Instead of relying on carbohydrates, some athletes can burn fat calories that their bodies are already carrying, Blende said. "The problem is, you can't just decide in your mind, OK, today I'm going to burn my fat instead of burning more carbohydrates. You have to train for that," she said.
To burn fat, some athletes eliminate all grain carbohydrates from their diets for a period of six to 10 weeks. During those weeks, people also keep their heart rates at a lower level by doing less intense, slower exercises. This allows the body to adapt to the change and build up fat-burning enzymes, Blende said. "Almost all ultramarathon runners have done some fat-burning training," she added. [How Many Calories Am I Burning? (Infographic)]
During fat-burning training, runners should also eat more cold-weather fruits than warm-weather, Blende recommends, because warm weather fruits, such as pineapple and papaya, have more sugars and carbohydrates.
During the actual run, Jurek and many ultra-runners consume "sports food," which is usually pre-digested, meaning all the fiber is removed, but the carbohydrates remain. The sports food comes in blocks, gels and drinks. As people run, their heart rates climb, and so athletes need to consume carbohydrate calories again, Blende said. But for people who have undergone fat-burning training, their bodies will be burning a higher amount of calories from fat than before, because they have built up fat-burning enzymes, she said.
But how do ultramarathoners remain focused for such long periods of time while doing high-intensity exercise?
As long as the body is functioning, the mind is free to wander while running. "Typically if you're going out for a 2-hour or 3-hour run, you can't focus the whole time - sometimes it's good to daydream," Honerkamp said.
But ultra-runners tend to be fiercely competitive, which can help them power through grueling races, said Jeff Brown, a psychologist at Harvard Medical School and author of the upcoming book "The Runner's Brain" (Rodale Books), which will be released in September.
"n ultra-marathoner typically possesses an ironclad identity characterized by a strong belief of completing any task that will face them in competition," Brown told Live Science in an email.
The reticular activating system, the part of the brain that regulates how we drift in and out of sleep, picks up on cues within and around the athlete to fortify or weaken that identity, which develops over time, Brown said.
During long races, it's also important for athletes to manage their emotions.
"In intensely challenging athletic events, an athlete will taste a broad range of emotion - anything from boredom or doubt to hope and exhilaration," Brown said. The limbic system, the part of the brain that controls basic emotions and drives, can also "play a role in post-competition blues that comes on like an unwanted depression following a big event."
Experienced athletes have control over their limbic systems "because smart athletes have intentionally practiced a variety of experiences - both positive and negative," Brown said.
Ultramarathons have skyrocketed in popularity in recent years. "It's probably become tenfold greater now than 20 years ago," Blende said. "There were hardly any 100-mile runs in the '80s and '90s, and now there are several hundred 100-mile runs."
And as more and more people run marathons, runners will likely continue to break records, Emmett said. There was a time when people doubted anyone would break a 4-minute mile, but runners did it, he added.
Part of the reason has to do with evolution, he added. The "theory of persistent hunting" explains that as humans evolved, the only way they could survive was to run down their food, and the quicker they could run, the sooner they could eat. Although humans can cover long distances, Emmett said, increasing speed is trickier, but he said he "thinks we're heading that way."
"Someone like Scott is - I want to say freak as a compliment - a freak of nature" in a way that inspires people to want to do what he does, Honerkamp said.
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