Eating salty food diminishes thirst while increasing hunger, due to a higher need for energy, according to new study carried out during a simulated mission to Mars.
The study found that "Cosmonauts" who ate more salt in the simulation retained more water, were not as thirsty and needed more energy.
Researchers, including those from the German Aerospace Centre (DLR) in Germany, sealed two groups of 10 male volunteers into a mock spaceship for two simulated flights to Mars.
The first group was examined for 105 days, the second over 205 days. Both groups had identical diets except that over periods lasting several weeks, they were given three different levels of salt in their food.
The results confirmed that eating more salt led to a higher salt content in urine.
The increase was not due to more drinking - in fact, a salty diet caused the subjects to drink less. Salt was triggering a mechanism to conserve water in the kidneys, researchers said.
The prevailing hypothesis had been that the charged sodium and chloride ions in salt grabbed onto water molecules and dragged them into the urine, researchers said.
However, the new results showed something different: salt stayed in the urine, while water moved back into the kidney and body.
"What alternative driving force could make water move back?" said Jens Titze, professor at University of Erlangen in Germany.
Experiments in mice hinted that urea might be involved. This substance is formed in muscles and the liver as a way of shedding nitrogen, researchers said.
In mice, urea was accumulating in the kidney, where it counteracts the water-drawing force of sodium and chloride.
However, synthesising urea takes a lot of energy, which explains why mice on a high-salt diet were eating more, researchers said.
Higher salt did not increase their thirst, but it did make them hungrier. Also the human "cosmonauts" receiving a salty diet complained about being hungry.
"It is not solely a waste product, as has been assumed. Instead, it turns out to be a very important osmolyte - a compound that binds to water and helps transport it," said Friedrich C Luft of Max Delbruck Centre for Molecular Medicine in Germany.
"Its function is to keep water in when our bodies get rid of salt. Nature has apparently found a way to conserve water that would otherwise be carried away into the urine by salt," Luft said.
The study was published in The Journal of Clinical Investigation.