Is there an upper limit on how many calories can be burned via exercise

I remember reading once that 2.5x the resting metabolic rate was the upper limit. Why is there an upper limit? Can the body just not repair itself enough after that much exertion, or does the body have some kind of fail safe so it shuts down when too much energy is being expended?

Lance Armstrong’s trainer has a diet book out. He was on one of the cable TV shows and was asked what Armstrong ate during a day. His answer was that for breakfast he had cereal, french toast and an omelett. A couple of hours before training he has some pasta and then in the evening after training some more pasta. Total calories was 10,000 (I believe that would exceed your 2.5x figure.)

The calories being burned are also producing heat (which helps keep us at 98.6°F). Also explains why we get hot & sweaty when working hard.

I would assume there would be a point where the number of calories being burned would produce so much heat that, despite the body’s cooling systems (like sweating), the body is overheated enough to go into a failure mode (heatstroke, maybe?).

But I have no idea how to actually calculate the number of calories to trigger this. And I’d guess that it varies by individual; a trained athelete like Lance Armstrong is probably much more efficient in using calories than your average obese couch potato.

If there is a limit, and I doubt it, the heat would not be a factor. To burn that many calories would take many hours, and if the body would not get overheated, unless other factors are present (such as a high heat index).

If there is a limit it would be due to the lack of fuel. Carbs and fats are used in most exercise, with carbs predominating except in very slow work. Carbs are exhausted relatively soon (muscle and liver glycogen stores are depleted). Free fatty acids are then depleted, forcing liver glycogenesis which is the production of glycogen from protein (amino acids) supplies. This is a slow process and it may not be able to keep up the demands if you continue to exercise.

In any event, once the glycogen stores are gone, you must slow down dramatically, which will seriously affect your ability to continue to exercise.

The amount of calories a humans can burn for sustained periods is limited by the amount of oxygen they can take in. Humans are in fact very good at this sort of thing since we’re built to run down prey on the African savannah basically by chasing it it until it collapses from exhaustion.

I believe the maximum aerobic capacity in humans is about 20 times the baseline resting rate. No other animal can beat this except for the dog family.

Using the above number and figuring 2500 calories per day for an adult male at rest, I get an upper limit of about 2000 calories per hour. Of course, this number is for elite athletes in peak physical condition. Think Tour de France, not jazzercise.

How high of a mountain of pasta does he eat? Because I can’t see this menu coming anywhere near 10,000 calories. Four-egg ham and cheese omelette, 600 calories. Large bowl of cereal and whole milk, 500 calories. Five slices of french toast, 750 calories. Half cup of syrup, 440 calories.

That leaves over 7,000 calories unexplained. 4 oz. of cooked spaghetti is 210 calories. Eight pounds of pasta?

I shudder to think what you’d have to do to get that kind of burn. I recall seeing a calorie expenditure chart on the 'net that listed “uphill skiing, maximum effort” as “only” 1126 calories per hour.

Well, they didn’t really specify what was in the omelette, so it could be loaded up with more than ham and cheese…maybe sausage, mushrooms, ham, three-cheese blend, peppers, onions, bacon, tomatoes, chicken, beef, etc., plus we don’t know how many eggs are involved. They also don’t mention any additional supplements, like a protein “weight gain” type of powder mix, which chould conceivably add a lot of calories without taking up a lot of stomach space.

Then again, maybe they just left out the part where he drinks blenderized pizzas.

/hungry now

In one of Tom Clancy’s books, a character recovering from an injury is undergoing physical therapy. The nurse advises him that his pulse is 220, and that he should back off a bit. He asks “What’s the record?” Her response: “Zero”.

That’s one limit!

10,000 calories is “only” 1.1 kg of fat. Add enough butter to your omlette of pasta (say, 2.4 pounds) and you’re there. He has to eat a lot, no doubt about it, but I’ve done worse on all-you-can-eat sushi night a the local Japanese restaurant.

We’re talking about high-end athletes’ caloric burn, right? So the answer is “uphill skiing, outrunning the guy who is giving maximum effort.”

I’ve also read – but am not sure if I believe – that the brain, which normally uses up so little caloric power that it’s “in the noise”, can run itself up to some prodigious amount of power consumption during high-stress events (dozens of calories per hour). An Olympian soccer player in the middle of a tight match, playing against his peers, might unknowingly “overclock” his brain to keep track of all the players and the ball, velocity vectors, changing field conditions, the score, the crowd noise, etc. and burn a huge amount of calories that would probably be written off as “he was playing World-class soccer for 90 minutes.”

Presumably, “world-class” students would also be able to put on a Game Brain for an exam, and burn extra heat during a particularly difficult real-time exam (like the SATs or MCATs/LSATs/The Bar). This might manifest as hunger after the fact, or cold fingers and toes, or sleepiness.

Noise?! Lies! The brain consumes 20% of your body’s energy, while making up only 2% of its mass. I don’t know how much this varies depending on what your brain is doing, but here’s a link:

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