It's easy to understand the links between exercise and bodily health. You can see muscles grow, or – if you're a scientist – slice out a tiny sample of muscle and measure the molecular changes triggered by your workout.
In contrast, it's not intuitively obvious why or how lifting weights or going for a bike ride should boost brain health. But as evidence of the strong links between physical activity and brain function keeps piling up, scientists are finding new ways of peering into the brain to unravel the mechanisms – and they're discovering that exercise has more ways of improving your neural wiring than previously suspected.
The latest finding, published this week in the journal Cell Reports by researchers at the Ottawa Hospital and the University of Ottawa, is that exercise helps kick-start a repair process that strengthens existing brain connections, in part by fixing the myelin "insulation" that protects nerve fibres in the brain. That's particularly intriguing because myelin damage is a hallmark of certain brain diseases such as multiple sclerosis.
The study, led by senior author David Picketts and his former doctoral student Matías Alvarez-Saavedra, involved mice that were genetically modified to have a brain condition called cerebellar ataxia that disrupted their balance and movement. These mice typically lived for only 25 to 40 days – unless they were given access to an exercise wheel.
"We found that exercise could help these animals survive," explains Picketts. The running mice lived for more than a year, and showed other signs of brain recovery such as improved balance. Examining the brains of the exercising mice showed that neurons in the damaged region had more myelin insulation.
The key difference? A protein called VGF, one of many molecules produced throughout the brain and body during exercise (the name stands for … nothing – it's "non-acronymic"). When the researchers used a non-replicating virus to inject VGF into the bloodstream of non-exercising mice, these mice also lived longer and had signs of myelin repair in their brains.
The Ottawa study isn't the only one pointing to links between exercise and healthy myelin, and multiple sclerosis isn't the only condition in which myelin plays an important role. Last month, South Korean researchers published results in the journal Neurorehabilitation and Neural Repair showing that running helped gerbils who had suffered a stroke to recover their memory, in part through the repair of myelin around neurons.
But the identification of VGF as the active molecule by the Ottawa researchers raises hopes that targeted pharmaceutical treatments can be developed for these and other conditions. While there's a big gap between experiments in mice and applications in humans, this is a long-term goal that Picketts and his colleagues are now pursuing with funding from the MS Society and the Canadian Partnership for Stroke Recovery.
Myelin, it turns out, also plays a crucial role in healthy brains. In 2014, scientists at University College London showed that when we learn complex new skills, our brains forge new connections among neurons, and myelin quickly forms a protective sheath around these new circuits. Disrupt myelin production, and we're unable to retain the new skills. So does this mean exercise enhances learning, even in the absence of any brain disease? When the Ottawa team measured myelin levels in healthy mice, running didn't seem to produce any significant change. "Generally, healthy people [already] have normal levels of myelin," Alvarez-Saavedra notes.
That means the main benefits of VGF-triggered myelin repair probably kick in when a disease is attacking the brain, Picketts says.
Of course, myelin isn't the only part of the brain that benefits from exercise. Other studies have linked exercise to changes such as increased blood flow and decreased inflammation in the brain. Perhaps most significantly, a review this month in the journal Behavioural Brain Research concludes that exercise likely plays a role in the growth of new neurons in the hippocampus, a brain region associated with memory, in adults – a regenerative process widely thought to be impossible until the 1990s.
That has potentially significant implications for warding off the cognitive decline associated with aging and with conditions such as Alzheimer's disease, Parkinson's disease and depression. If none of that convinces you that the benefits of exercise on the brain are worth pursuing while you're still healthy, then it's worth considering one of the caveats noted by the South Korean researchers studying myelin and stroke recovery. Running on a treadmill starting a few days after a stroke, they noted, did help their gerbils' brains recover from strokes. But the protective effects were far greater in gerbils who started running before their stroke.
For your brain's sake, in other words, the safe bet is to start (or continue) exercising now.
Alex Hutchinson's latest book is Which Comes First, Cardio or Weights? Follow him on Twitter @sweatscience.