The internet seems to think the answer is yes but we spent forever debating this and no one is 100% convinced yet so here we are, settling the bet, so to speak.
FAQ
1. What if it's bad or dangerous for other reasons besides dehydration?
Only dehydration matters for this market. Imagine the snow is clean, you have enough food, enough warmth, etc. Is it still a bad idea to eat snow? Specifically, does your hydration level go down by doing so?
2. What if the answer depends on the temperature?
If you have to go below the coldest temperature ever recorded on earth (-89.2 °C) before the answer is yes, then this resolves NO. If the answer is yes all the way up to 0 °C then this resolves YES. If the snow temperature below which eating snow dehydrates you is between those temperatures then this resolves proportionally. For example, if it's true for snow temperature below -44.6 °C then we'll call this half true and resolve to 50% YES.
3. Are you sweating at all?
No, it's just a friendly wager. Oh, you mean in the snow-eating scenario. Also no: if you were sweating then eating the snow would help you retain moisture simply by cooling you off and helping you sweat less. That would be cheating so we're stipulating that you're not sweating.
I won't resolve this without consensus among those of us debating out loud and will be transparent about our reasoning and recruit experts if there are reasonable objections in the comments. Note that I'm trading in this market myself.
Related questions
exhaled breath contains about the same amount by volume of co2 and water vapor. Burning carbs produces the same amount by volume of co2 and water vapor. Burning carbs roughly breaks even though perhaps your muscles will use more protein in the process and contribute to a slight increase in urea. I guess 5% of the carb calories is the extra protein breakdown, and one ninth of that protein becomes urea, and then 50x the urea is water excreted in urine, so the extra protein increases your water consumption by 5/18 as much as the breathing. So burning extra carbs is very slightly a net negative for hydration - each gram of carbs burned would be a loss of a fraction of a gram of water.. But Melting a gram of water only takes 0.08 kilocalories versus the 4 you get from burning a gram of carbs. That’s 50x so there’s zero chance that eating ice is a net negative for hydration. Even if you had to warm it up from absolute zero to body temp using 0.4kcal/g (with suitable insulation to avoid frostbite) you still come out way ahead.
I have almost no faith in the following but here's GPT4 doing a bunch of math to try to estimate the threshold temperature below which eating an ice cube could result in a net loss of hydration:
https://chat.openai.com/share/8f376127-eb89-4ca7-ae72-0320627c434d
Its bottom line is -231.8 °C which is cold enough to liquify the air. So that's a NO vote from GPT4, for whatever that's worth.
The case for YES: melting the snow takes calories which slightly increases your heart rate which slightly increases your breathing, from which you lose moisture. Other biological processes involved in maintaining your body temperature also involve moving more fluids around your body, and the more that happens the more ends up, say, in your bladder.
So I think theoretically you could eat an ice cube so cold that the water you lose in the process of melting it is more than you gain from the melted ice.
I guess I better add a clarification to the market description in case the answer is "it depends on the temperature of the snow". [now done]
@dreev I haven’t done any math on the calories it takes to melt it, but it also would depend a lot on your body temperature and the environment around you. If you are hot and sweating, cooling you down would help retain moisture instead of cost it.
@tr Good point. I think we should hold things like that constant. So we should say you're not sweating at all. The idea being that we don't want to bias the answer towards either yes or no. Let me add another FAQ item for this... [and done]
My prediction so far is that eating snow helps surprisingly little because you need calories to melt it and melting enough of it inside you to make much of a difference hydration-wise could risk hypothermia. Still, those are just tradeoffs -- it's not, I predict, that it's literally dehydrating you. If you're (a) dying of thirst, (b) have plenty of calories, (c) can keep warm enough, and (d) don't have a way to melt the snow outside your body, then you should 100% go cookie-monster on the snow.