Exercise long enough and everyone hits the same wall, but the human body still allows you to work hard now and rest later.

Share on Pinterest
Endurance events can stress the body in extreme ways. Getty Images

Athletes competing in endurance events like the Ironman triathlon or ultramarathons routinely push their body beyond its limits.

These events are grueling, but they take place over a single day.

Now imagine running the Race Across the USA, a 3,000-mile race from California to Washington, D.C. — runners basically do six marathons a week for 20 weeks.

A new study found that when people sustain this level of exertion for this long, they all hit the same metabolic limit.

The study was published June 5 in Science Advances.

At this point, their body can only burn calories at 2.5 times their resting metabolic rate without needing to break down its own tissues for energy.

The researchers say this may mark the limit for what level of physical activity people can sustain over the long term.

Brent Ruby, PhD, director of the Montana Center for Work Physiology and Exercise Metabolism at the University of Montana, who wasn’t involved in the study, described the limit on energy expenditure as this: “If I’m going to do this kind of effort every day for a year, what’s the maximum energy expenditure that I can maintain without losing weight?”

Ruby pointed out that this isn’t the upper limit for energy expenditure.

A study by Ruby and his colleagues found that athletes in an Ironman had a total energy expenditure of 9.4 times their resting metabolic rate. For athletes in a 100-mile ultramarathon, it was 8.5 times their resting metabolic rate.

The new paper, though, shows that the energy expenditure that people can sustain decreases with the duration of the event, plateauing at around 2.5 times the resting metabolic rate.

They suggest that this limit is determined by the ability of the digestive system to break down food and absorb nutrients to fuel physical activity.

Peter Weyand, PhD, a professor in applied physiology and biomechanics at Southern Methodist University in Dallas, who wasn’t involved in the study, thinks this idea is “certainly plausible.”

“In this scenario, getting the calories into the bloodstream via the gut or intestines becomes all-important,” said Weyand.

He said research in this area has led to experiments on how to speed up getting calories — in the form of simple carbohydrates — from the gut to the blood to the muscles. Several sports nutritional products are based on this.

The new study focused on how much exertion people can sustain over long periods such as 20 weeks or more without losing weight.

But athletes in shorter endurance events routinely burn more calories during the event than they take in — creating an energy deficit.

In Ruby’s study, athletes competing in the Ironman or ultramarathon lost 2.5 kilograms and 1.5 kilograms of their body weight, respectively. He said this isn’t really that bad considering that they burned around 9,000 and 16,000 calories during the race.

If athletes are burning 9,000 calories or more during a race, there’s no way they can eat enough food to keep up — but they can catch up after the race.

“Competing in a single Ironman or ultramarathon is going to create an enormous energy deficit crater,” said Ruby. “But then over the next several days, you just lay low and eat whatever you can find, and you’re back to normal — very quickly.”

There are other factors that may affect the level of exertion that you can sustain. One is getting rid of excess body heat.

The new study also looked at people competing in arctic trekking — a cold environment.

The researchers didn’t find a difference in maximal energy expenditure in athletes competing in cold or warm conditions.

They write in the paper that this may be because endurance events aren’t usually held under very high temperatures. If they were, not being able to get rid of body heat might affect performance and energy expenditure.

Ruby said that talking about energy expenditure without considering the fluid demands doesn’t give the whole picture.

In his study, athletes competing in the 100-mile ultramarathon lost 87 percent of their initial total body water.

“Just contemplate that for a moment,” said Ruby. “Consider losing and attempting to replace that much fluid in less than 48 hours.”

The Ironman and ultramarathon athletes included in Ruby’s study were all non-elite racers.

This shows that with the right training, nutrition, and mental stamina, many people can compete in these races.

“Every one of us has the capability to do this,” said Ruby.

Meghan Laws, an ultramarathon coach, offered some tips on training for an endurance event without “hitting the wall:”

  • Don’t force the training. If you really struggle with motivation, it’s time to take more than a few days off from training, and instead focus on rest, relaxation, and good nutrition.
  • Alternate easy and hard days. Think of the easy day as a reward for the hard work.
  • Avoid the FOMO trap. Instead of watching social media for how your friends are training, focus on yourself and what works best for you.
  • Plan your training to the race. Try to optimize your training so that when you pin the race number on, you’ll have your best performance.