Title says it all. Just curious
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They don’t, much, but insofar as they do it’s just a result of the fluid warming or cooling to body temperature.
Your body weighs, if you’re a typical human, about 70 kg. (Perhaps more if you’re tall or overweight, or a bit less if you’re quite small.) Your body’s heat capacity is pretty similar to that of water, in no small part thanks to the fact that your body *is* mostly water. In other words, heating your body by 1 degree C takes about as much energy as heating 70 kg of water by 1 degree C would.
Suppose you drink a full liter of ice-cold water at 0 C (that is, it’s cold enough to freeze but hasn’t yet, or just thawed from ice). Your normal body temperature is about 37 C, but let’s assume you’re overheated at about 39 C (that is, you’re hot enough to be in some physical danger). Since you outweigh (and since your heat capacity is similar, out-heat-capacity) the water by a factor of ~70, it can cool you by ~1/70th the difference, or by about a half a degree C. That’s meaningful, but you’ll still be somewhat overheated even after chugging quite a lot of ice water.
Similarly, for hypothermia at 35 C body temp and a scalding hot drink at 60 C, you’d only warm up by about a third of a degree C. And again, that’s if you chug an entire liter of quite hot drink (I’m pretty sure actually chugging that much at that temperature would burn your mouth and throat quite severely). A safer-to-chug value of 45 C would only heat you by ~0.15 degrees C.
For comparison, the typical energy production of a normal human body is around 100 watts at rest, which corresponds to the liter of cold water equalling about 25 minutes of passive heat dissipation. But if you’re that overheated, you’re not passively dissipating heat: your body is actively engaging lots of mechanisms to dissipate it faster. Sweating can add a few hundred more watts of cooling, meaning that your cold drink on a very hot day is probably only contributing 5 or 10 minutes worth. Not nothing, but not a ton…if the temperature of the drink is all we care about.
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The much more important effect, especially in heat – and it’s May, so I’m guessing that’s what’s on your mind – is hydration.
Water takes a *lot* of energy to evaporate. Evaporating water takes ~5 times as much energy as it does to heat water from freezing to boiling – about 2,500 kJ/kg. So if your cold drink ends up being sweat that evaporates – and in hot temperatures it absolutely will – you’ll get ~(37/70) = ~0.5 degrees worth of cooling from heating it to body temperature, but ~(**500**/70) = ~7.1 degrees worth of cooling from sweating it out. And *that* is a lot – seven degrees C of body temperature is enormous.
So the real answer, in heat dry enough for sweat to evaporate readily, is “the temperature of your water barely matters, you could drink scalding hot water and still cool down by a lot”.
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