Theories on salt may be wrong

THE salt equation taught to doctors for more than 200 years is not hard to understand.

The body relies on this essential mineral for a variety of functions, including blood pressure and the transmission of nerve impulses. Sodium levels in the blood must be maintained.

If you eat a lot of salt, you will become thirsty and drink water, diluting your blood enough to maintain the proper concentration of sodium. Ultimately, you will excrete much of the excess salt and water in urine.

The theory is intuitive and simple. And it may be completely wrong. New studies of Russian cosmonauts, held in isolation to simulate space travel, show that eating more salt made them less thirsty but somehow hungrier. Subsequent experiments found that mice burned more calories when they got more salt, eating 25 percent more just to maintain their weight.

The research, published recently in two dense papers in the Journal of Clinical Investigation, contradicts much of the conventional wisdom about how the body handles salt and suggests that high levels may play a role in weight loss.

The findings have stunned kidney specialists.

“This is just very novel and fascinating,” said Melanie Hoenig, a professor of medicine at Harvard Medical School. “The work was meticulously done.”

James Johnston, a professor at the University of Pittsburgh, marked each unexpected finding in the margins of the two papers. The studies were covered with scribbles by the time he was done.

“Really cool”, he said, although he added that the findings need to be replicated.

Still, Jens Titze, a kidney specialist, said he would not advise eating a lot of salt to lose weight. If his results are correct, more salt will make you hungrier in the long run, so you would have to be sure you did not eat more food to make up for the extra calories burned.

And, Titze said, high glucocorticoid levels are linked to such conditions as osteoporosis, muscle loss, Type 2 diabetes and other metabolic problems.

But what about liquids? Everyone knows that salty foods make you thirsty. How could it be that a high-salt diet made the cosmonauts less thirsty?

In reality, said Zeidel, people and animals get thirsty because salt-detecting neurons in the mouth stimulate an urge to drink. This kind of “thirst” may have nothing to do with the body’s actual need for water.

These findings have opened up an array of puzzling questions, experts said.

But now, Hoenig said, “I suspect that when it comes to the adverse effects of high sodium intake, we are right for all the wrong reasons.”