Exercise can be good for brain health -- but too much can be bad for your heart. David Marcu

Scientists have a compelling explanation for why adults have difficulty remembering their early childhood: a process called neurogenesis, which is responsible for creating new neurons. In 2014, a surprising study found that exercise-induced neurogenesis results in memory loss. Now, a team of scientists from Texas A&M College of Medicine replicated the study to find the previous study’s results to be invalid. Or, at the very least, questionable.

The 2014 study, conducted on mice, was counterintuitive to experts since a wealth of research associated exercise with a wide array of cognitive benefits. But this study found an association between exercise and a degradation in memories.

"It stunned the field of hippocampal neurogenesis,” said Ashok K. Shetty, a researcher in the new study and a professor at Texas A&M College of Medicine. "It was a very well-done study, so it caused some concern that exercise might in some way be detrimental for memory."

To understand how exercise factors in, it is important to understand how the brain creates and stores memories. It all starts in the brain’s hippocampus--a region responsible for learning, memory and mood regulation--where neurons are created.

“When new neurons are born and mature and integrate into these pre-existing neural circuits, they’re going to change the connections that existed before,” said Katherine Akers, lead author of the 2014 study, to The New York Times. “Changing these connections might degrade the integrity of the pre-existing memories.”

Shetty’s team tackled the study, which they published in the Journal of Neuroscience, in a new manner by using rats instead of mice. While this change may seem minute, rats are considered to be more physiologically alike to humans with similar neural networks. The study found there was no memory loss when the test subjects exercised.

"We had completely contradictory findings from the 2014 study," said Maheedhar Kodali, first author of the study and a postdoctoral fellow at the Institute for Regenerative Medicine, in a statement.

The researchers trained the rats to complete a task over four days. Half the trained animals were put in a group that exercised (utilized running wheels in cages) and the other half served as a control group that was sedentary. The rats with running wheels in their cages ran an average of 48 miles over the course of four weeks and were seen to have had a higher rate of neurogenesis. Most importantly, the rats who ran had a similar level of recall as those in the control group.

"This is pretty clear evidence that exercise greatly increases neurogenesis in the hippocampus, which has functional implications," said Maheedhar Kodali, first author of the study and a postdoctoral fellow at the Institute for Regenerative Medicine. "Neurogenesis is important for maintaining normal mood function, as well as for learning and creating new memories."

The takeaway message, says Shetty, is that exercise is good for you and your brain. As for the study, more research needs to be done to understand what is at play.

"Now we need to study other species to fully understand this phenomenon," said Kodali.