Eating more salt has long been associated with feeling thirstier and drinking more water, but a new study of Russian cosmonauts training for life in space has turned that well-established idea on its head.
The research found that within 24 hours, increased salt intake actually made the trainees less thirsty, as their bodies started to conserve and produce their own water.
Not only could that help in preparing diets for people travelling in space, it could also give us insights into improving treatments for obesity, diabetes, and heart disease, according to scientists from the US and Germany.
The two new studies, one on the trainee cosmonauts and one a follow-up experiment on mice, build on decades of research by Jens Titze, from the Vanderbilt University Medical Center of Tennessee.
Titze first started studying trainee cosmonauts in 1994, reports Gina Kolata at The New York Times, noticing that sodium levels and urine production were not as closely linked as they should be.
Scientists have thought that taking on more salt (sodium chloride) increases thirst, and then increases the amount of water we drink, which does the important job of balancing out the levels of sodium in the body.
However, that's not what Titze was seeing in his time with trainee cosmonauts, and the two latest studies were designed to take a closer look at what was happening.
In the first, ten cosmonauts in a simulated long-term mission to Mars were given a controlled diet, with salt intake varying between 6, 9 and 12 grams a day.
Many of the findings were as expected: when the crew ate more salt, they excreted more salt. Urine volume increased and sodium levels in the body remained steady, which is all as it should be.
However, the fluid intake was the surprise – the crew started drinking less as their salt intake increased.
"The body most likely had generated or produced water when salt intake was high," Titze told The New York Times.
To add to the mystery, the trainees reported feeling hungrier when levels of salt in their food were increased, even though they were eating the same amount of food overall.
The second study, on mice, was put together to try and work out why the cosmonauts had begun drinking less and feeling hungrier when they were given more salt, and sure enough the mice also drank less water in response to more salt.
What seems to be happening in both the cosmonauts and mice is that the increased levels of salt increase the levels of glucocorticoid hormones produced in the body – hormones which influence both metabolism and the immune system.
These glucocorticoids apparently break down muscle protein which is converted into urea. Urea usually helps get waste out of the body, but in this case, helps to retain water.
Scientists had previously thought that charged sodium and chloride ions in salt dragged water molecules into the urine, but the urea seems to block this from happening.
In other words, the kidney swings into action when salt intake is high to make sure a balance of water is maintained – so you don't necessarily need to swig more water with your salted peanuts, because your body does a similar job for you.
"Nature has apparently found a way to conserve water that would otherwise be carried away into the urine by salt," explained one of the researchers, Friedrich C. Luft from the Max-Delbrueck Centre for Molecular Medicine in Germany.
That process of freeing up water for the body also needs more energy, which explains the feelings of hunger. In the mice test, the animals ate 20-30 percent more food on the high-salt diet.
That's all the science, so what can we learn from it?
We might have to re-examine the links between high-salt diets and weight gain, for start. If we're not actually drinking more, the basis for the argument that high-salt diets lead to increased weight through more fluid intake may be misguided.
What's more, high glucocorticoid levels are linked with conditions like type 2 diabetes and muscle loss, so we need to understand more about the dangers of having too much salt in our diet, something doctors already warn about.
The research shows the function of urea to be more important than previously thought, and offers new insight into the process of water homeostasis in the body, the way that water levels are kept in balance.
It also gives us more pointers about how much salt and water we're going to have to pack for a mission to Mars – which was where this research started in the first place.
Most significant though, is the news that salt may not make us drink more after all – so it could be time to rewrite the textbooks again.
The findings have been published in two papers in The Journal of Clinical Investigation, here and here.