How Running a Marathon Affects the Body

— -- From the start of the race to the finish, running a marathon can take a toll on even the most experienced runners. Here's a look at how your body can be affected by running 26.2 miles.

Your Body Reroutes Blood Flow

As a runner starts a race, the body immediately sends blood to the areas in use.

"The body is very smart it starts sending oxygen and blood to where it's most needed," said Dr. Dennis Cardone, the chief of primary care sports medicine at the Department of Orthopedic Surgery at NYU Langone Medical Center in New York City. "The blood flow to the stomach and gastrointestinal system starts to decrease and increasingly blood flow goes to working muscles."

As a result of decreased blood flow to the stomach, Cardone advises runners to avoid eating anything that can disrupt the stomach.

"It's why we tell people don't have a big meal, don't have anti-inflammatory medication," Cardone explained. Discomfort "can get more pronounced."

All of Your Muscles Get a Workout. ALL of Them

For the marathon distance, the body uses every single type of muscle fiber to help get through the race.

This means using fast-twitch muscle fibers normally only used for sprinting. As a result of tapping into all this muscle fiber, carbohydrates stored in the muscle start to deplete, meaning a runner might need a mid-race snack.

Dr. Lewis G. Maharam, the chairman of the International Marathon Medical Directors Association, told ABC News in an earlier interview that lower levels of carbohydrates can complicate a run for marathoners.

“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]."

Drinking Too Much Water Can Affect Your Sodium Levels

While sports medicine experts used to warn runners to constantly drink water so they are never thirsty, they now tell runners to drink enough water but not too much. Too much water can diminish the sodium concentration in the blood. Some runners are so anxious to stay hydrated that they drink too much water, thereby diluting sodium levels, sometimes to an extreme degree leading to a condition called hyponatremia, Cardone said.

"Muscles start to function improperly and [runners] can start to have some mental status changes," Cardone explained.

Your Legs May Swell Post-Run

After running a full marathon, blood can start to pool in the lower extremities as the heart also starts to slow down. Called "venous pooling" the condition usually reverses quickly, but sometimes runners can pass out at the finish line as they become lightheaded.

"With that blood pooling you may notice that your feet or even your hands become stiff," Cardone said.