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Like so many other conclusions about eating
and health, however, this dietary system was
erected on a shaky foundation of ecological
studies. Researchers compared the incidence of
hypertension and cardiovascular diseases among
people in less developed areas (where only about
one in 100 adults have high blood pressure) to their
incidence in those living in industrialized areas
(where about a third of adults have hypertension).
Among the many dietary differences between the
populations, salt seemed a likely suspect.
Yet intervention studies that have encouraged
subjects to lower the sodium in their diets (or
have fed them low-salt food directly) have turned
in results that are both controversial and mixed.
On the one hand, people who already have high
blood pressure may benefit from reduced-salt
diets. One study found that hypertensive people
were more likely to be able to reduce or eliminate
their high blood pressure medication after they
started eating low-sodium diets. And two large
studies that included more than 3,000 subjects
with elevated blood pressure found a 25%
reduction in the risk of heart-related medical
problems among those on low-sodium diets. The
benefits persisted for years after the intervention
had ended.
On the other hand, some clinical trials have also
found that reductions in salt intake do not seem to
strongly affect blood pressure if it is already within
normal limits. A handful of studies seem to show
that the amount of salt one eats has no bearing on
one’s risk of dying of heart disease. And several
analyses of health data collected from thousands
of people representative of the U.S. population as
a whole have shown that those with the lowest salt
intakes have higher rates of death from cardiovascular
problems and from all causes.
These varied results illustrate that there’s no
one-size-fits-all dose of salt that ensures good
health. Yet health professionals with impressive
credentials insist nonetheless that we should all
reduce our intake (see Legislating Salt, next page).
The study of salt’s impact on health is one of the
most bitterly controversial in all of science. In an
award-winning article titled “The (Political)
Science of Salt” that appeared in the journal
Science in 1998, journalist Gary Taubes described
the acrimonious debates between scientists that
study salt. According to Taubes, “The controversy
over the benefits, if any, of salt reduction now
constitutes one of the longest running, most
vitriolic, and surreal disputes in all of medicine.”
Little has changed since this article appeared.
A 2009 meta-analysis of salt studies done from
1966 through 2008 found that risk of stroke was
higher in those who ate the most salt, but the 95%
confidence interval allowed that the risk might
still be low (1.06–1.43). The risk of cardiovascular
disease (95% confidence interval: 0.99–1.32) was
lower than the risk of stroke. Many observers
would interpret these confidence intervals as
indicating a low risk of stroke and no significant
risk of cardiovascular disease. But in the
controversy-filled world of salt research, these
findings are unlikely to alter many opinions.
More recently, a few scientists have been
exploring the intriguing possibility that our
increased rates of hypertension in the rich world
are due not to an excess of salt in our diets but
rather to a deficiency of potassium. Processed
foods tend to be high in sodium but low in potassium.
Fruits and vegetables, in contrast, are
usually low in sodium and high in potassium.
The potassium hypothesis gained traction when
a large study called INTERSALT reported that
subjects who excreted less potassium in their urine
(indicating that they had ingested less) had higher
blood pressure than their peers and that those
with both high sodium and low potassium were
most likely to have hypertension. A clinical study
provided some support for the idea: participants
who ate their usual portion of sodium but less
potassium saw their blood pressure rise.
Or perhaps the real answer is that what matters
for maintaining normal blood pressure is eating
a mix of healthful nutrients. The Dietary Approaches
to Stop Hypertension (DASH) trials fed
subjects either typical U.S. diets or better diets,
rich in fruit, vegetables, potassium, and low-fat
dairy. Within weeks, the latter group enjoyed a
significant drop in blood pressure despite eating
just as much salt (more than the U.S. recommended
daily allowance) as their peers.
These data speak well of eating a diet rich in
fresh produce. But there’s little to suggest that
those of us with normal blood pressure will benefit,
in the long term, from hiding the salt shaker.
C ONTR O V E RSIE S
Legislating Salt
SCIENCE POLICY
In 2009, New York City health department commissioner
Thomas R. Frieden (later appointed to head the U.S. Centers
for Disease Control and Prevention) asked manufacturers of
packaged and mass-produced restaurant food to reduce the
amount of sodium in their products by one quarter over five
years. In the following five years, Frieden wanted to see
another 25% reduction.
He claims that cutting by half the salt in products like
these, which are allegedly responsible for 80% of the salt in
the average diet, will save 150,000 lives each year. (That
figure is often repeated but rarely with a citation to its
source, which actually refers to a rough estimate of lives that
might be saved by sodium reduction across all 300 million
people in the United States.)
“If there’s not progress in a few years, we’ll have to consider
other options, like legislation,” Frieden said, according
to The New York Times.
The sense of urgency seems misplaced when the best
evidence suggests that sodium reductions are ineffective at
reducing blood pressure by a significant amount in most
people with normal blood pressure. Low-sodium diets may
help some people with hypertension control their blood
pressure. But why legislate the salt consumption of millions
T IMELINE
Salt and Heart Disease: Advice and Evidence
1979 The Surgeon General’s report asserts
that salt clearly causes high blood pressure
1980 The U.S. Department of Agriculture
issues dietary guidelines that caution against
eating too much salt
1988 The INTERSALT
study of more than
10,000 people finds that
salt consumption is not
strongly related to
blood pressure
1997 The DASH trial
observes that adherents to
a diet high in fruit, vegetables,
and low-fat dairy experience
significant drops in blood
pressure, even without changing
salt intake
of people for the benefit of a few?
Frieden responded to that question
with a line of reasoning often
used by advocates of dietary
systems. Most Americans eat
twice the recommended amount
of sodium, he asserted, so eating
less salt will result in some blood
pressure reduction, which will
reduce the risk of heart attack
and stroke.
As the DASH studies have shown, however, eating more
fruits, vegetables, and low-fat dairy lowers blood pressure
more effectively than low-sodium diets do. Unfortunately,
regulating several positive additions to people’s diets seems
much more difficult than demonizing a single ingredient.
Frieden suggests that consumers won’t notice a gradual,
50% reduction in sodium over a decade, the sort of change
that he likes to call “stealth health.” Perhaps, but this sort of
activist health legislation done in advance of firm scientific
knowledge seems unjustified—and possibly dangerous if it
encourages food manufacturers to search for new flavor
enhancers that might be less benign.
2001 A second
DASH trial shows that
those who both
adhere to the diet
and reduce salt have
the largest drops in
blood pressure
2005 The U.S. government dietary guidelines state that
“on average, the higher an individual’s salt intake, the higher
an individual’s blood pressure. Nearly all Americans
consume substantially more salt than they need. Decreasing
salt intake is advisable to reduce the risk of elevated blood
pressure”
2004 A meta-analysis of 57 randomized, controlled
trials finds that healthy subjects without
hypertension experience only modest decreases in
blood pressure when they reduce salt consumption
2009 A meta-analysis of salt consumption studies
from 1966 to 2008 finds that those who ate the most
salt had little or no increased risk of heart disease
compared to those who ate the least salt
236 VOLUME 1 · HISTORY AND FUNDAMENTALS FOOD AND HEALTH 237