We’ve all heard it: eating salty foods makes you thirstier. But what sounds like good nutritional advice turns out to be an old-wives’ tale. In a study carried out during a simulated mission to Mars, an international group of scientists has found exactly the opposite to be true. “Cosmonauts” who ate more salt retained more water, weren’t as thirsty, and needed more energy.
The studies were carried out by Natalia Rakova (MD, PhD) of the Charité and MDC and her colleagues. The subjects were two groups of 10 male volunteers sealed into a mock spaceship for two simulated flights to Mars.
The first group was examined for 105 days; the second over 205 days. They had identical diets except that over periods lasting several weeks, they were given three different levels of salt in their food.
The results brought some surprises – over the long term. Short-term, the results confirmed the bartender’s wisdom: salted peanuts will increase your beverage sales. Eating more salt also leads to a higher salt content in urine – no surprise there. Nor was there any surprise in a correlation between amounts of salt and overall quantity of urine. But the increase was not due to more drinking – in fact, a salty diet caused the subjects to drink less. The reason: Salt was triggering a mechanism to conserve water in the kidneys.
Before the study, the prevailing hypothesis had been that the charged sodium and chloride ions in salt grabbed onto water molecules and dragged them into the urine. The new results showed something different: salt stayed in the urine, while water moved back into the kidney and body. This was completely puzzling to Prof. Jens Titze, MD of the University of Erlangen-Nürnberg and Vanderbilt University Medical Center and his colleagues. “What alternative driving force could make water move back?” Titze asked.