It is well established that exercise bolsters the structure and function
of the brain. Multiple animal and human studies have shown that a few
months of moderate exercise can create new neurons, lift mood and hone
memory and thinking.
But few studies have gone on to examine what happens next. Are these
desirable brain changes permanent? Or, if someone begins exercising but
then stops, does the brain revert to its former state, much like unused
muscles slacken?
The question may be particularly relevant at this time of year, when
so many people start new exercise programs. Helpfully, two recent animal
studies that were presented at the 2012 annual meeting of the Society for Neuroscience in New Orleans have taken on the issue and may have relevance for people, though the results are disquieting.
Of the two experiments, the more dramatic looked at what happens to the brain’s memory center when exercise is stopped.
Researchers from the University of Sao Paulo in Brazil began by
allowing half of a group of healthy, adult rats to run at will on
running wheels. Rats enjoy that activity and, for a week, they
enthusiastically skittered on their wheels. The animals were also
injected with a substance that marks newborn neurons in the hippocampus,
or memory center of the brain, so that the scientists would be able to
track how many cells had been created. Inactive animals, including
people, create new brain cells, but exercise is known to spark the
creation of two or three times as many new hippocampal neurons.
A separate control group was housed in cages with locked wheels, so
that they remained sedentary. They were also monitored for new brain
cell growth.
After a week, the runners’ wheels were locked and they, too, became inactive.
A week later, some of the exercised and control rats completed memory
testing that required them to find, then remember, the location of a
platform placed along the wall of a small swimming pool. (Rats aren’t
fond of being in the water, and the platform allowed them to clamber
out.) Those with better memories remembered and paddled to the platform
more quickly.
The remaining animals completed the same memory test after either three weeks or six weeks of inactivity.
Afterward, the researchers compared the animals’ performance on the
memory test, as well as the number of new brain cells in the hippocampus
of each group of rats.
They found that, after only a week of inactivity, the rats that had
run were much faster on the water maze test than the control animals.
They also had at least twice as many newborn neurons in the hippocampus.
But those advantages faded after several more weeks of not running.
The brains of the animals that had been inactive for three weeks
contained far fewer newborn neurons than the brains of the animals that
had rested for only one week. The brains of the animals that had been
inactive for six weeks had fewer still.
The animals inactive for three or six weeks also performed far worse
on the water maze test than the animals that had been inactive for only a
single week. In fact, their memories were about as porous of those of
the control animals, “indicating,” the authors write, “that the
exercise-induced benefits may be transient.”
The other new study of exercise-induced brain changes found that they
were similarly fragile, although this study explored the impacts of
exercise on mood.
In earlier experiments by the same group of scientists,
from McMaster University in Hamilton, Ontario, rats given access to a
running wheel, toys and other types of environmental enrichment were
able to use serotonin, a neurotransmitter involved in anxiety and other
moods, more efficiently. After several months of exercise, the exercised
animals became noticeably less anxious and more resilient to stress
during behavioral testing. But that savoir-faire dissipated rapidly if
they were removed from the cages with running wheels and toys.
In their latest experiment, also presented at the Society for
Neuroscience meeting, the researchers reported that after 10 weeks of
running, followed by three weeks of inactivity, the running rats’ brains
were almost indistinguishable from those of animals that had never
exercised. They had almost comparable levels of an enzyme in the brain
that affects the synthesis and uptake of serotonin. It was as if they
had never run.
In other words, the brain benefits “wear off quickly,” said Dr.
Michael Mazurek, a professor of neurology at McMaster, who oversaw the
study. “This is analogous to what happens to muscle bulk or heart rate
following exercise withdrawal.”
Gilberto Xavier, a professor of psychology at the University of Sao
Paulo and senior author of the study of hippocampal neurons, agrees.
“Brain changes are not maintained when regular physical exercise is
interrupted,” he said, adding that, “though our observations are
restricted to rats, indirect evidence suggests that the same phenomenon
occurs in human beings.”
Meaning that the lessons of both studies point in the same direction.
For the ongoing health of our minds, as well as for the plentiful other
health benefits of exercise, it might be wise to stick to those New
Year’s exercise resolutions.
