“I think the training regimes were quite close to what humans might do,” says study lead Miriam Nokia. The rats doing endurance training, for example, ran on a treadmill three times a week for half an hour. The rat version of weight lifting was a little different–tiny weights were tied to the rats’ tails while they climbed up a ladder–but like weight-lifting humans, the rats were noticeably buffer after eight weeks of working out.
But the weight-lifters didn’t show signs of neurogenesis, or new brain cells. By the end of the study, the rats that jogged on a treadmill had significantly more new brain cells than those that lifted weights or were sedentary. They also fared much better than those that did high-intensity training–spurts of sprinting, then jogging, repeated for 20 minutes.
“[High intensity training] is in nature more stressful which could have dampened its effects on adult hippocampal neurogenesis,” says Nokia. “Stress is in general considered to decrease adult neurogenesis.” On the other hand, the rats on the treadmills were prodded on by tiny shocks–also stressful–and the weight-training rats weren’t, so stress alone doesn’t explain the differences. Nokia also points out that mild stress may actually increase the number of new brain cells.
The study also found that some rats were more likely to benefit from running than others. Those who were selectively bred for their ability to get fit–and who jogged on their running wheels voluntarily, often for miles every day–ended up also seeing the biggest increase in neurogenesis.
None of this proves that the same thing would happen in humans. But a 2014 study of elderly women suggested something similar: aerobic exercise made the part of the brain responsible for memory grow, and weight lifting didn’t have any effect.
Weight lifting might still have benefits for the brain, but likely not new brain cells. “The effects of anaerobic training on the brain are something I definitely wish to study more,” says Nokia.