A first of its kind study has revealed that salty food diminished thirst and increased hunger, caused by a higher need for energy.
It is commonly believed that salt in diet makes people more thirsty and less hungry.
In a simulated mission to Mars, an international group of scientists found that the exact opposite was true. Cosmonauts who ate more salt retained more water, were not as thirsty, and needed more energy.
The studies were conducted by Dr Natalia Rakova from the Max Delbrück Centre for Molecular Medicine and colleagues and involved male participants who were divided into two groups of 10. They were sealed into a mock spaceship for two simulated flights to Mars. One group was examined for 105 days.
The second group's 'flight to Mars' covered 205 days.
Both the groups were given identical diets and later they were given three different levels of salt in their food.
As expected, the results confirmed that higher salt intake corresponded with higher salt content in urine. The researchers also found a correlation between amounts of salt and overall quantity of urine.
Scientists had known that higher salt intake stimulated the production of more urine - it had simply been assumed that the extra fluid came from drinking.
However, the increase was not due to more drinking - in fact, a salty diet caused the subjects to drink less, which triggered a mechanism to conserve water in the kidneys.
The results, published in the Journal of Clinical Investigation, showed that salt stayed in the urine, while water moved back into the kidney and body.
According to the prevailing hypothesis prior to the study, charged sodium and chloride ions in salt grabbed onto water molecules and dragged them into the urine. But the new results showed salt stayed in the urine, while water moved back into the kidney and body. Professor Jens Titze MD of the University of Erlangen and Vanderbilt University Medical Center and his colleagues found this quite puzzling. Titze wondered what alternative driving force could make water move back.
Experiments in mice had indicated that urea might be involved. The substance is formed in muscles and the liver as a way of shedding nitrogen. In mice, urea was accumulating in the kidney, where it counteracted the water-drawing force of sodium and chloride.
But synthesising urea took a lot of energy, which explained why mice on a high-salt diet were eating more. Higher salt did not increase their thirst, but made them hungrier. The human "cosmonauts" who had received a salty diet also complained about being hungry.
According to one of the researchers Friedrich C Luft from Delbrück Center for Molecular Medicine in Germany, urea was not solely a waste product as had been assumed.
''Instead, it turns out to be a very important osmolyte -- a compound that binds to water and helps transport it. 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 added.