# Calories burnt maintaining body temp

If you put two physically (and metabolically) similar people in rooms, one 70 degrees and the other 60, how many more calories would the person in the lower temp room burn percentage wise?

Assume, the person in the colder room doesn’t shiver much or at all, he merely feels colder than the person in the warmer room.

I think any answer depends on how much insulation (read: clothing) the two have. If both are equipped with sweaters, pants, socks, hats, mittens, etc, there’s hardly going to be any difference at all.

Assuming no insulation(fun!), and using http://hyperphysics.phy-astr.gsu.edu/Hbase/thermo/bodcon.html#c1 as a guideline, I get 15.17W of conduction heat transfer for the 70F room temperature, and 20.5W for the 60F room. A difference of 5.3W, or 1.3cal/sec (that’s “calories” with a little c; for food calories, divide by 1000)

Of course, this is just conductive heat transfer. When you factor in convection due to buoyancy (or forced convection in the case of a breeze) and sweat evaporation, it increases that number.

That is a very interesting link, but all it really tells me is the amount of thermal energy that is being absorbed or expelled by a human body as the ambient temperature changes. It doesn’t tell me how much chemical food energy the body uses to achieve thermoregulation. For example does it require more energy to cool the body or to heat it?

Assuming 100% digestive efficiency, I would guess that the two are pretty much the same, assuming you’re just trying to find the delta in energy input between the two thermal environments. EnergyIn=EnergyOut. Any difference in expelled heat (Energy out) is going to have to be made up with exactly the same calories used.

However, digestive efficiency does not appear to be 100% efficient, per this thread.

If, for example, digestive efficiency is only 40%, you can multiply my original answer (0.0013 Cal/sec) by 1/.4 to get 0.00325 Cal/sec = 11.7 Cal/hr.

Hopefully, Stranger will be along to tell me just how wrong I am.

MILITARY RATIONS

The roughly 3060 extra Calories a day isn’t put in there to help winter soldiers get fat.

So you’re saying it takes one Calorie of food to either generate one Calorie of thermal energy inside the body, or expel one Calorie of thermal energy from the body?

That seems amazingly efficient. The body is constricting vessels, contracting muscles, pumping hormones and whatnot, and all these things to generate and preserve heat and all the energy you feed into the system turns into heat? Isn’t any of it transformed into another form of energy?

Also, why should it be that expelling heat costs the same as generating it. Physiologically its a very different process involving revving up the sweat glands. Won’t some of the food energy be lost as another form of energy? A droplet of sweat falling from your face carries some potential energy, wasn’t that energy originally a food calorie spent moving the fluid to a pore and pushing it out?

Lastly, the forces of evaporation, thermal conduction, convection and radiation are all either making the body’s task of heat regulation easier or harder, depending on the temperate differentials. Can it be that the relevant burden or benefit is equal so that it will always take one Calorie of food to make or move one Calorie of heat from the body regardless of the ambient temperature?

I think it’s a lot more complicated than measuring the heat transfer. In a 70 degree room your skin will be warmer than in a 60 degree room. Your body does work in constricting your blood vessels, but it saves energy in heat loss. Until you get to the point of shivering you may actually save energy in a cold room.

The same in a hot room. Sweating cools by evaporation, which varies in efficiency with the humidity in the environment. The whole question of how much does it take to maintain body temperature is subject to so many variables that a simple answer is impossible.

Well, that assumes 100% efficient digestion, which isn’t correct, as I mentioned.

Also, when I was talking about expelling heat, it was in the context of keeping the body warm, per the OP. Cooling through evaporation much easier energy-wise, I believe.

What other form of energy? Electricity? Sonic? Nuclear? Assuming the guy is just sitting in a chilly room, you’ve got heat loss from skin temperature/sweat evaporation and heat loss from respiration. Sure, there are plenty of other energy expendiatures for the body, but I don’t know if any of them are temperature dependent (at least for 60F ambient vs. 70F). You’re right, you have potential energy lost from moving/expelling sweat, but what’s the difference in sweating between someone sitting in 60F vs. 70F?

Anyway, this is all just a WAG on my part. I’m sure the actual answer is nowhere close, especially seeing Squink’s winter rations figure. However, I imagine a lot of that has to do with actually doing work in cold weather as well as being exposed to thermal conditions far worse than the OP (freezing rain, wind, etc.)

I’d be interested to see someone else’s take on it.

Search on beer and icewater and you will find debates on a subject that is closely related and in which some cites are given. The short answer I believe is that it depends. There will be no simple answer because the body has various strategies for maintence of core temperature in cold, only one of which is increasing metabolic rate. They other biggie is (in effect) increasing insulation by vasoconstriction to reduce thermal loss, resulting in maintenance of core temperature without burning additional calories at all.

Nitpick, but it takes calories to constrict the vessels. I simply can’t believe there is no answer to this question. No one knows how many calories the body regularly expends to maintain a constant temperature?

The mechanism of thermogenesis in mammals without brown fat is still a bit of a mystery. Last I heard it probably has something to do with futile cycling of the Na/K ATPase, but without a precise mechanism, it’s hard to know exactly what to measure.

It just seems odd that every single Calorie of food your body burns is converted into thermal energy. Won’t some of that energy end up as a free radical bouncing around the body, or in used in a non exothermic metabolic reaction?