Dean Karnazes: the man who can run for ever

From club runners to Olympians,
every athlete has a limit.
Scientifically, this limit is defined
as the body's lactate threshold and
when you exercise beyond it,
running rapidly becomes
unpleasant. We've all experienced
that burning feeling – heart
pounding, lungs gasping for air –
as your muscles begin to fatigue,
eventually locking up altogether as
your body shuts down. However,
there is one man whose
physiological performance defies
all convention: Dean Karnazes is
an ultrarunner from California
and, at times, it seems as if he can
run forever.
Karnazes has completed some of
the toughest endurance events on
the planet, from a marathon to the
South Pole in temperatures of -25C
to the legendary Marathon des
Sables, but in his entire life he has
never experienced any form of
muscle burn or cramp, even during
runs exceeding 100 miles. It means
his only limits are in the mind.
"At a certain level of intensity, I do
feel like I can go a long way
without tiring," he says. "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."
When running, you break down
glucose for energy, producing
lactate as a byproduct and an
additional source of fuel that can
also be converted back into energy.
However, when you exceed your
lactate threshold, your body is no
longer able to convert the lactate as
rapidly as it is being produced,
leading to a buildup of acidity in
the muscles. It is your body's way
of telling you when to stop – but
Karnazes never receives such
"To be honest, what eventually
happens is that I get sleepy. I've
run through three nights without
sleep and the third night of
sleepless running was a bit
psychotic. I actually experienced
bouts of 'sleep running', where I
was falling asleep while in motion,
and I just willed myself to keep
While supreme willpower is a
common trait among ultrarunners,
Karnazes first realised that he was
actually biologically different
when preparing to run 50
marathons in 50 days across the US
back in 2006. "I was sent to a
testing center in Colorado," he
recalls. "First, they performed an
aerobic capacity test in which they
found my results consistent with
those of other highly trained
athletes, but nothing
extraordinary. Next, they
performed a lactate threshold test.
They said the test would take 15
minutes, tops. Finally, after an
hour, they stopped the test. They
said they'd never seen anything
like this before."
As Laurent Messonnier from the
University of Savoie explains, the
difference is that your aerobic
capacity is a measure of your
cardiovascular system
performance, while your lactate
threshold is your ability to clear
lactate from your blood and
convert it back into energy.
"If you take a high-level runner
and you train that guy for a long
time, his cardiovascular system will
improve until a certain point
where it will be very difficult to
improve it further, as it's
determined by the heart and the
blood vessels. So if you carry on
training that guy, you will not
improve his aerobic capacity but
his performance will still improve,
because the lactate threshold is not
limited by the cardiovascular
system – it's determined by the
quality of the muscles."
Your body clears lactate from the
blood via a series of chemical
reactions driven by the
mitochondria in your muscle cells.
These reactions transform lactate
back to glucose again and they are
enhanced by specific enzymes. The
clearance process also works more
efficiently if your mitochondria
have a larger capacity, increasing
their ability to use lactate as a fuel.
Years of training will improve both
your enzymes and mitochondria
and so improve your clearance, but
there is a limit to how much you
can improve your lactate threshold
by training alone. If you inherit
these enzymes and a larger mass of
mitochondria genetically, your
personal limits will be far higher.
Karnazes fell in love with running
from an early age, and at high
school he began to show endurance
capabilities which far surpassed
those of his peers. At one charity
fundraiser, while his fellow
runners were able to manage 15
laps of the track at most, Karnazes
completed 105. But in his mid-teens
he stopped altogether until
experiencing an epiphany on his
30th birthday. Gripped by a
powerful desire to run once more,
he set off into the night.
After 15 years of no training, most
of us would not have been
physically capable of getting too
far, but Karnazes did not stop until
30 miles later. Although the blisters
were excruciating, his muscles
showed little sign of fatigue.
"Many elite distance runners will
show some improvements in their
ability to clear lactic acid from the
system due to the 'training effect',
but that only goes so far," he says.
"The rest, as I am told, is left up to
heredity. They say the best thing
you can do as a long-distance
runner is to choose your parents
However, genetics alone does not
tell the full story. Karnazes
believes that his lactate clearance
abilities could also be down to low
body fat, low sweat rate, a highly
alkaline diet and low exposure to
environmental toxins. Genetics can
give you the propensity for a
natural advantage but you express
your genes differently depending
on your environment and your
The intriguing question is whether
Karnazes' lactate clearance
abilities would be the same now if
he had not done so much running
at an early age.
"If you take two twins – one grows
up in Africa and one grows up in
northern Europe – their athletic
performance will potentially be
very different, because they will
express their genes differently as
the environment, food, everything
is different," Messonnier says.
An interesting experiment could be
to repeat the lactate threshold test
with Karnazes' brother.
"He plays competitive volleyball
but has never really done an
extensive amount of running,"
Karnazes says. "I would be curious
if he exhibits some of those same
abilities to clear lactic acid from
his system."