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Many marathon runners know what can be stimulated by bursting an energy gel halfway through. (Mmmm, goop high in calories.) But according to new research published in Progress of scienceWith regard to endurance events, there is a limit to the amount of energy that the human body can derive by breaking down or metabolizing food. This metabolic limit depends on the duration of your tortu … – uh, the event you endure lasts. But above all, after a certain point of time, these are plateaus suggesting to humans a universal ceiling for the amount of energy that their bodies can absorb.
Extra juice
It all started with a race. Not in any race, though – a transcontinental 140-day race across the United States called, rightly, Race Across USA (RAUSA). Starting in the Los Angeles area and ending in Washington DC, participants ran essentially a marathon a day for more than four months in 2015 to raise awareness of obesity in America. Bryce Carlson, one of RAUSA's organizers and director of research, invited Herman Pontzer, an evolution anthropologist from Duke University, and his team to accompany the event's science team.
Caitlin Thurber, one of the Master's students at Pontzer, first author of the Progress of science paper, took action from six of the runners. At different times before and during the race, she calculated athletes' daily energy expenditure, which is the amount of energy a person uses in a day.
To do this, the team relied on something called the double tagged
water method (DLW). DLW replaces hydrogen and oxygen in ordinary water by
different isotopes, or variations, of each element. (For isotope nerds on
there: They used oxygen 18 and hydrogen 2, also called deuterium.
Pontzer, the use of less common isotopes creates what amounts to a metabolic follow-up
technical.
When our body metabolizes carbon, which is found in just about everything we eat, one of the waste byproducts is carbon dioxide (CO2). Of the two CO2 oxygen atoms, one can be attributed to the food we ate. But this second oxygen comes from the water. So, if you change the isotopes in the water that someone drinks, the isotopes act as a chemical label that can help you locate and measure metabolic waste CO2. "This, in turn, provides an accurate measure of calories burned," says Pontzer in an email.
By calculating the number of calories that the runners have burned, each
day, Pontzer and his team were also able to calculate something called
metabolic reach. It's the relationship between daily energy expenditure and something
called resting metabolic rate (CMA) – the energy that a person needs to exercise
basic functions, rest, such as breathing and circulation. Basically, your
The metabolic scope is how much juice you have left for things that
are not essential to survive, such as growing up, breeding or … running six
marathons a week for more than four months, as do all healthy-minded humans. They found that
on average, runners' metabolic scopes were about 3.24 times their CMAs.
But the researchers wanted to know more. "We were curious to see
how high levels of spending in RAUSA runners have accumulated against
other measures of other extreme events, "writes Pontzer. So they turned to
Literature.
Wear it to the metabolic limit
They reviewed previous research papers and amassed information on a range of other endurance activities. These included Ironman competitions (a non-stop triathlon consisting of a 2.4km race, a 112km bike race and a marathon, completed in about half a day); the Tour de France (where cyclists travel about 2,200 miles in 23 days); 50 and 95 day treks across the Antarctic; farmers working during the 120 days of harvest in The Gambia; even pregnancy (I mean, women develop a new person over a period of about 280 days).
After
plotting the average metabolic scopes for these events, the researchers noticed
a trend. The longer an endurance event lasts, the lower the metabolism of a person.
reach obtained. At around 275 days and beyond, the metabolic reach reached a plateau of 2.5 x
CMA.
Pontzer
and his team assumed that this limit was related to the amount of food that someone ate. They
looked again at other studies that were either about overeating – eating more
calories that you use, then do a minimal physical activity in the following
days – and studies where people ate what they wanted but also participated
in the endurance events.
It does not matter that people are eating too much or that they are experiencing grueling events, when Pontzer and one of his coauthors, John Speakman, were calculating the amount of energy actually absorbed by the participants in the foods, it still reached the limit of 2.5 x CMAs.
"If you want to spend energy above 2.5 x [RMR]you can only do
that if you burn body fat and other body reserves, "says Speakman, a
Biologist at the University of Aberdeen and at the Chinese Academy of Sciences. "Yes
you spend less than 2.5%, you can keep it indefinitely from the food.
An open challenge
While this may sound like an awesome factoid, knowing this metabolic limit is particularly interesting for experts whose bread and butter push the boundaries of human endurance. In particular, says Speakman, it is "important for operations such as military operations and other endurance activities, for which we expect people to perform for long periods of time with a high level of expense" .
But it could also shed light on our understanding of other phenomena, such as how our ancestral roots might play. The 2.5 x CMA limit on the metabolic reach of endurance athletes is not much higher than the metabolic limit achieved by women during pregnancy, which is approximately 2.2 x CMAs. These similarities suggest that the amount of extra energy we have left for non-essential functions is rooted in evolution. But it's a question of future research. Meanwhile, Pontzer is open to the possibility of missing out on the target with its metabolic limit. "I guess it's a challenge for endurance endurance athletes," Pontzer says in a press release. "Science works when we are wrong. Maybe someone will exceed that ceiling one day and show us what we are missing. "
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