It took decades of research to persuade scientists to give up their long-held belief that new neurons could not be formed in the brains of adults, but there is no longer any doubt about it. It is now well-established that strenuous physical exercise stimulates the birth of new neurons in part of the brain that is critical for memory, the hippocampus. The molecular and cellular details explaining how exercise stimulates the birth of new brain cells have been worked out now in great detail. Immature non-neuronal cells in the adult brain (glia) respond to protein growth factors that are generated in the body during robust physical activity. These growth factors stimulate the mother cells to spawn new neurons in the hippocampus. Amazingly, these nubile neurons then migrate through brain tissue to find their proper place in the neural circuitry. Even more remarkable, new research proves that the new neurons are then able to wire themselves into the existing network of connections to boost performance in memory, just like adding RAM chips does for a laptop. But why? Why should pumping muscles build more brain cells?
This is the question addressed by Gerd Kempermann and colleagues at Stanford, the University of Zurich and Dresden, Germany, in their recent paper published in the journal "Frontiers in Neuroscience." To understand the answer, you are going to have to suspend reality for a moment and imagine that rather than spending your day engaged in intellectual stimulation in front of your computer, you are instead living in the wild like our caveman ancestors.
Back then, human activity could be divided into two states, lounging and looking (for food). The purpose of memory back then, as it is today, is to integrate novel information that is likely to be important to our survival in the future. Back when natural selection was picking which genes our ancestors would pass down to the human race of today, searching for food was the intellectual arena of cognitive challenge. It was on these often lengthy and strenuous excursions from the familiar home site that novel information was most likely to be encountered. Our ancestors walked vast distances in search of food and better habitat, crossing through unfamiliar and dangerously challenging terrain and transcending distances that we now cover sitting on our gluteus maximus behind the wheel of a car. This, the scientists suggest, is why the body hatches new neurons in the memory region of the brain when we exercise -- to better equip us for the cognitive demands of the excursion. If their theory is correct, we should remember an excursion far better if we had peddled our way over the road rather than motored over it effortlessly nudging the wheel in the directions commanded by our GPS. "Make a legal U-turn if possible," (you've zoned out again and missed your exit).
This theory might explain the odd connection between burning calories and birthing neurons. Although the body of city-dwelling humans today was engineered to excel in the environment of our distant past, these ancient mechanisms built into our biology can be extremely useful to humans in modern times. Building brains by exercise has been shown to provide animals with an increased cognitive reserve, meaning that after brain injury or disease that kills or damages healthy neurons, animals that have been forced to do reps on the exercise wheel before a brain injury, do far better in recovering. The animals forced to work out also have much slower cognitive decline in aging compared to sedentary cage-mates, because the loss of brain cells is a normal process of aging.
Surprisingly, the same drugs used to treat chronic depression have been found to stimulate the birth of new neurons in the hippocampus. This ancient biological connection between muscle and brain can account for how pumping iron could benefit our mental health as well as our cognitive health; not to mention the side effect of shaping legs and flattening bellies.