It's kind of sad to me how slowly science can move from the laboratory to the classroom (perhaps less critically important than moving from the lab to the bedside, but still pretty critical). I'd like to think with the popularity of the internet and the spread of science communication direct to your screens (hello!), we'll be moving the latest science to the classroom, but when I was but a sprog, things were different. For example, I was raised in high school and college to think that we were born with all the neurons we would ever have. It turns out, all that time, science knew full well that wasn't the case.
And here's another. In college, heck, in grad school, I learned that neurons only eat glucose. Only glucose. Disdain all other molecules. But it turns out, that's not the case either, and we've known for more than a decade! Neurons aren't such picky eaters after all, they'll eat glycogen, the secondary form of energy storage which we usually think of as being used by muscle.
But it gets wilder than that. A reader pointed me to a recent column from Tara Parker Pope, showing that not only does the brain use glycogen as a source of energy during exercise, but that it can "train" to store more glycogen when the body receives exercise training! Being a running addict as I am (and taking a grudging day off right now for the sake of my knees), I wanted to take a look.
And the best part? Rats on treadmills.
(I should note they seem less than enthusiastic. Usually rats really enjoy running in things like wheels. Can't say as I blame them though, I hate treadmills too.)
Matsui et al. "Brain glycogen decreases during prolonged exercise" Journal of Physiology, 2011.
Matsui et al. "Brain glycogen supercompensation following exhaustive exercise"Journal of Physiology, 2012.
Most of us know that during exercise, your muscles use up the readily available glucose (the preferred cellular fuel) in the bloodstream first. Then they move on to stored glycogen, stored to some extent in the muscle and with more stored in the liver. Glycogen can be broken down quickly to lactate and works fairly well as an alternate energy source. When you run out of glycogen stores, you hit physical exhaustion, where you just can't keep going. Athletes call it "hitting the wall" (running out of muscle glycogen) or "bonking" (running out of liver glycogen). Both of these are unpleasant. There are often tears.
Of course, this happens in your skeletal muscle, the muscles doing most of the moving. But what about your brain? It takes a lot of activity in your brain to do something as simple as running, there is a lot of coordinated firing to keep muscle coordination going, keep the legs moving, and any extra conversation with your running partner? Bonus. We used to think of the brain as being a glucose hog. It turns out it still is, but it also will utilize local stores of glycogen, produced by neuronal support cells in the brain known as astrocytes.
What the authors showed in their first paper was that, in the brain as in the muscle, working out to exhaustion will deplete glycogen stores in both places.
What you can see here are glucose (top left) and glycogen measures (all the rest) in either sedentary rats or rats that ran on a treadmill for 120 minutes (TWO HOURS of moderate speed running. I'm impressed). You can see that glycogen stores were well depleted in the muscle and the liver, and also depleted in various areas of the brain, especially the cortex, cerebellum, and hippocampus. It turns out that it has to be exhaustive exercise. 30-60 minutes of treadmill running isn't going to do it.
But that's just one long run, followed immediately after by the measurements. What if you let your rats recover and refuel?
What you can see here are the recovery curves for glycogen stores in the brain, liver, and muscle. You can see that the brain is indeed a greedy organ, recovering glycogen stores more speedily than the muscle or liver. In general, the brain fully recovered glycogen stores, even overcompensating a little, in 6 hours, while muscle followed behind at 24 hours to peak, and liver coming in last at 48 hours.
But what about if you put your rats in "training"? We know that when muscles are trained, they actually increase the amount of glycogen they can store, becoming more resistant to fatigue. What about the brain? The authors put the rats through a three week regimen of 60 min runs, 5 days a week, running about 20 m/min (that's 1.2 kilometers, or a little less than a mile, and considering the size of the rat, a pretty long jog). They then looked at glycogen stores.
While all the trained rats lost weight and fat compared to sedentary rats, they GAINED glycogen stores, in the muscle, and in the brain. The authors hypothesize that this long term overcompensation could be the brain and body's response to the increased energy demands associated with a long term exercise regiment. But the article in the NY Times (and other studies looking at glycogen demands and memory formation) goes further, speculating that this may lead to the increased cognitive function associated with exercise. I feel like we'd need another study to prove that, something that linked exercise with increased basal cognitive function as well as increased use of glycogen stores during that cognitive function, but it's a very interesting idea. In the meantime, think of your brain glycogen stores while you exercise. Your muscles aren't the only thing being trained.
Matsui T, Ishikawa T, Ito H, Okamoto M, Inoue K, Lee MC, Fujikawa T, Ichitani Y, Kawanaka K, & Soya H (2012). Brain glycogen supercompensation following exhaustive exercise. The Journal of physiology, 590 (Pt 3), 607-16 PMID: 22063629
Matsui T, Soya S, Okamoto M, Ichitani Y, Kawanaka K, & Soya H (2011). Brain glycogen decreases during prolonged exercise. The Journal of physiology, 589 (Pt 13), 3383-93 PMID: 21521757