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Many mountain hikers recommend drinking hot fluids, because they provide a lot of energy. According to the discussion "Does eating snow help dehydration?", the actual calorie gain is minimal, however there are calories available practically immediately.

However, how hot can water/fluid be to be able to safely drink it (how many degrees)? Too hot water would burn your lips, but if the water is not hot enough to burn the lips, is it safe to swallow? Does the large amount of hot fluid in the stomach pose any threat to the body, or I can assume that anything I can swallow without getting burned is safe for me?

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    I find this a strange question. Pragmatically speaking don't you already have experience with this? People regularly drink hot tea, coffee, chocolate, etc. that is as hot as comfortable without ill effect. When you say "burn" do you mean more than uncomfortably hot? – Mr.Wizard Aug 11 '13 at 10:06
  • @Mr.Wizard I've made a misswording, I'm asking about temperature in degrees, 'hot' is a vague term. For someone hot is 40 degrees, for someone else 60... – Danubian Sailor Aug 11 '13 at 16:24
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    @DanubianSailor I think there you answered your own question, the "hotness" of the drink will be based on your experiences with hot drinks. So to try and get a fixed temperature would not work. – AquaAlex Jan 26 '16 at 10:51
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    (It is similar to asking what temperature is a hot shower.) – AquaAlex Jan 26 '16 at 10:52
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I think the answer is highly personal, as an avid coffee-drinker, hot is for me what scalding might be for someone else. But if I am to give some kind of benchmark, I would say 45°C is a pretty good temperature to aim for.

Not as hot as to scald your mouth, but hot enough to give you some warmth if drank in sufficient quantities. But if you want to carry "optimal"-heating potential, I suggest you try to keep your water boiling hot, and mix it with colder water when drinking. Since you don't have to carry around 4 litres of lukewarm water.

Hopefully this will shed some light on your question.

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Many mountain hikers recommend drinking hot fluids, because they provide a lot of energy.

Not really. The ideal core body temperature has traditionally been stated as 37.7°C. We reduce human to jar of 80kg of fluid content. Imagine what happens if you pour 0.2kg of 45°C fluid into the "human jar". It is pointless. The drank mass and the available temperature difference is futile.

However, as @Chris H mentioned: the fluid goes directly to the core and its temperature could be higher than the 45°C. This makes the energy from hot drinks slightly less insignificant.

So it does not provide energy because it is hot. The most proper reason why to drink hot drink is "placebo" effect. Because of that and obvious security reasons I would suggest the following rule:

Aim for the lowest temperature that makes you feel warm.

What is pretty consistent with great answer from Marcus Wigert.

To obtain energy from hot drinks, it is suggested (thanks to @Chris H for pointing that out) to put directly a source of energy into the hot drink. In my personal experience, I would recommend combinations of following:

  • unflavoured whey protein concentrate
  • cocoa powder
  • butter
  • coconut oil
  • honey (in extreme situations when immediate energy is required)
  • (1) that heat is delivered directly to the core, where it's most needed; (2) 45C is way too cool this paper discusses avoiding burns and reckons that most people drink hot drinks around 60C. What matters is the temperature difference between drink and human, 3x what you assume. – Chris H May 17 at 6:39
  • @ChrisH Fair points! To consider your points in the model I proposed, we should reduce human jar content (about 15%?) and slightly increase the temperature of the incoming fluid. So we can get somewhere to +0.1°C (if the transfer will be done instantly - it will not). Seems still more like a placebo to me. But thanks for the comment. – matousc May 17 at 6:48
  • Yes, I made it about 0.2 C for one drink by a conservative estimate, so around 1/10 of the drop in body temperature that leads to mild hypothermia. Health authorities do recommend hot drinks as well as energy snacks, so I'm looking into the research. – Chris H May 17 at 7:02
  • I found a study and it supports your view though hot drinks are also a good way of delivering nutrition without cooling. They also measured skin temperature to assess warming despite having internal temperature data – Chris H May 17 at 7:10
  • @ChrisH Great work! I will update my answer to match your comments. Thanks for the notes. I admire how curious you are. – matousc May 17 at 7:37

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