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.
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In the second paper, the researchers replicated their findings in mice. In these experiments, they did find that mice on high-salt diets ate more food than those on low-salt diets. They also found that high-salt diets were associated with a breakdown of muscle protein. The protein was converted into urea, a chemical that enables the kidneys to reabsorb fluid and prevent water loss while salt is excreted.
What’s more, the breakdown process was fueled by an increase in glucocorticoids—compounds that, in humans, have been linked to the development of diabetes, obesity, heart disease, and osteoporosis. That’s significant, says Dr. Titze, because scientists to-date have mainly focused on how sodium contributes to high blood pressure.
“Our findings suggest that there is much more to know,” he says. If a high-salt diet triggers an increase in glucocorticoids, he says, it could predispose people to other chronic health problems—even in the absence of blood pressure changes. And it could potentially raise the risk of metabolic syndrome, a combination of three or more risk factors for heart disease and diabetes.
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Dr. Titze says that, when it comes to the short-term effects of salty foods, “bartender’s wisdom” still holds true. “If you put salted peanuts down in front of your customers, they’re absolutely going to consume more drinks,” he says. “But our research showed that, over several months and even over 24 hours, they’re going to conserve more water and actually consume less.”
In an accompanying commentary article, Mark Zeidel, MD, professor of medicine at Harvard Medical School, wrote that the new studies challenge common beliefs about how sodium and fluid levels are balanced in the body. They also demonstrate that an adjustment in dietary salt “changes protein and fat metabolism, and alters eating and drinking habits,” among other physiological changes in the body.
Learning more about these changes may help scientists develop new treatments for conditions like high blood pressure and congestive heart failure, Dr. Zeidel wrote. Dr. Titze says it may also help doctors better understand the connection between salt and weight gain.
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Of course, most sodium in the typical American diet doesn’t come from table salt; it comes from restaurant meals and processed foods that also tend to be high in sugar, saturated fats, and simple carbs—so there’s already plenty of reason to limit these in your diet. These new studies may suggest one more.
Dr. Titze says that if his team’s theories hold up, reductions in sodium content across the packaged food and restaurant industries could potentially prevent some of these harmful effects on metabolism and appetite. Until then, he takes a simple approach to reducing sodium and managing his weight: “If you eat less of everything, you will automatically eat less sodium,” he says. “So my take is to exercise a bit more and eat less in general.”