In 2014, a study published in the journal Science claimed that new neurons created through physical activity erase old memories. Now, new research by scientists from Texas A&M College of Medicine finds this is not the case.
Published in the Journal of Neuroscience, the study reveals that physical activity does fuel the development of new neurons - or nerve cells - in a brain region crucial to memory, but that these newly formed neurons do not hinder memory recall.
It is generally accepted that exercise is beneficial for cognitive function, due to the abundance of studies that have demonstrated as such.
But in May 2014, research from the University of Toronto in Canada suggested that, when it comes to memory, physical activity could do more harm than good.
"It stunned the field of hippocampal neurogenesis," says Ashok K. Shetty, Ph.D., co-author of this latest research and a professor in the Texas A&M College of Medicine Department of Molecular and Cellular Medicine. "It was a very well-done study, so it caused some concern that exercise might in some way be detrimental for memory."
In the 2014 study, researchers found mice that exercised demonstrated greater development of new neurons - known as neurogenesis - than those that did not exercise.
However, they also found that memories the mice formed prior to exercising were erased after exercising. Upon removing the newly formed neurons, these old memories were restored.
"The mice who exercised had a large number of new neurons," notes Shetty, "but somehow that seemed to break down the old connections, making them forget what they knew."
Exercise fuels neurogenesis, but it does not hinder memory
For the new research, Shetty and team replicated the earlier study, but they used rats instead of mice. This is because the neuronal function of rats more closely resembles that of humans.
Firstly, the researchers trained rats in a water maze over a 4-day period, and for a further several days, the rats repeatedly completed the water maze, in order to induce memory consolidation.
Next, half of the trained rats were placed in cages with running wheels for 4 weeks, while the remaining half were placed in standard cages, making them sedentary.
Over the 4-week period, the exercising rats ran around 48 miles. Compared with the sedentary rats, those that exercised experienced a 1.5 to 2.1-fold increase in neurogenesis in the hippocampus of the brain - the region responsible for learning and memory.
"This is pretty clear evidence that exercise greatly increases neurogenesis in the hippocampus, which has functional implications," says first author Maheedhar Kodali, Ph.D., of the Institute for Regenerative Medicine at Texas A&M. "Neurogenesis is important for maintaining normal mood function, as well as for learning and creating new memories."
The researchers then placed both groups of rats back in the water maze, in order to test whether they could remember how to navigate it.
The team found that the memory recall of the exercising rats was comparable with that of sedentary rats, suggesting that exercise has no detrimental effect on memory. This effect was the same for moderate and brisk runners.
The authors say their findings contradict those of the 2014 study, and they should come as welcome news for individuals who believed their morning run might have negative outcomes for memory.
"Exercise is not at all harmful. It doesn't cause any memory problems, and there are many studies proving its benefits for making new memories and maintaining good mood. Now, our study showed that exercise does not interfere with memory recall ability. Keep exercising, and don't worry about losing your old memories."
Ashok K. Shetty, Ph.D.