Does sucking your own sweat help against dehydration?

No, drinking your own sweat would not stave off dehydration. Since sweat is so salty, it would be like drinking ocean or salt water, which dehydrates you more. If you are in a situation in which there is no fresh water, or in which you cannot make a fire and boil water, your best option is to drink your own urine.

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I must disagree with the above answer:

Drinking your own sweat (if you could collect it in sufficient quantity) would indeed help protect one against dehydration, and would certainly not be harmful as would drinking ocean water. The ionic strength of sweat is much less than that of intra- and extracellular bodily fluids, making it hypoosmotic. This means that in 1ml of sweat there is much less salts than in 1ml of blood plasma, etc. Drinking your own sweat would replace water that had been lost by the body.

Alternatively, drinking your own urine would not be a good idea if one were facing dehydration, as your body concentrates the salts in urine, making it hyperosmotic. This means that in 1ml of urine there is much more salt than in 1ml of blood plasma, etc. Drinking your own urine would replace salts that had been removed by the body.

Do not confuse the above statements as saying that no salts are lost in sweat, as some salts are lost, many being important ions necessary for proper bodily function. This is why someone who sweats profusely (athletes) needs to consume beverages like Gatorade rather than pure water, to replenish ions. I guess if an athelete were to drink their own sweat they would replenish the ions as well as the water lost, a good thing.