As every thirsty person knows, swallowing your spit doesn't do the trick. But why is that? Saliva is about 98 percent water, so shouldn't it be hydrating?
Not necessarily, said Dr. Len Horovitz, an internist at Lenox Hill Hospital in New York City.
"First of all, let's think about what saliva is," Horovitz said. "It's a very concentrated fluid that has proteins and enzymes. It's much more concentrated than water." [The 7 Biggest Mysteries of the Human Body]
Freshwater is much less concentrated with salts and other solutes than the fluids in the human body, and that's why it's so refreshing to drink when you're thirsty, he said. That's because of osmosis, when water flows toward higher concentrations. If you drink a less-concentrated substance, your body can absorb its fluids and become hydrated. But if you drink a liquid with a higher concentration than your body's, the water in your body will flow toward the substance, and make you dehydrated.
The fluids in many parts of the human body have the approximate concentration of the saline you pick up at the drug store — think of artificial teardrops or saline nasal sprays.
Saliva, however, is more concentrated than saline, so it would make you thirstier if you drank it, Horovitz said. That's because it would cause the fluids in your body to flow toward the concentrated saliva, and not toward your dehydrated cells.
"In other words, saliva is not watery enough," Horovitz told Live Science.
If someone is severely dehydrated, health care workers will give him or her saline through an intravenous (IV) drip, especially if that person can't take liquids orally.
"When you want to hydrate somebody, you give them saline, because we're made of saline," Horovitz said. The saline solution also ensures that a person's cells won't burst. (If a person is given pure water through an IV, the water would rush into the concentrated cells, making them so engorged that they could pop.)
If doctors are treating patients orally, they "usually give them water or something like Gatorade, which has electrolytes," Horovitz said.
In fact, when you're thirsty, your saliva gets even more concentrated because your body has less water to spare, he said.
But saliva isn't the most concentrated fluid in the body. For instance, blood is more concentrated, and so is pus, Horovitz said. [How Much Blood Is in the Human Body?]
Saliva may not satiate thirst, but it serves a number of useful purposes. It lubricates food to make it easier to chew and digest. It also contains enzymes that help break down carbohydrates, fats and proteins. In addition, it has antibacterial qualities, and preserves periodontal health by coating the gums and teeth, Horovitz said.
He noted one way for people to gauge whether they're hydrated.
"The best way to tell if you're dehydrated is to look at the color of your urine," Horovitz said. When your body is lacking fluids, it goes into survival mode and holds on to all of its water, meaning your urine will be very concentrated and less watery, he explained.
"If your urine is very dark and looks like iced tea, you're probably very dehydrated," Horovitz said. "If it's pale yellow, your hydration is probably very admirable."
Live Science newsletter
Stay up to date on the latest science news by signing up for our Essentials newsletter.
Laura is the archaeology and Life's Little Mysteries editor at Live Science. She also reports on general science, including paleontology. Her work has appeared in The New York Times, Scholastic, Popular Science and Spectrum, a site on autism research. She has won multiple awards from the Society of Professional Journalists and the Washington Newspaper Publishers Association for her reporting at a weekly newspaper near Seattle. Laura holds a bachelor's degree in English literature and psychology from Washington University in St. Louis and a master's degree in science writing from NYU.