This Is What Happens to Your Body When You Run a Marathon

Boston marathoners can expect their bodies to take quite a beating.

"Everyone has pain," said Dr. Lewis G. Maharam, the chairman of the International Marathon Medical Directors Association. "It's part of the deal."

Muscles take the brunt of the damage, he said.

Normally, the body only needs slow twitch muscle fibers to drive it forward. But for the marathon distance, the body recruits every single type of muscle fiber, including the fast twitch fibers normally only used for sprinting, he said. That uses up a lot of blood and almost all of the carbohydrate energy supplies stored in the muscles and blood.

“When you exhaust glycogen stores, the body’s preferred source of sugar, you start breaking down body fat and muscle protein,” he said. "That’s when you’re in danger of [hitting the wall]."

He added that can lead to psychological symptoms like confusion and disorientation.

On the other hand, runners who drink too much are in danger of developing hyponatremia, an imbalance of electrolytes, Maharam said. High sodium concentrations in the blood can be so severe they lead to brain swelling.

“No one knows why, but hyponatremic runners lose their ability to remember numbers,” he said. “When you ask them where they live they can tell you the street but not the house number.”

The runners who make it to the finish line can expect some muscle soreness for up to a week, Maharam said.

“It’s by inflammation and microscopic muscle tears,” he added. “That same pounding can also cause joint pain and tightness in the tissues that connect bone to muscle.”

Running a marathon can compromise the immune system for several months afterwards, leaving marathoners susceptible to colds and infections. But most of the other effects will disappear after a drink of water and a good meal, Karp said.

So why do runners put their bodies through all this? Maharam said most runners he’s talked to expect some suffering but ultimately feel it’s worth it. And as one recent study in the journal Memory suggested, the pain of a marathon, like the pain of childbirth, is the kind of pain you forget.