’m not asking for any particular reason, but: Is the process by which the body digests food analogous in any way to combustion, in the sense that the amount of energy released is the same? I’d be inclined to guess yes; combustion is, of course, an oxidation whereby the carbon molecules in the food combine with oxygen from air into carbon dioxide, releasing the chemical energy that was present in the fuel. I suppose digestion is, similarly, an oxidation, albeit probably a slower one?
Look up Bomb Calorimeter, it’s how food calories are calculated. They dry the food product, burn it in a special container surrounded by water and calculate the caloric value of the food by seeing how much it raises the temperature of the water.
So, on one level, burning and digesting are analogous with respect to energy content. Is this what you had in mind?
The word really would be metabolism because that’s where the energy is actually converted, not in digestion.
Metabolism can be aerobic or anaerobic. Aerobic metabolism is like combusion, as you said, for the reasons you said.
Anaerobic metabolism does not consume oxygen, and it doesn’t produce CO2 and water. It produces lactate and ethanol to generate energy, so it’s not really like combustion at all. However, lactate and ethanol themselves do eventually get processed into things that can be oxidized.
So in the final analysis, all food ends up getting converted into things that can be oxidatively metabolized, but some of the important energy-generating steps aren’t oxidative at all.
It is how food calories were calculated. The numbers you see now are calculations based on the Atwater System. Which does owe its numbers to bomb calorimeter testing. Wikipedia’s entry is pretty good:
To the OP itself, yes the processes are analogous.
How much we each, and there is individual variation, absorb the energy available in each food stuff, and how much energy is expended doing it, is a separate factor. The classic case is whole nuts such as almonds: we don’t absorb all of the calories there; and we use some energy up in the process.
Combustion will burn the fiber, which we don’t digest (although our gut flora can eat some of it, which you may have experienced), so that will cause a difference between the calorimeter and what we can get out of food.