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 In one study that looked at death and heart disease, researchers
found that extremely low-salt diets may not be as beneficial as many
experts believe, and might even pose a hazard.
The other, focused on blood pressure effects, found that people with
a moderate salt intake didn't benefit from reducing their
consumption as much as people in the high-salt group.
"Previously it was believed that the lower you go the better. What
these studies show collectively is that there is an optimal level,
and lower is not necessarily better," Dr. Andrew Mente of McMaster
University in Hamilton, Ontario, chief author of the blood pressure
study, told Reuters Health by phone.
On the other hand, a third international study supports the
conventional wisdom that there's a direct link between less salt and
better health, and found no evidence that extremely-low levels of
sodium are dangerous.
All three reports are published in the Aug. 14 New England Journal
of Medicine.
One thing all the studies confirm: too much salt is bad for you.
High amounts contribute to high blood pressure, stroke, heart
attack, kidney problems and heart failure.
That third study concluded that high salt intake was responsible for
1.65 million heart disease deaths worldwide in 2010. It's based on
computer models, surveys of salt use in 66 countries and 107
published studies.
"These are staggering numbers when you actually think about it," Dr.
Elliott Antman, president of the American Heart Association, told
Reuters Health.
Worldwide, daily consumption is usually 3 to 6 grams of sodium,
which translates to 7.5 to 15.0 grams of salt. That's well above the
limit of 1.5 to 2.4 grams of sodium each day recommend by the World
Health Organization, the American Heart Association and other
organizations.
"Even at 4 grams per day, we found half a million deaths per year,"
chief author Dr. Dariush Mozaffarian, dean of the Friedman School of
Nutrition Science and Policy at Tufts University, told Reuters
Health.
Mozaffarian said his analysis uncovered no danger at low levels or
below, and he argued that even Mente's blood pressure study showed
no risk there.
But Mente said the relationship seems to be more complicated than
previously thought, and the health benefits of lowering salt intake
don't fall at the same pace for everyone.
"If people are eating a very high level of sodium and they reduce
their intake, you get a large reduction in blood pressure," he said.
"But if you're eating a moderate level of sodium - about what most
North Americans eat - and you reduce it to a lower level, you're not
really getting much in return as far as blood pressure reduction is
concerned."
The most dramatic evidence was the study that looked at the link
between sodium intake and death, heart attacks and strokes using
urine samples to estimate sodium consumption.
Led by Mente's colleague at McMaster, Dr. Martin O'Donnell, it found
that consuming less than 3 grams of sodium per day increased the
risk of death or major cardiovascular events by 27 percent compared
to people who consumed 4 to 6 grams daily.
The optimum sodium level was 3 to 6 grams per day. "Both higher and
lower levels of estimated sodium excretion were associated with
increased risk," the O'Donnell team concluded.
That study shows "there are unintended consequences of going lower"
Mente said in a telephone interview.
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Currently only about 1 percent of the people worldwide "were
consuming less than 2 grams per day," he said. "What we're
recommending right now is what almost nobody around the world eats,
apart from maybe a few hunter-gatherer societies. We're making
recommendations where the human experience is extremely sparse.
These findings strongly question our current recommendation."
Yet Mozaffarian argued that they don't. Both the O'Donnell and Mente
studies derived their results from the same research project, known
as the PURE study.
"When you have a single study like the PURE study, that's one study
in the context of many," said Dr. Mozaffarian. Some have suggested a
risk for very-low salt intake while others have not.
Not only does the weight of the evidence suggest that there is no
risk in aggressive salt reduction, Mozaffarian said. "You don't have
any plausible biology to explain why you'd get an uptick in risk
with low sodium intake."
There's another reason for skepticism. The PURE study assessed
sodium consumption based on a single urine sample collected each
morning. Antman called that a "very unreliable method" for measuring
salt intake.
"The gold standard is 24-hour urine collection, and they didn't use
it. And even then your sodium intake may vary a lot from day to
day," he told Reuters Health.
"The bottom line is, the American Heart Association position has not
changed in response to the PURE paper," said Antman, a
cardiovascular expert at Brigham and Women's Hospital in Boston. The
association recommends less that 1.5 grams of sodium daily.
Nonetheless, Dr. Suzanne Oparil of the University of Alabama at
Birmingham contends in a Journal editorial that, "Both high and low
levels of sodium excretion may be associated with an increased risk
of death and cardiovascular-disease outcomes and (the tests suggest)
that increasing the urinary potassium excretion counterbalances the
adverse effect of high sodium excretion."
That idea should be confirmed in a large study, she said, but "in
the absence of such a trial, the results argue against reduction of
dietary sodium as an isolated public health recommendation."
"The big picture is that high sodium is bad and should be reversed,
and there's just some controversy over how low you should go," said
Mozaffarian, adding that doctors and patients should be pushing for
a strong national policy to reduce the sodium content of processed
food.
"Whether it should be 2 grams or 1.5 grams or 2.5 grams per day,
that's all theory." he said. "Right now it's close to 4 grams per
day. Let's get it down below 3, and then we can argue how low it
should go. But right now it's clearly way too high."
SOURCES: http://bit.ly/1uIrwAc;
http://bit.ly/1oFemTU and
http://bit.ly/1pLK1Cc;New England Journal of Medicine, online
August 13, 2014.
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