Sam, go read my links and try again. It is very very easy on this topic to fall into the trap of ignoring the fact that body heat by and large derives as a byproduct of normal activity.
Even Cecil falls into the trap.
It is like saying that the level in a dam must must rise and fall in a direct relationship with evaporation and rainfall, while ignoring the fact that there is a river flowing into it that keeps the dam full to overflowing 99% of the time.
Actually, no.
Your body, at 98.6, would diffuse away any appreciable heat in a matter of relatively few minutes, precisely because it is excess heat over and above you body’s native temperature. Under normal conditions, your body does not burn any extra calories to stay warm. The energy that keeps it warm is the waste heat from the chemical reactions it must engage in to stay alive
Excess thermal energy is of no more use to the body than the stuff dissolved in urine. It’s excess. It’s unusual for the human body to actually burn chemical fuel to generate heat, under normal conditions, as I said, you are already radiating waste. Under normal conditions, you have no use for more heat. You get enough by doing the important stuff, like pumping blood, synthesizing proteins, etc.
Think of it this way: you have to breathe to stay alive, even in a body temperature enrvironment. That oxygen is used in a form of extremely controlled combustion to produce ATP and other energy compounds that are mistly used almost immediately. The products of that combustion is exactly what you’d expect if you burned glucose (blood sugar, the primary fuel for most cells) or ketones [the primary fuel for heart muscle and brain neurons] directly. Glucose (C6H12O6) and ketones are made of carbon, hydrogen and oxygen. That’s it. Burning the carbon creates CO2, the hydrogen is oxidized to water.
When you eat sugar (sucrose, alpha-D-glucosido-beta-D-fructofuranoside, or C12H22012), it is split into glucose and fructose (as implied by the big word above) which is then converted to glucose. That glucose can be stored as fat or glycogen, converted to ketones, used as an ingredient in other reactions, and many other essential functions. These are the calories we count,
When you drink hot water, you’re just adding random thermal heat that can’t do appreciable biological work due to its entropy in relation to the system. It can’t really do anything but be radiated away.
I don’t know why you fell I’d consider it “puzzling to think heat is related to calories”. Before medical school, I was a molecular biologist, with more years of painful biochemistry classes than I care to discuss in polite company. I assure you that chemists use that fact at least 100 times a day. It’s not puzzling. It’s the definition!
Instead of fixating on ‘heat’, give more consideration to entropy and enthalpy - i.e. what makes the energy of a chemical bond different from that same energy as random thermal motion. Temperature is, by definition, random motion [unless you’re into extreme cryogenics or astrophysics] Crudely put: you can’t reverse randomness to do useful work, and doing useful work generates the random heat as an inevitable waste product.
[The gods of biochemistry are going to punish me for how much I simplified, but the same principles would apply if I went through all the equations, which would take about 300 pages more]
Oops! THis post never went through when I meant to post it. I came back hours later, found my answer still unsent, and hit ‘post’ without previewing again. My apologies to those who had already made my points -quite effectively!- in the interim
Someday, with the help of KP, I’m gonna get Cec to admit that column of his linked above is wrong.
It seems to me that there’s a slightly different way to answer the OP as well. Heating some foods (e.g., vegetables) makes their carbohydrates more available for quick breakdown into available glucose (i.e., raises the glycemic index of the food. This means that heating the food makes more of its stored energy available in the form of less-complex carbohydrates, right?
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The energy released in breaking down ATP is partly manifest as heat. But, that energy is also
used by our cells to preform all of our life functions.
When you start your car, gasoline is burned, and the parts get hot, but would you bother
having a car if that’s all that happened? If you used a blowtorch to heat your engine block,
would it run without other fuel?
It seems to me, plain common sense tells one that heating food, if anything, would decrease
the [sub]calories[/sub] because the chemical bonds have already started breaking, releasing energy
before reaching the body.
Okay, I’m ready, show me how wrong I am. 
Nope, and I hope I didn’t imply that in my post above.
It depends which “chemical bonds” you’re talking about. ATP production comes about from breaking the bonds in simple sugar molecules. Other molecules (proteins, fats, carbohydrates) must also first be broken down into smaller component molecules before the body can use them for ATP production, and this process of breaking down molecules is part of the process of digestion. So if you, say, heat a carbohydrate food and break some of the bonds so that they become simple sugars instead, you’ll save your body the energy otherwise spent in digestion – but the body will still get the same amount of ATP from the composite sugar molecules. It just will expend less energy to digest your heated material.
Now, if you fully burn that carbohydrate, releasing the energy before the body has a chance to get to it, you simply won’t get any nutrition at all from it. That’s a case where you’d be breaking the chemical bonds and releasing energy before it reaches the body.
Again, full disclaimer – i’m an undergrad, not a biologist. I hope if there are any errors in this post someone will let me know.
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I was being ironic…
Indeed, cooking does cause breakdown, making digestion somewhat easier. How much breakdown depends on the cook
Please don’t make me review the Krebs cycle!
After reading all these posts, I feel an urge to summarize what I have understood from them:
The body gets energy from breaking certain chemical bonds in the food. This, and only this, kind of energy is the one that the body can use for whatever task needs to be done. Among other (more important) things, the body can turn this energy into heat, should the need arise. The body cannot convert heat into the kind of chemical energy it uses as “fuel”.
Certainly it is true that the body will need less energy if it has to perform fewer or less strenuous tasks. In the rare condition that the body is turning chemical energy into heat in order to keep itself warm, any external source of heat will reduce the energy needed by the body. One example is drinking warm fluids, but putting on better clothes or moving to a heated room will have the same effect. If we consider motion instead of heat (just another form of energy which the body happens to be able to produce by using its “fuel”), riding a car will reduce my need for energy compared to running.
In both cases let’s not confuse reduced energy consumption with energy intake.
Did I miss anything?