Salty diet reduces thirst, increases hunger

A salty diet causes people to drink less water while increasing hunger due to a higher need for energy, suggests new research.

The findings, published in the Journal of Clinical Investigation, are based on a study carried out during a simulated mission to Mars.

"Cosmonauts" who ate more salt retained more water, were not as thirsty, and needed more energy, the results showed.

What does salt have to do with Mars? Nothing, really, except that on a long space voyage conserving every drop of water might be crucial.

The researchers said that the findings should be applicable whether a body is being sent to Mars or not.

In the study carried out by Natalia Rakova from Max-Delbrueck Centre for Molecular Medicine, Berlin in Germany and her colleagues, the participants 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 and 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 confirmed that eating more salt led to a higher salt content in urine. Higher amounts of salt also increased overall quantity of urine.

But the increase was not due to more drinking -- in fact, a salty diet caused the participants to drink less. Salt was triggering a mechanism to conserve water in the kidneys.

"This water-conserving mechanism of dietary salt excretion relies on urea transporter-driven urea recycling by the kidneys and on urea production by liver and skeletal muscle," the researchers said.

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.