Beverages of any type generally contain some measure of water. However, in the case of some liquids, the non-water components take more water from the human body to filter out than the water contained in the beverage.

The most-known example of this is salt-water from the ocean - there is too much salt so the body tries to use water from the body to "water down" the salt water.

Given a beverage with unknown additives, how would one determine if the beverage would actually hydrate or dehydrate a person?

If you knew the ingredients (and the amounts), could you tell that way?

comments seem to indicate that it depends on the solutes and tonicity of the solution: are there any other factors to be aware of? What (common) solutes will have the largest affect on whether the human body will be hydrated and why?

if you need "common" to be defined, consider it defined as: contained in many[more than 3] store-bought beverages

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    $\begingroup$ It will depend on the tonicity of the fluid, I guess. Sea water is hypertonic while distilled water is hypotonic. Hypotonic fluids would move into the body faster due to osmosis. By the same reasoning hypertonic fluids would pull water from the body into digestive tract till sufficient electrolytes have been absorbed. $\endgroup$ – One Face Mar 2 '15 at 22:40
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    $\begingroup$ @CRags Thanks for the info! I guess what I'm looking for is how to measure the tonicity of a fluid. A quick google search didn't come up with much, but I didn't have much time to look. I'll have to check it out later. $\endgroup$ – DoubleDouble Mar 2 '15 at 23:04
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    $\begingroup$ It depends on the solutes. Glucose can be metabolized. Some things inhibit vasopressin (anti-diuretic hormone) excretion, others stimulate it, influencing the outcome. This question might be too broad. $\endgroup$ – anongoodnurse Mar 3 '15 at 0:40
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    $\begingroup$ Yes it depends on the solutes; it is not just tonicity. Note that marine fishes do drink seawater; the salt is actively filtered out. @anongoodnurse You can expand on your comment and post it as an answer. So the question "Is it possible to know if some beverage is hydrating or not" can be at least answered by "no". $\endgroup$ – WYSIWYG Mar 3 '15 at 5:30
  • $\begingroup$ I think at the least a list of things that will determine if a drink will cause hydration or dehydration can be provided for this question. It is an interesting one after all! $\endgroup$ – One Face Mar 3 '15 at 13:37