WASHINGTON (AP) -- Take a slow deep breath, then exhale just as
slowly. Can you take fewer than 10 breaths a minute?

Research suggests breathing that slowly for a few minutes a day
is enough to help some people nudge down bad blood pressure.

Why would that brief interlude of calm really work? A scientist
at the National Institutes of Health thinks how we breathe may hold
a key to how the body regulates blood pressure -- and that it has
less to do with relaxation than with breaking down all that salt
most of us eat.

Now Dr. David Anderson is trying to prove it, with the help of a
special gadget that trains volunteers with hypertension to
slow-breathe.

If he's right, the work could shed new light on the intersection
between hypertension, stress and diet.

"If you sit there under-breathing all day and you have a high
salt intake, your kidneys may be less effective at getting rid of
that salt than if you're out hiking in the woods," said Anderson,
who heads research into behavior and hypertension at the NIH's
National Institute on Aging.

An estimated 65 million Americans have high blood pressure,
putting them at increased risk of heart attacks, strokes, kidney
damage, blindness and dementia. Many don't know it. Hypertension is
often called the silent killer, because patients may notice no
symptoms until it already has done serious damage.

Anyone can get high blood pressure, measured as a level of 140
over 90 or more. But being overweight and inactive, and eating too
much salt -- Americans eat nearly double the upper limit for good
health -- all increase the risk. Indeed, losing weight, physical
activity and cutting sodium are the most effective lifestyle
changes people can make to lower blood pressure. Still, most
hypertension patients need medications, too.

While they know risk factors, scientists don't fully understand
the root causes of hypertension: What skews the body's usually
finely tuned mechanisms for regulating the force of blood pounding
against artery walls, until it can't compensate for some extra
pounds on a couch potato? Understanding those mechanisms could
point to better ways to prevent and treat hypertension.

Enter breathing.

Meditation, yoga and similar relaxation techniques that
incorporate slow, deep breathing have long been thought to aid
blood pressure, although research to prove an effect has been
spotty.

Then in 2002, the Food and Drug Administration cleared the
nonprescription sale of a medical device called RESPeRATE, to help
lower blood pressure by pacing breathing. The Internet-sold device
counts breaths by sensing chest or abdominal movement, and sounds
gradually slowing chimes that signal when to inhale and exhale.
Users follow the tone until their breathing slows from the usual 16
to 19 breaths a minute to 10 or fewer.

In clinical trials funded by maker InterCure Inc., people who
used the slow-breathing device for 15 minutes a day for two months
saw their blood pressure drop 10 to 15 points. It's not supposed to
be a substitute for diet, exercise or medication, but an addition
to standard treatment.

Why slow-breathing works "is still a bit of a black box," says
Dr. William J. Elliott of Chicago's Rush University Medical Center,
who headed some of that research and was surprised at the effect.
Slow, deep breathing does relax and dilate blood vessels
temporarily, but that's not enough to explain a lasting drop in
blood pressure, says NIH's Anderson.

So, in a laboratory at Baltimore's Harbor Hospital, Anderson is
using the machine to test his own theory: When under chronic
stress, people tend to take shallow breaths and unconsciously hold
them, what Anderson calls inhibitory breathing. Holding a breath
diverts more blood to the brain to increase alertness -- good if the
boss is yelling -- but it knocks off kilter the blood's chemical
balance. More acidic blood in turn makes the kidneys less efficient
at pumping out sodium.

In animals, Anderson's experiments have shown that inhibitory
breathing delays salt excretion enough to raise blood pressure. Now
he's testing if better breathing helps people reverse that effect.
"They may be changing their blood gases and the way their
kidneys are regulating salt," he says.

If Anderson's right, it would offer another explanation for why
hypertension is what he calls "a disease of civilization and a
sedentary lifestyle."