A new study has shown that eating salty food does not make you thirsty, but rather, it diminishes thirst. What eating salty food does do is increase the need for energy and therefore food.
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Everybody is familiar with the conventional belief that salty foods make you thirsty, increase the need to drink and lead to more urine production. However, after putting this theory to the test, researchers have shown the opposite to be true.
In a study carried out by an international group of scientists who simulated a mission to Mars, “cosmonauts” who ate a salty diet retained more water, became less thirsty and needed more energy, compared with control subjects.
The researchers, who were from the German Aerospace Center (DLR), the Max Delbrück Center for Molecular Medicine (MDC), Vanderbilt University and elsewhere around the globe, have published their findings in The Journal of Clinical Investigation.
Until now, no one has examined the relationship between salt intake and drinking habits in a long-term study. Scientists do know that increased salt consumption increases urine production, although it had simply been assumed that this was a result of drinking more water.
For the current study, scientist Natalia Rakova from MDC and colleagues sealed two groups of 10 male participants into a mock, cosmonaut-like environment for two simulated flights to Mars. The men were fed diets that were identical, aside form the level of salt they contained. The first group were assessed for 105 days and the second group for 205 days.
As expected, comparison of the two groups showed that a higher salt intake led to more salt in the urine and more urine production. However, the increased urine production was not a result of more drinking and, in fact, participants on the salty diet drank less.
The researchers found that salt was inducing a mechanism that would conserve water and produce urea, the synthesis of which is an energy-consuming process that triggers hunger, not thirst.
Scientists had previously assumed that charged sodium and chloride ions in salt recruited water molecules and dragged them into the urine to be expelled by the body. The results of this study showed that salt did stay in the urine and get expelled, but that water moved back into the kidneys and body.
Further analysis using mouse models showed that urea accumulated in the kidney and counteracted the water-grabbing force of sodium and chloride. Since urea production requires energy, a salty diet caused the mice to eat more, without making them thirstier. Similarly, the men in the cosmonaut study who were on a high-salt diet complained of hunger.
The results suggest the urea is more than simply a waste product, but an important osmolyte that binds to water and helps transport it, in order 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,”
Co-author Friedrich Luft, MDC.
The findings also introduce a new take on how the body achieves water homeostasis – the amount and balance of water in the body, with the implication being that the process involves activity of the liver, muscles and kidneys.
Furthermore, senior author Jens Titze (Vanderbilt University Medical Center) points out that although cardiovascular aspects such as blood pressure were not directly examined, “it's also clear that their functions are tightly connected to water homeostasis and energy metabolism."