Brush aside conventional wisdom on salt — eating food that’s full of sodium doesn’t lead to dehydration, says a new study.
In fact, the consumption of salty foods has the opposite effect. Instead of drying one out, eating more sodium results in the body retaining more water while decreasing thirst and fueling hunger.
That’s what a collaborative group of researchers from Germany and the United States discovered when they monitored the physiologic results of salt consumption on a group of 20 men who participated in a simulated trip to outer space.
“For some reason, no one had ever carried out a long-term study to determine the relationship between the amount of salt in a person’s diet and his drinking habit,” report the study authors from the Max Delbruck Center for Molecular Medicine in Germany.
They note that “the idea that increasing salt intake increases drinking and urine volume is widely accepted,” yet scientific evidence had never confirmed the claim. Now it appears the medical community — and food consumers everywhere — should revise their preconceptions about sodium intake.
For the study, the researchers tracked two groups of 10 men who took part in a simulated voyage to Mars. The monitoring period lasted up to 205 days, and the researchers carefully delivered different levels of salt in the study participants’ food.
They found that some pieces of conventional wisdom about salt consumption held true. For instance, eating more salt results in elevated sodium content in a person’s urine, and people eating more salt produce more urine overall.
But the big surprise came when the researchers took a closer look at the reason behind the higher urine content. It wasn’t a result of drinking more water; the researchers found that high-salt dieters consumed less water than normal. Instead, they found that “salt was triggering a mechanism to conserve water in the kidneys.”
In a related study on mice, the researchers learned that urea, a substance formed in muscles and the liver, was gathering in the kidneys, which appeared to “counteract the water-drawing force of sodium and chloride.” The researchers hypothesize that, because synthesizing urea uses a lot of energy, the body may be demanding more calories — and people who eat a lot of salt are hungrier than normal. The twin studies are helping to create a new portrait of urea.
“It’s not solely a waste product, as has been assumed,” said Dr. Friedrich C. Luft, a researcher with the Max Delbruck Center for Molecular Medicine. “Instead, it turns out to be a very important osmolyte – a compound that binds to water and helps transport it. Its function is to keep water in when our bodies get rid of salt. Nature has apparently found a way to conserve water that would otherwise be carried away into the urine by salt.”
The study also provides important insight into how the body achieves the right balance of water retention.
“We now have to see this process as a concerted activity of the liver, muscle and kidney,” said Jens Titze of the University Medical Center Mainz in Germany. “While we didn’t directly address blood pressure and other aspects of the cardiovascular system, it’s also clear that their functions are tightly connected to water homeostasis and energy metabolism.”
Both studies appear in The Journal of Clinical Investigation.