He has run a marathon to the South Pole in temperatures of −13 °F, completed fifty marathons in fifty days - one in every state - and most famously sprinted 350 miles in just 80 hours and 44 minutes without sleep.
Yet unstoppable marathon runner has never had a cramp or even experienced his muscles seizing up.
Now Dean Karnazes has revealed that his superhuman abilities are down to a quirk of his physiology. The 53-year-old has a rare condition which allows his body to rapidly flush lactic acid from his system.
Typically, as we exercise, the body converts glucose to energy which produces lactic acid as a by-product. As that builds up in the muscles, it begins causing cramps and fatigue as a signal to stop.
Karnazes never receives those signals. As a result, he is able to keep on running without stopping and compete in some of the toughest endurance races in the world.
'At a certain level of intensity, I do feel like I can go a long way without tiring,' he told the Guardian previously.'No matter how hard I push, my muscles never seize up. That's kind of a nice thing if I plan to run a long way.'
During one incredible feat, he was able to keep running for three days and three nights without sleep - although he admits that the third night took all his willpower.
Even then he experienced bouts of 'sleep running' - falling asleep while he continued to jog.
'To run that far and that long is a super-human feat,' Nicole Marie Pinto, an exercise physiologist at the University of California San Francisco Medical Center, told Inside Edition
Karnazes began running as a kid and began exhibiting extraordinary endurance skills at an early age.
At a high school fundraiser, his classmates completed 15 laps round the track, while he finished at 105.
Despite his abilities, Karnazes stopped running after high school and did not return to the sport until his 30th birthday.
Since then, there has been no looking back. The endurance athlete has single-handedly completed 'The Relay', a 200-mile run from Calistoga to Santa Cruz, eleven times.
He also completed a marathon to the South Pole in −13 °F without snowshoes in 2002, while he ran 350 miles in 80 hours in 2005.
Karnazes said he first realized he was different from other runners while training for another stunt, running 50 marathons in 50 days across 50 states back in 2006.
Doctors performed a lactate test to analyze how long it took for him to reach his lactic acid threshold.
The test typically takes around 15 minutes. Analysts eventually gave up on the test after more than an hour.
While years of running can help to improve an athlete's lactic acid threshold, Karnazes' abilities come from his unique, innate ability to flush his body during exercise.
He also believes that his very low body fat, and his high alkaline, paleo-style diet also helps his superhuman running skills.
Karnazes, who regularly runs the equivalent of a marathon before breakfast, said his focus is always on endurance.
'I don't care how fast I go,' he told IE. 'I care about how far I go.'
When he is not running, Karnazes has written several books on the subject of running and healthy eating and has raised two children with his wife Julie.