I’m sure you’ve heard that eating salty foods will make you thirsty. This assumption turns out to be false.
Although it has been known for a long time that an increased salt intake stimulates the production of more urine, scientists have actually never done research to determine the relationship between the amount of salt in a person’s diet and their drinking habits. It was always assumed that the extra fluid comes from drinking.
What appears to be good nutritional advice has now been found not to be true. An international group of researcher conducted a study during a simulated mission to Mars and found that the opposite is in fact true. Cosmonauts who ate more salt weren’t as thirsty, retained more water and needed more energy.
Scientists from the Max Delbrück Center for Molecular Medicine (MDC), Vanderbilt University, the German Aerospace Center (DLR) and colleagues around the world recently used the opportunity to put the old adage to the test during a simulated mission to Mars.
During a long space voyage, it becomes critical to conserve every drop of water. The relationship between salt intake and drinking could have an impact on space flight calculations. It would not be good if an interplanetary traveler died because he or she liked an occasional pinch of salt on their food. The real benefit of the simulation was however that it provided an environment in which every aspect of a person’s water consumption, nutrition and salt intake could be measured and controlled.
Natalia Rakova (MD, PhD) of the Charité and MDC and her colleagues did the research using two groups consisting of 10 male volunteers each. The participants were sealed into a simulated spaceship for two flights to Mars. The first group was monitored for 105 days, while the second was monitored for more than 205 days.
The groups were fed identical diets over the study periods, except that the salt level in the food was varied between groups. The results were not surprising in that it confirmed that eating more salt led to a higher salt content in urine, while the correlation between amounts of salt and overall quantity of urine was also as expected.
The surprise was that the increased quantity of urine was not caused by more drinking. The opposite was in fact true – a diet with more salt resulted in the subjects drinking less. Salt was triggering some unknown mechanism to keep water in the kidneys.
Prior to this study, scientists believed that the charged chloride and sodium ions in salt latched onto water molecules and dragged them into urine. The new results pointed to something completely different – salt stayed in the urine, but water moved back into the body and kidneys.
Prof. Jens Titze, MD of the University of Erlangen and Vanderbilt University Medical Center and his colleagues were perplexed by this result and wanted to find out what force could make water move back.
Experiments conducted with mice pointed to the possibility that urea might be involved. Urea is formed in the liver and muscles as a mechanism to shed nitrogen. The urea accumulated in the kidneys of mice, where it neutralizes the water-drawing force of chloride and sodium. Synthesizing urea does however use a lot of energy, which explains why mice eating a diet with a high salt content were eating more. More salt made them hungrier, not thirstier. The “cosmonauts” that had eaten a salty diet also complained about being hungry.
Scientists’ understanding of the function of urea in our bodies was changed by the outcome of this project. Prof. Friedrich C. Luft, MD of the Charité and MDC noted that urea is not merely a waste product as has been thought, but it turns out that it is an important osmolyte. Osmolytes bind to water and helps transport it, keeping water in when our bodies get rid of salt. Nature has apparently developed a mechanism that conserves water that would otherwise be carried away into the urine by salt.
The research results has changed the way scientists think about how our bodies achieve water homeostasis, the process of maintaining a proper amount and balance. This happens whether a body is being sent to Mars or not.
Titze explained that this process has to be seen as a combined activity of the muscle, liver and kidneys. He added that although the team did not look at blood pressure and other aspects of the cardiovascular system directly, it is clear that their functions are tightly interwoven with energy metabolism and water homeostasis.