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Running a marathon: What happens to your body?

Excitement, pain, regret, acceptance and relief.

These could be considered the five stages of running the full 26.2-miles. But emotions aside, a runner's body experiences some dramatic physical changes over the course of the race.

From the start to the finish line, Dr. Sanjey Gupta — vice chairman of emergency medicine at Long Island Jewish Medical Center and a longtime runner — walks us through the different ways your body reacts to running a marathon. He also shares his tips for how to avoid "bonking" and other common pitfalls.

Pre-race jitters

As you wait at the starting line, you
Photo Credit: Patrick E. McCarthy

As you wait at the starting line, you might feel nervous. "It's the anxiety that goes along with starting this big challenge," Gupta says. Your sympathetic nervous system is activated, which can cause your heart rate and breathing rate to increase, your blood vessels to constrict and your blood pressure to rise.

"Your heart is pumping more blood to your muscles in preparation of the race starting," Gupta said.

The start

Then, the starting horn goes off, and
Photo Credit: Patrick E. McCarthy

Then, the starting horn goes off, and "once you start moving, that's a whole different thing," Gupta said.

Since you're running in the middle of a large crowd, you'll have the tendency to become hyperalert, he said. Your attention sharpens, and even your eyes begin to work harder to "make sure that your next step is going to be a safe one." Your "fight or flight" response kicks in, getting your adrenaline pumping, he said, but added, "Hopefully, you don't go out too fast." And, of course, your muscles, your heart and your respiratory system are all working harder now that you're exercising.

Miles 1 to 3: Settling in

Within the first mile of the marathon, most
Photo Credit: Instagram / tarakconry

Within the first mile of the marathon, most runners will relax a little and settle into their pace, Gupta said. (Usually, you'll be grouped with others who run at the same speed.)

"Your heart rate is going to be elevated, but the benefit of all your training is to make your body efficient," he said.

Assuming you trained appropriately and are sticking to your pace, he said, your breathing should settle into a normal rate by this point in the race.

Miles 3 to 5: The high

If you've put in the training, you should
Photo Credit: Richard T. Slattery

If you've put in the training, you should be feeling pretty good around this point in the race.

Your body's sole purpose right now is to maximize the amount of blood it has to pump to necessary systems including your muscles, which means it will siphon it off from others not vital to the task at hand, such as your gastrointestinal tract, he explained. (This is one reason why you should avoid eating any foods during the race that might be difficult to digest, he added.)

Miles 5 to 10: Fuel in the tank

Most runners will be about an hour into
Photo Credit: Ed Betz

Most runners will be about an hour into the race at this point, and your body is running on glycogen stores, about 500 grams, for fuel. This is the energy from that bagel you had earlier in the morning or anything else you ate before the race, Gupta said.

Typically, the body requires about 100 calories per hour, he added, so "most people are good for at least the first half of the race with glycogen stores."

Gupta recommends eating complex carbohydrates beforehand, because they break down slower, giving your body more sustained energy.

Miles 10 to 15: Feeling dizzy?

It's important that you hydrate throughout the race,
Photo Credit: Mike Stobe

It's important that you hydrate throughout the race, Gupta says.

"If people haven't taken enough water, once the body is depleted, that's when you're starting to get cramps," he explained.

But too much of a good thing can also be bad.

"If you take in more water than you're losing, you can become hyponatremic," which means you've diluted the amount of sodium in your body, he said.

The symptoms of this would be dizziness and weakness, Gupta said, which you might start feeling around 10 to 15 miles into the race.

Bottom line: Adjust your water intake based on how much fluids you appear to be losing via sweating, he said.

Miles 15 to 20: 'The wall'

Although it varies by runner, many marathoners say
Photo Credit: Mike Stobe

Although it varies by runner, many marathoners say they "hit the wall" somewhere in the 15- to 20-mile range of the race. What's happening, Gupta explains, is that your body has fully depleted its glycogen stores.

"You've burned up any fuel you've taken in earlier in the day, so you're relying on your body to power you through by breaking down muscle and fat tissue," he said.

At this point, you've probably built up a lot of lactic acid in your body, your legs are locking up and your electrolytes and phosphorus levels may also be low. And you're starting to get fatigued.

"You might feel like your brain is in a fog," said Gupta, referring to this experience as "bonking."

"Bonking starts at the top," he said. "If your brain is 'bonking,' your body is going to 'bonk,' too."

Eating a banana can help. Some runners also turn to energy gel packs. Gupta also recommends taking in a little protein, but make sure it's easy to digest.

Miles 20 to 25: Ouch

Your legs, your feet ... everything hurts at
Photo Credit: Mike Stobe

Your legs, your feet ... everything hurts at this point.

You're more than halfway through and yet you still have a 10K to go as you cross the 20-mile marker. If you haven't hit the "wall" yet, you're probably close to smacking into it at this point.

"The soreness in your legs is just from the repetitive motion of running," Gupta said. "And your feet have been taking a huge pounding, so you might be developing blisters or hot spots."

One way to reduce foot issues is to stick with the same sneakers and socks you used during training; never try something new, he added.

But if you're experiencing some new aches and pains, they could be the result of your running form deteriorating as your muscles start fatiguing, he said.

"You may not even realize it until you start getting pain in your calf muscle," he said. "This is where your attitude on finishing really kicks in."

Miles 25-26.2: The final kick

You're in the home stretch now with just
Photo Credit: Instagram / tarakconry

You're in the home stretch now with just a little over a mile to go.

And with the end in sight, your body rallies to push you to the finish line.

"Once your brain decides it's no longer fatigued, it somehow convinces the body it isn't either," Gupta said. "This is why some people have a 'kick' at the end."

The finish

You summon all you have left to surge
Photo Credit: Joseph D. Sullivan

You summon all you have left to surge across the finish line, but your body is not always ready to celebrate -- at least not right away.

"There are a lot of reasons people collapse at the finish line," Gupta said, "but it's mainly for benign reasons."

All this blood has been pumping to your muscles, so when you start slowing down, your core temperature goes up and your body wants to release this heat, so the vessels in the skin dilate, your blood pressure drops and you have a momentarily passing out until things go back to normal, he explained.

Some runners even vomit after they cross the finish line.

Gupta said this is usually the result of water or energy gels just sitting in their stomach due to lack of blood flow to their gut. When they push themselves to run harder to finish the race strong, the exertion causes them to vomit the contents of their belly.

Post-race recovery

Show off your medal proudly, indulge in your
Photo Credit: Instagram / aemerie

Show off your medal proudly, indulge in your favorite meal and give your body much-needed rest and recovery.

"Your body has taken a pounding, there's a lot of strain on your muscles, and you're going to have to deal with the consequences of a lot of muscle breakdown," Gupta says.

Those effects include plenty of soreness, lactic acid buildup and muscle pain, he said.

He recommends staying hydrated, eating protein and allowing your muscles time to heal.

"After I do a long-distance event, I don't do anything," he added.


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