Why Everything We Know About 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 carefully maintained.
If
you eat a lot of salt — sodium chloride — 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 Dr. Melanie Hoenig, an
assistant professor of medicine at Harvard Medical School. “The work was
meticulously done.”
Dr.
James R. 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.
The
new studies are the culmination of a decades-long quest by a determined
scientist, Dr. Jens Titze, now a kidney specialist at Vanderbilt University Medical Center and the Interdisciplinary Center for Clinical Research in Erlangen, Germany.
In
1991, as a medical student in Berlin, he took a class on human
physiology in extreme environments. The professor who taught the course
worked with the European space program and presented data from a
simulated 28-day mission in which a crew lived in a small capsule.
The
main goal was to learn how the crew members would get along. But the
scientists also had collected the astronauts’ urine and other
physiological markers.
Dr.
Titze noticed something puzzling in the crew members’ data: Their urine
volumes went up and down in a seven-day cycle. That contradicted all
he’d been taught in medical school: There should be no such temporal
cycle.
In
1994, the Russian space program decided to do a 135-day simulation of
life on the Mir space station. Dr. Titze arranged to go to Russia to
study urine patterns among the crew members and how these were affected
by salt in the diet.
A
striking finding emerged: a 28-day rhythm in the amount of sodium the
cosmonauts’ bodies retained that was not linked to the amount of urine
they produced. And the sodium rhythms were much more pronounced than the
urine patterns.
The
sodium levels should have been rising and falling with the volume of
urine. Although the study wasn’t perfect — the crew members’ sodium
intake was not precisely calibrated — Dr. Titze was convinced something
other than fluid intake was influencing sodium stores in the crew’s
bodies.
The conclusion, he realized, “was heresy.”
In
2006, the Russian space program announced two more simulation studies,
one lasting 105 days and the other 520 days. Dr. Titze saw a chance to
figure out whether his anomalous findings were real.
In
the shorter simulation, the cosmonauts ate a diet containing 12 grams
of salt daily, followed by nine grams daily, and then a low-salt diet of
six grams daily, each for a 28-day period. In the longer mission, the
cosmonauts also ate an additional cycle of 12 grams of salt daily.
Like
most of us, the cosmonauts liked their salt. Oliver Knickel, 33, a
German citizen participating in the program who is now an automotive
engineer in Stuttgart, recalled that even the food that supplied 12
grams a day was not salty enough for him.
When the salt level got down to six grams, he said, “It didn’t taste good.”
The
real shocker came when Dr. Titze measured the amount of sodium excreted
in the crew’s urine, the volume of their urine, and the amount of
sodium in their blood.
The mysterious patterns in urine volume
persisted, but everything seemed to proceed according to the textbooks.
When the crew ate more salt, they excreted more salt; the amount of
sodium in their blood remained constant, and their urine volume
increased.
“But then we had a look at fluid intake, and were more than surprised,” he said.
Instead
of drinking more, the crew were drinking less in the long run when
getting more salt. So where was the excreted water coming from?
“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.”
