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.
Combustion also burns some hydrocarbons we cannot digest, such as mineral oil.
(Apology to Ruken if the fiber they mention was meant to include this.)
Something that I wonder about is that human poop would certainly burn. So there has to be some significant % of caloric value that digestion doesn’t utilize.
A lot of feces is dead colon bacteria cells, which are basically indigestible compounds in food rearranged as living bacterial cells, so also as flammable as any other dead cells.
Normally, feces are made up of 75 percent water and 25 percent solid matter. About 30 percent of the solid matter consists of dead bacteria; about 30 percent consists of indigestible food matter such as cellulose; 10 to 20 percent is cholesterol and other fats; 10 to 20 percent is inorganic substances such as calcium phosphate and iron phosphate; and 2 to 3 percent is protein.
I recently heard a definition that opened my eyes: fiber is carbohydrate content we don’t digest. I think I knew that all along but it helped click that things like grass clippings, nutshells, and sawdust are still carbohydrates, even though they don’t count for dietary calories … in people. Of course, cattle, rabbits, and shipworms eat that stuff right up.
Cecil Adams on the subject.
Not everything we eat can be completely oxidized by the body. Fiber has been mentioned, but protein is worth discussing as well. You have probably heard that protein has 4 kilocalories per gram. That is a rough average of how much energy the human body can extract from protein. In fact there is closer to 5.5 kilocalories per gram of chemical energy available in protein. The difference is partly because it takes energy for the body to digest protein, and partly because breakdown products, mostly urea, contain energy that the human body can’t use, and partly because a small proportion of protein passes straight through the digestive system without being digested.
Nearly any time you have a compound that contains nitrogen, rather than gaseous N2, there’s energy tied up in that that could be released. Sometimes quite emphatically. Nitrogen really wants to be a gas.
I used to do a roughly similar activity with my 7th and 8th grade science students. They’d impale an almond on a pin, stand the pin in a cork and put the arrangement under a small beaker of water with a thermometer in it and light the almond on fire. When the nut was burned, they could calculate the number of calories released by the combustion. The weight of the water in grams times the rise in temperature (C) gave the number of calories. BUT, the calories that food is measured in is kilocalories, that is, 1000 calories each. So, if the nut raised 40 grams of water 5 degrees that would be 200 calories. But only .2 food calories (usually denoted by the capital C). Still, the point was made (I hope) that the calorie is not a measure of weight, as in “all those calories caused me to gain 10 pounds” but a measure of heat/energy, energy that a person could roughly get from that food via metabolism rather than fire. And, of course, food that is not converted to energy is frequently just stored by the body, thereby adding weight.