Everyone knows that salty foods make you thirsty. But according to new research, when people increase their sodium intake long-term, they actually drink less water. And that’s not the study’s only surprising finding: High sodium levels also increase feelings of hunger, the authors say, which may suggest that high-salt diets contribute to weight gain.
Experts say this counterintuitive discovery—that dietary salt boosts appetite but decreases thirst—upends more than 100 years of conventional scientific wisdom. The findings are published this week as a set of two papers in the Journal of Clinical Investigation.
In the first paper, German and American researchers report on 10 Russian cosmonauts who participated in flight simulation programs from 2009 to 2011. The men were living in a tightly controlled environment for months at a time, so they were ideal for nutritional and metabolic research.
The authors wanted to see what would happen when they gradually decreased the cosmonauts’ dietary salt intake from 12 grams a day (similar to an average Russian diet) to 6 grams a day (the recommendation of most national health experts). Prevailing science suggested that the men would be less thirsty, and drink less water, as their sodium levels decreased.
But that’s not what happened. Instead, the men drank less water when they were on the high-salt diet—suggesting that their bodies were either conserving or producing more water, not flushing it out with the salt, as previously suspected.
Senior author Jens Titze, MD, associate professor of medicine and of molecular physiology and biophysics at Vanderbilt University, says the findings were unexpected, but not entirely surprising.
“It makes sense that on a high-salt diet, the body wants to prevent water loss,” he says. “So the kidneys have to find a way to increase water content—and if you have more water content in your body, you’re going to be less thirsty.”
The men also reported feeling hungrier when their salt levels were higher, even though they were getting the same amount of calories and nutrients. This may be because it takes extra energy for the body to conserve water, explains Dr. Titze. “I do think that if we’d offered the cosmonauts more food, they would have overeaten and gained weight,” he says.