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Extreme Calorie Burn

Extreme Calorie Burn

You’ll learn: When you body begins to use fat to fuel exercise and when you simply can’t eat enough food to keep up with your exercise.


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In 2014, I did a 24-hour endurance event with GORUCK, called a GORUCK Heavy.

We covered 58 miles wearing 35-pound rucks and group-carried other heavy items.

When I got home and weighed myself, I’d lost about 10 pounds.

Much of that was likely water weight. But I wondered just how many calories one single, long effort can burn.

In Monday’s post, we explored how exercise impacts weight loss and maintenance. Read the piece here.

We learned that you can out-exercise a less-than-perfect diet—but it requires a lot of exercise. And you’ll have to maintain that exercise to maintain your weight if you don’t change your diet.

Backing this idea, we had a wonderful comment on that piece from reader Dana Margulies. She explained how she’s able to “outrun” her diet ever since she started training for ultramarathons seven years ago.

But, she noted, “I know it won't last forever. If I ever have to reduce my activity, I know I will need to watch what I eat.”

Today, we’re diving into the science of extreme calorie burn.

Most of us aren’t training for ultra marathons or doing 24-hour endurance events. But new research on the upper limits of endurance can tell us:

  • How many calories you can burn in a single bout of exercise.
  • When your body begins to burn fat during exercise.
  • The upper bounds of human calorie burn (i.e., at what point you can’t eat enough to fuel your exercise and begin to draw on tissues.)
  • What happens to your metabolism when you exercise extreme amounts often.

Let’s roll …

The new science of extreme effort

In short

New research shows that people can burn thousands upon thousands of calories during long endurance events.

The details

Online calculators suggested I burned 8,000 to 12,000 calories during my 24-hour effort. That’s about three to four days of my normal diet.

Yet online calorie calculators are unreliable. To learn more, I texted Mike Roussell, a PhD performance nutritionist and friend of 2%. He ran some back-of-hand math and guessed that I burned 10,000.

But he admitted that this was just an estimate.

There wasn’t much research on how many calories a person can burn in one big physical moonshot.

But that was then, and this is now. Wild new experiments are showing the upper bounds of human calorie burn.