What Happens to Your Body When You Run a Marathon?
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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.
A typical runner needs between 15 ounces (for a smaller runner) and 30 ounces (for a larger runner) of fluid every hour, Dr. Harrast says. To get a more precise measurement of how much water you need, weigh yourself before and after an hour-long training run to see how much fluid you lost.
As far as food, you’ll want to start taking in calories in the form of simple carbohydrates after an hour to an hour-and-a-half of running, Statuta says. Simple carbs are sugars that break down quickly in the body, rapidly providing you with fuel in the form of glucose, per the Cleveland Clinic. Energy bars, energy gels, crackers, and pretzels all qualify as simple carbs.
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Mile 10
The miles are flying by until your stomach starts to cramp — ouch!
Specific foods may cause digestive upset in runners, too, Statuta says, so experiment during training to find what bothers your stomach the least. Still, there’s no promise your gut will behave. GI distress can happen at any time on a course, Statuta says, including before you cross the starting line (due to race-day jitters).
RELATED: 6 Quick Tips for Running Your Best Marathon
Miles 12 to 15
As the miles increase, waste products (byproducts of the body’s energy-making processes, including lactic acid) start to build in your muscles, and the pounding of feet on pavement (or trail) starts to induce fatigue and soreness, according to Statuta.
You don’t use your muscles as effectively when fatigue sets in, Harrast says, which can lead to strains, injuries, and cramping. The best solution is prevention — be well-trained for race day. Harrast also recommends changing your stride for short bursts every mile or so to give your body a new stimulus. Take bigger steps for a few seconds, and then take smaller steps.
Hydration and fueling also affect the comfort of your muscles, so stick to your food and water plan.
Miles 18 to 20
Things are going great, and then suddenly your legs turn to lead. Each step feels like running uphill. You’ve hit “the wall,” as runners like to call it. Some argue that the wall occurs when you deplete your glycogen stores (glycogen is the stored form of carbohydrates), while others, like Statuta, suggest that your body just moves slower as you fatigue.
There’s also a mental factor to consider. This is likely the farthest distance you went in training, you’ve still got miles to go, and your brain is screaming, “Why are we still doing this?!”
The Finish Line
Your legs aren’t the only thing exhausted — your immune system can weaken during the three to 72 hours after a marathon, Harrast notes. But your immune system being down isn’t enough to give you a cold — you still need to be exposed. All those people hugging you and high-fiving you at the end? They could be passing on a virus. Healthy practices like getting enough sleep, eating well, avoiding crowds, and practicing good hand-washing habits are key to protecting yourself from post-race sniffles, Harrast says.
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