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Why Exercise Makes Us Feel
Good
By GRETCHEN
REYNOLDS, NYTimes.com
Why does exercise make us happy and calm?
Almost everyone agrees that it generally does,
a conclusion supported by research. A survey
by Norwegian researchers published this month,
for instance, found that those who engaged
in any exercise, even a small amount, reported
improved mental health compared with Norwegians
who, despite the tempting nearness of mountains
and fjords, never got out and exercised. A
separate study, presented last month at the
annual meeting of the American College of
Sports Medicine, showed that six weeks of
bicycle riding or weight training eased symptoms
in women who’d received a diagnosis
of anxiety disorder. The weight training was
especially effective at reducing feelings
of irritability, perhaps (and this is my own
interpretation) because the women felt capable
now of pounding whomever or whatever was irritating
them.
But
just how, at a deep, cellular level, exercise
affects anxiety and other moods has been difficult
to pin down. The brain is physically inaccessible
and dauntingly complex. But a recent animal
study from researchers at the National Institute
of Mental Health provides some intriguing
new clues into how exercise intertwines with
emotions, along with the soothing message
that it may not require much physical activity
to provide lasting emotional resilience.
For
the experiment, researchers at the institute
gathered two types of male mice. Some were
strong and aggressive; the others were less
so. The alpha mice got private cages. Male
mice in the wild are territorial loners. So
when then the punier mice were later slipped
into the same cages as the aggressive rodents,
separated only by a clear partition, the big
mice acted like thugs. They employed every
animal intimidation technique and, during
daily, five-minute periods when the partition
was removed, had to be restrained from harming
the smaller mice, which, in the face of such
treatment, became predictably twitchy and
submissive.
After
two weeks of cohabitation, many of these weaker
mice were nervous wrecks. When the researchers
tested them in a series of stressful situations
away from the cages, the mice responded with,
as the scientists call it, “anxiety-like
behavior.” They froze or ran for dark
corners. Everything upset them. “We
don’t use words like ‘depressed’
to describe the animals’ condition,”
said Michael L. Lehmann, a postdoctoral fellow
at the institute and lead author of the study.
But in effect, those mice had responded to
the repeated stress by becoming depressed.
But
that was not true for a subgroup of mice that
had been allowed access to running wheels
and nifty, explorable tubes in their cages
for several weeks before they were housed
with the aggressive mice. These mice, although
wisely submissive when confronted by the bullies,
rallied nicely when away from them. They didn’t
freeze or cling to dark spaces in unfamiliar
situations. They explored. They appeared to
be, Dr. Lehmann said, “stress-resistant.”
“In
people, we know that repeated applications
of stress can lead to anxiety disorders and
depression,” Dr. Lehmann said. “But
one of the mysteries” of mental illness
“is why some people respond pathologically
to stress and some seem to be stress-resistant.”
To
discern what was different, physiologically,
about the stress-resistant mice, the scientists
looked at brain cells using stains and other
techniques. They determined that neurons in
part of the rodents’ medial prefrontal
cortex, an area of the brain involved in emotional
processing in animals and people, had been
firing often and rapidly in recent weeks,
as had neurons in other, linked parts of the
brain, including the amygdala, which is known
to handle feelings of fear and anxiety.
The
animals that had not run before moving in
with the mean mice showed much less neuronal
activity in these portions of the brain.
Dr.
Lehmann said that he believed that the running
was key to the exercised animals’ ability
to bounce back from their unpleasant housing
conditions.
Of
course, as we all know, mice are not people.
But the scientists believe that this particular
experiment is a fair representation of human
interpersonal relations, Dr. Lehmann said.
Hierarchies, marked by bullying and resulting
stress, are found among people all the time.
Think of your own most dysfunctional office
job. (Interestingly, the same experiment cannot
be conducted on female mice, who like being
housed together, Dr. Lehmann said, so he and
his colleagues are testing a female-centric
version, in which “cage mates are swapped
out continuously,” to the consternation
and grief of the female mice left behind.)
Perhaps
best of all, Dr. Lehmann does not believe
that hours of daily exercise are needed or
desirable to achieve emotional resilience.
The mice in his lab ran only when and for
as long as they wished, over the course of
several weeks. Other animal experiments have
intimated that too much exercise could contribute
to anxiety, and Dr. Lehmann agrees that that
outcome is possible. Moderate levels of exercise
seem to provide the most stress-relieving
benefits, he said. Dr. Lehmann does not have
a car and walks everywhere, and although he
lives in Washington, a cauldron of stress
induction, he describes himself as a “pretty
calm guy.” |
