Ever wonder what happens to your brain when you exercise? No? Well that might perhaps be because you exercise too much and it’s killing your brain.

Okay, now that I’ve gotten that bit of extreme journalistic sensationalism out of the way, I’ll admit what I know about the effects of exercise on the brain are somewhat limited. Exercise is generally healthy for the brain, this we know. Working out can also strengthen the connections of your nerves, improve your mood, and keep the right kind of nutrients flowing into your brain. In a recent article I wrote about how many sports associated with impact can also be detrimental to your brain, but that’s the only potential problem with exercising and my brain, right? So long as I don’t play football or soccer I should be good.

Well that might not be the whole picture, especially for long distance runners. In a recent study in BMC Medicine, researchers wanted to look at some of the effects of endurance training on the brain. Apparently, it is fairly well documented that exercise induced hyponatremia can cause acute encephalopathy and brain edema. That sounded terrifying to me, but mainly because I needed to look up a few of those words. Basically, low salt in your blood can reduce brain function and cause excessive fluid or swelling in the brain. This can be worsened by taking an NSAID (Non-Steroidal Anti-Inflammatory Drug) such as Ibuprofen which many endurance athletes make a (bad) habit of. In some cases the edema has killed marathon runners. But that’s not all. Embolism and even brain lesions are possible, but less common from endurance exercise. And here I was considering hitting a treadmill later on.

The researchers in this recent study looked at participants in an extreme ultramarathon. This race took two months to complete and was nearing 3,000 miles in length. If brain damage was going to be caused by any kind of race, it would be this one. They then compared the results of the participants’ MRIs with other populations who experience brain volume changes and lesions. The average person loses about 0.2% of their gray matter annually as a byproduct of aging. Those with multiple sclerosis experience about 0.5% loss, and Alzheimer’s disease has been associated with up to 2% gray matter loss per year. The runners of this ultramarathon lost 6%. That’s not a typo. Six percent in two months. Not a year, two months. They also lost about the same amount of body weight incidentally.

This change wasn’t due solely to losses in water, as you may expect. While water loss could account for both changes in the athlete’s body weight and brain volume, they can only account for a fraction of what was shown.

Here’s the silver lining in that cloud. Researchers also looked at brain lesions, and found no new brain lesions at all in any of the athletes studied. Also, after a six-month follow up, the brain volume had returned to normal, so the issue was thankfully reversible. Changes in sodium, catabolism (the breakdown of tissue), and disturbances in proteins were all cited as possible causes of the findings among other things, but nothing conclusive was determined.

Going forward, if you are a marathoner yourself it is wise to ensure proper nutrition while you are running marathon distances. Keep yourself hydrated, keep an intake of sodium and proteins, and stay healthy. Although some of the aspects of the article may sound frightening keep in mind the bottom line: all exercise is ultimately good for the brain. Make sure you are exercising responsibly and keep checking back for follow ups to this study.