Everyone knows that salty foods make you thirsty. But according to new research, when people increase their sodium intake in the long term, they actually drink less water. And that is not the study just surprising: High sodium levels also increase feelings of hunger, the authors say, which can suggest that high-salt diets contribute to weight gain.
Experts say this contraindictive discovery - that diet salt enhances appetite, but reduces thirst-upends more than 100 years of conventional scientific wisdom. The results are published this week as a set of two papers in the Journal of Clinical Investigation .
In the first work, German and American researchers report on 10 Russian cosmonauts who participated in flight simulation programs from 2009 to 2011. The men lived in a strictly controlled environment every month, so they were ideal for nutritional and metabolic research
The authors wanted to see what would happen if they gradually reduced the dietary intake of cosmonauts from 12 grams per day (similar to an average Russian diet) to 6 grams per day (the recommendation of most national health experts). The prevailing science suggested that men would be less thirsty and drink less water because their sodium levels decreased.
But that's not what happened. Instead, the men drank less water than they were at the high salt diet, suggesting that their bodies either conserve or produce more water and not rinse it with the salt, as was previously suspected.
Older author Jens Titze, MD, Associate Professor of Medicine and Molecular Physiology and Biophysics at Vanderbilt University, says the results were unexpected but not quite surprising.
"It makes sense that in a high-salt diet the body wants to prevent water loss," he says. "So the kidneys must find a way to increase the water content - and if you have more water in your body, you will be less thirsty."
The men also reported that they were hungry when their salt levels were higher, even though they got the same amount of calories and nutrients. That may be because it takes extra energy for the body to save water, explains Dr. Titze. "I think if we offered the cosmonauts more food, they would have exaggerated and gained weight," he says.
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In the second work, the researchers replicated their results in mice. In these experiments they found that mice on high-salt diets ate more food than those with 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 allows the kidneys to absorb fluid and prevent water loss while salt is excreted.
What is more, the collapse has been fueled by an increase in the glucocorticoide compounds associated with the development of diabetes, obesity, heart disease, and osteoporosis in humans. This is significant, says Dr. Titze, as the scientists have concentrated on how sodium contributes to high blood pressure.
"Our results suggest that there is much more to know," he says. When a high-salt diet triggers an increase in glucocorticoids, he says that it could predispose people to other chronic health problems - even in the absence of blood pressure changes. And it could potentially increase the risk of a 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 wisdom" is still true. "If you put salted peanuts in front of your customers, they will consume absolutely more drinks," he says. "But our research showed that over several months and even over 24 hours they will conserve more water and actually consume less."
In an accompanying commentary, Mark Zeidel, MD, professor of medicine at the Harvard Medical School, wrote that the new studies have balanced common beliefs about the question of how sodium and fluid levels in the body. They also show that an adaptation to dietary salt "changes the protein and fat metabolism and changes the eating and drinking habits", among other physiological changes in the body.
Learning more about these changes can help scientists develop new treatments for conditions such as high blood pressure and congestive heart failure, wrote Dr. Zeidel. Dr. Titze says that it can also help doctors to better understand the link between salt and weight gain.
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Of course, most of the sodium in the typical American diet does not come from table salt; It comes from restaurant meals and processed foods that also tend to be high in sugars, saturated fats and simple carbohydrates-so there is already plenty of reason to limit these in your diet. These new studies can still suggest something
Dr. Titze says that if his team theories hold down reductions in sodium content over the packaged food and catering industry could possibly prevent some of these harmful effects on metabolism and appetite. Until then, he takes a simple approach to reduce sodium and manage his weight: "If you eat less of anything, you will automatically eat less sodium," he says. "So my taking is to exercise a little more and eat less."
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