“There
was only one way to explain this phenomenon,” Dr. Titze said. “The body
most likely had generated or produced water when salt intake was high.”
Another
puzzle: The crew complained that they were always hungry on the
high-salt diet. Dr. Titze assured them that they were getting exactly
enough food to maintain their weights, and were eating the same amount
on the lower-salt diets, when hunger did not seem to be problem.
But
urine tests suggested another explanation. The crew members were
increasing production of glucocorticoid hormones, which influence both
metabolism and immune function.
To
get further insight, Dr. Titze began a study of mice in the laboratory.
Sure enough, the more salt he added to the animals’ diet, the less
water they drank. And he saw why.
The
animals were getting water — but not by drinking it. The increased
levels of glucocorticoid hormones broke down fat and muscle in their own
bodies. This freed up water for the body to use.
But that process requires energy, Dr. Titze also found, which is why the mice ate 25 percent more food on a high-salt diet. The hormones also may be a cause of the strange long-term fluctuations in urine volume.
Scientists
knew that a starving body will burn its own fat and muscle for
sustenance. But the realization that something similar happens on a
salty diet has come as a revelation.
People do what camels do, noted Dr. Mark Zeidel, a nephrologist at Harvard Medical School who wrote an editorial
accompanying Dr. Titze’s studies. A camel traveling through the desert
that has no water to drink gets water instead by breaking down the fat
in its hump.
One
of the many implications of this finding is that salt may be involved
in weight loss. Generally, scientists have assumed that a high-salt diet
encourages a greater intake of fluids, which increases weight.
But if balancing a higher salt intake requires the body to break down tissue, it may also increase energy expenditure.
Still,
Dr. Titze 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, Dr. 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 Dr. 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.
“The work suggests that we really do not understand the effect of sodium chloride on the body,” said Dr. Hoenig.
“These
effects may be far more complex and far-reaching than the relatively
simple laws that dictate movement of fluid, based on pressures and
particles.”
She and others have not abandoned their conviction that high-salt diets can raise blood pressure in some people.
But
now, Dr. Hoenig said, “I suspect that when it comes to the adverse
effects of high sodium intake, we are right for all the wrong reasons.”