Moving the body demands a lot from the brain. Exercise activates countless neurons, which generate, receive and interpret repeated, rapid-fire messages from the nervous system, coordinating muscle contractions, vision, balance, organ function and all of the complex interactions of bodily systems that allow you to take one step, then another.
This increase in brain activity naturally increases the brain’s need for nutrients, but until recently, scientists hadn’t fully understood how neurons fuel themselves during exercise. Now a series of animal studies from Japan suggest that the exercising brain has unique methods of keeping itself fuelled. What’s more, the finely honed energy balance that occurs in the brain appears to have implications not only for how well the brain functions during exercise, but also for how well our thinking and memory work the rest of the time.Earlier, scientists had believed that the brain, which is a very hungry organ, subsisted only on glucose, or blood sugar, which it absorbed from the bloodstream. But about 10 years ago, some neuroscientists found that specialised cells in the brain, known as astrocytes, that act as support cells for neurons actually contained small stores of glycogen (stored carbohydrates) critical for the health of all brain cells.
In petri dishes, when neurons, which do not have energy stores of their own, are starved of blood sugar, their neighbouring astrocytes undergo a complex physiological process that results in those cells’ stores of glycogen being broken down into a form easily burned by neurons. This substance is released into the space between the cells and the neurons swallow it, maintaining their energy levels.
To study the brain’s stored energy use in live conditions, outside of petri dishes, Japanese researchers used high-powered microwave irradiation to instantly freeze glycogen levels at death. This allowed them to accurately assess just how much brain glycogen remained in the astrocytes or had recently been used.
In the first of their new experiments, published last year in The Journal of Physiology, scientists at the University of Tsukuba gathered two groups of adult male rats and had one group start a treadmill running programme, while the other group sat for the same period each day on unmoving treadmills. The aim was to determine how much the level of brain glycogen changed during and after exercise.
They found prolonged exercise significantly lowered the brain’s stores of energy, and it was especially noticeable in certain areas of the brain, that are involved in thinking and memory, as well as in the mechanics of moving.
The findings of their follow-up experiment, however, were even more intriguing. In that study, which appears in the current issue of The Journal of Physiology, the researchers studied animals after a single bout of exercise and also after four weeks of regular, moderate-intensity running.
After the single session on the treadmill, the animals were allowed to rest and feed, and then their brain glycogen levels were studied. The food went directly to their heads; their brain levels of glycogen had been restored to what they had been before the workout. Rather it had increased by as much as a 60% in the frontal cortex and hippocampus and slightly less in other parts of the brain. The astrocytes had “overcompensated”. The levels, however, had dropped back to normal within 24 hours.
That was not the case, though, if the animals continued to exercise. In those rats that ran for four weeks, the “supercompensation” became the new normal, with their baseline levels of glycogen showing substantial increases compared with the sedentary animals.
It’s not just about rats
The increases were especially notable in, again, those portions of the brain critical to learning and memory formation — the cortex and the hippocampus. Which is why the findings are potentially so meaningful — and not just for rats.
While a brain with more fuel reserves is potentially a brain that can sustain and direct movement longer, it also “may be a key mechanism underlying exercise-enhanced brain function," says Dr Hideaki Soya, a professor of exercise biochemistry at the University of Tsukuba and senior author of the studies, since supercompensation occurs most strikingly in the parts of the brain that allow us better to think and to remember. As a result, Dr Soya says, “increased storage and utility of brain glycogen in the cortex and hippocampus might be involved in the development” of a better, sharper brain.
For people, it means that after a run or other exercise that is prolonged or strenuous enough to leave you tired, a bottle of chocolate milk or a banana might be just the thing your brain needs.
The New York Times