What Happens to Your Body When You Run a Marathon?


It’s the starting line for your first — or maybe tenth — marathon. You’ve got at least six months of consistent training under your belt, and you’ve managed to run 20 miles at least a couple of times. Your body knows what it’s like to run for several hours, and so does your brain.

But what exactly does your body go through once the starting gun fires? Here’s a mile-by-mile breakdown of what happens when you run a marathon.

The First 5 Miles

The excitement of a crowd means most runners start fast. That’s a problem, says Siobhan M. Statuta, MD, a sports medicine primary care specialist at UVA Family Medicine in Charlottesville, Virginia, because you’ll end up burning through your body’s energy stores earlier and fatiguing faster. Not great when you still have (gulp) more than 20 miles to go. “Slow and steady gets the job done,” Dr. Statuta says.

Unlike shorter running events (like a 5K, or 3.1 miles), where you don’t have to worry much about food or water, longer distances mean increased sweat and a greater calorie burn because you’re out there longer, according to Mark A. Harrast, MD, a sports medicine physician at UW Medicine and medical director of the Sports Medicine Center at Husky Stadium in Seattle, Washington.


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