SALT AFFECTS ORGANS , EVEN IN ABSENCE OF HYPERTENSION
You may think you're
one of the lucky ones who can eat all the salty snacks and convenience foods
you want and still register low numbers on the blood pressure cuff. But, new
research suggests you may not be so lucky after all
A review paper
co-authored by two faculty members in the University of Delaware College of
Health Sciences and two physicians at Christiana Care Health System provides
evidence that even in the absence of an increase in blood pressure, excess
dietary sodium can adversely affect target organs, including the blood vessels,
heart, kidneys and brain.
Authors of the paper,
"Dietary Sodium and Health: More Than Just Blood Pressure," include
William Farquhar and David Edwards in UD's Department of Kinesiology and
Applied Physiology; William Weintraub, chief of cardiology at Christiana Care;
and Claudine Jurkovitz, a nephrologist epidemiologist and senior scientist in
the Value Institute Center for Outcomes Research at Christiana Care.
The paper was
published in the March 17 issue of the Journal of the American College
of Cardiology.
"Blood pressure
responses to alterations in dietary sodium vary widely, which has led to the
concept of 'salt-sensitive' blood pressure," says Farquhar. "There
are no standardized guidelines for classifying individuals as having salt-sensitive
blood pressure, but if blood pressure increases during a period of high dietary
sodium or decreases during a low-sodium period, the person is considered salt
sensitive. If there's no change in blood pressure with sodium restriction, an
individual is considered salt resistant."
However, the research
cited in the paper points to evidence of adverse effects on multiple target
organs and tissues, even for people who are salt resistant.
Bad news for the body
Potential effects on
the arteries include reduced function of the endothelium, which is the inner
lining of blood vessels. Endothelial cells mediate a number of processes,
including coagulation, platelet adhesion and immune function. Elevated dietary
sodium can also increase arterial stiffness.
Farquhar and Edwards
have done previous work in this area, with one study showing that excess salt
intake in humans impairs endothelium-dependent dilation and another
demonstrating that dietary sodium loading impairs microvascular function. In
both cases, the effects are independent of changes in blood pressure.
They review their work
and the growing body of evidence to support a deleterious effect of dietary
salt on vascular function independent of blood pressure in a recent invited
paper in Current Opinion in Nephrology and Hypertension.
"High dietary
sodium can also lead to left ventricular hypertrophy, or enlargement of the
muscle tissue that makes up the wall of the heart's main pumping chamber,"
Edwards says. "As the walls of the chamber grow thicker, they become less compliant
and eventually are unable to pump as forcefully as a healthy heart."
Regarding the kidneys,
evidence suggests that high sodium is associated with reduced renal function, a
decline observed with only a minimal increase in blood pressure.
Finally, sodium may
also affect the sympathetic nervous system, which activates what is often
termed the fight-or-flight response.
"Chronically
elevated dietary sodium may 'sensitize' sympathetic neurons in the brain,
causing a greater response to a variety of stimuli, including skeletal muscle
contraction," Farquhar says. "Again, even if blood pressure isn't
increased, chronically increased sympathetic outflow may have harmful effects
on target organs."
Difficult detective
work
Jurkovitz points out
that studying the effects of salt restriction on clinical outcomes is not easy.
Challenges include accurate assessment of intake, long-term maintenance on a
defined salt regimen, and the need for large numbers of patients and extended
follow-up to obtain enough outcomes for meaningful analysis.
However, she says,
"A large body of evidence confirms the biological plausibility of the
association between high sodium intake and increases in blood pressure and
cardiovascular events."
This evidence has
resulted in the American Heart Association's recommendation that we consume
less than 1,500 mg of sodium a day.
Shaking the salt habit
Taking the salt shaker
off the table is a good way to start, but it's probably not enough, says
Weintraub, whose work focuses on cardiology outcomes.
"Approximately 70
percent of the sodium in our diets comes from processed foods, including items
that we don't typically think of as salty such as breads and cereals," he
says. "Also, restaurant food typically contains more salt than dishes
prepared at home, so eating out less can help reduce salt intake, especially if
herbs and spices -- instead of salt -- are used to add flavor to home-cooked
meals."
But the authors
acknowledge that shaking the salt habit won't be easy, and it won't happen
overnight.
"Reducing sodium
will take a coordinated effort involving organizations like the AHA, food
producers and processors, restaurants, and public policy aimed at
education," Weintraub says.
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