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Phys Ed Gretchen Reynolds on the science of fitness.

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.