The idea that salty foods make people thirstier has been around for a long time. A new study, however, suggests that this is nothing but a myth, as salty diets can actually make people hungrier.
The study was carried out during a simulated mission to Mars by an international group of scientists.
The results were published in the April issue of The Journal of Clinical Investigation.
Salty Foods Cause Hunger, Not Thirst
During the mission, the cosmonauts who ate saltier foods retained more water and did not get thirsty. Instead, they needed more energy.
There has never been a longitudinal study that confirmed the connection between the amount of salt consumed by a person and the drinking habits, but scientists have always known that people who eat saltier foods produce more urine.
It was previously thought that the extra fluid simply came from drinking, but researchers from the German Aerospace Center (DLR), the Max Delbrück Center for Molecular Medicine (MDC), Vanderbilt University and their colleagues have proven otherwise.
The study was not inherently connected to the simulated Mars voyage, except for the fact that such a long trip would require conserving water as much as possible.
The scientific interest in conducting this research on the cosmonauts in the simulated mission was that it provided an environment where water consumption, dietary habits, and overall nutrition can be easily measured and controlled.
The research was conducted by Natalia Rakova and her colleagues. As part of the studies, two groups of 10 men were sealed into a mock spaceship for two simulated flights to Mars.
The first group was analyzed for 105 days, and the second one for 205 days. The two groups followed the same diets, except for several weeks during which they were fed different amounts of salt.
The results of the research confirmed that a higher amount of salt does increase the urine production. However, the second conclusion took the scientific community by surprise.
The urine was not produced due to drinking more water. In fact, salt triggered a water-conserving mechanism in the kidneys. Researchers once assumed that chloride ions and charged sodium in the salt dragged water molecules into the urine. The new study, however, showed that salt remained in the urine, while water went back to the kidneys.
"The physiological response of this water conservation process during salt excretion can be summarized as natriuretic-ureotelic regulation for endogenous free water accrual within the renal urine concentration process to prevent body water loss. In addition, salt can induce a glucocorticoid-driven catabolic state with increased urea osmolyte and metabolic water generation," noted the research.
High Levels Of Urine
While there is not a definite answer to this puzzling mechanism, researchers believe that the urea could be involved. Experiments in rodent subjects showed that urea was accumulating in the kidney, where it counterbalances chloride and sodium, stopping them from turning the water into urine.
Instead, as synthesizing urea requires high levels of energy, mice on a more salty diet were found to have an increased appetite. The higher levels of salt in their diet didn't cause thirst but hunger. This result was confirmed in the simulated Mars voyage, as part of which cosmonauts on the salty diet complained of being hungry.