Just for a moment, let's look at today's Pittsburgh Marathon as an amoeba.

If you were suspended high above the Strip District starting line today, the massed marathon runners would indeed look like a giant single-celled organism with 21,000 legs.

Like any other living being, the marathon has to move. It has to burn energy. It has to respire. And it has to maintain its temperature.

And from our lofty vantage point, the Pittsburgh Marathon turns out to be one mighty impressive creature.

Based on the best estimates from exercise physiologists, it appears that the 10,500 marathon and half-marathon runners today will burn roughly 20 million calories.

More than 15 million of those calories will be given off as heat. Also, our creature will exude 18,160 gallons of sweat and will exhale 13,900 pounds of carbon dioxide.

That's one big energy-burning organism, and it raises the question of just what other uses all that work might be put to if it weren't being spent pounding the pavement over 26.2 miles.

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For one thing, the marathon calories expended are equivalent to burning 636 gallons of gasoline. That's enough for a Toyota Prius to make five round trips between Los Angeles and New York City.

Or, said Bopaya Bidanda, chairman of the industrial engineering department at the University of Pittsburgh, it's enough energy to pull a 35-ton tractor-trailer from Pittsburgh to Phoenix and back.

The marathon output also equals more than 23,000 kilowatt hours of electricity, which would meet the electricity needs of more than 26 Pennsylvania homes for a month, according to Dr. Bidanda, who has run two Pittsburgh marathons and worked on his calculations with Martin Gaussin, a visiting student from the French Institute of Advanced Mechanics.

Another equivalent: Since it takes 3,500 calories to burn off a pound of body fat, he said, the marathon could easily qualify for "The Biggest Loser," with the power to shed about 5,700 pounds.

The starting point for all these numbers is the remarkable factory known as the human body.

David Swain, an exercise science professor at Old Dominion University in Norfolk, Va., and associate editor of the journal "Medicine & Science in Sports & Exercise," said a marathon runner will basically chew up 100 calories a mile, or 2,600 for the full course, above and beyond the "resting state" calories he normally would use.

For some reason, sports studies have based their estimates on a 136-pound runner. For the Pittsburgh Marathon, we've bumped it up to a more robust 150-pound average.

Jie Kang, an exercise science professor at the College of New Jersey who did his doctoral training at the University of Pittsburgh, calculated calorie burning based on speed and gender, and came up with a slightly lower estimate.

But Dr. Swain pointed out that not only is this an inexact science, but faster runners who burn more calories per mile will be balanced out by slower runners who burn fewer calories but spend much more time finishing the course.

Many of the slower runners may walk part of the marathon, and when they do, they will burn about half the number of calories per mile, he said.

Why such a big differential, when the same mass is being moved over the same distance?

It's because 50 percent of the energy used by a runner is actually expended "leaping into the air," Dr. Swain said, rather than propelling the body forward.

Our physiological factories are also not terribly efficient. Only 22 percent to 24 percent of the calories runners burn are actually converted to useful work. The rest of those calories are given off as heat.

With the marathon runners radiating nearly 80 million British thermal units, it might seem they would warm up the air at least a little bit.

But "the air" is a big place, points out Todd Miner, a meteorologist at Penn State University.

"Given how many cubic yards of air space there are around Pittsburgh," he said, "I think you'd find any heat effect would be very, very small. It would have little, if any. impact a block away.

"But if any of the runners stopped and stood next to you, you'd probably feel the heat coming off them."

You also can't blame the marathoners for any significant contribution to global warming, even though they will collectively exhale nearly seven tons of carbon dioxide.

To put that in perspective, consider the fact that a typical coal burning power plant kicks out 5,200 tons of carbon dioxide per day.

When it comes to sweat output, the expert is David Dzombak, a civil and environmental engineering professor at Carnegie Mellon University.

He calculated that a runner will pump out a little more than half a gallon of sweat every hour, making the combined output more than 18,000 gallons. That, he gleefully said " would fill a circular swimming pool that is 20 feet in diameter with 7.7 feet of sweat!"

Dr. Kang estimated that at least 85 percent of the calories burned during a marathon come from carbohydrates, which are primarily stored as glycogen in the muscles. The rest comes from fat and a little protein.

It takes twice as much oxygen to burn fat as it does to burn carbs, Dr. Swain said, which is why the body uses up carbohydrates first when its energy demand is high.

When we're at rest or only mildly active, he said, our bodies tend to burn fat. Just sitting on the couch burns 62 calories an hour, he said, assuming we don't offset that with potato chips or doughnuts.

For the less kinetically inclined -- such as all the folks who will be watching the marathon from the sidelines or on TV -- this could be viewed as a silver lining.

In fact, based on those figures, it would take about 81,000 static spectators to equal the energy output of the marathon runners. In Pittsburgh, we might just be able to hit that goal.

Carbs, anyone?