BTW, this is a pretty good article on what the body can do when properly trained; forget standards, like you burn x-calories doing x-activity. The body is far more complex, but also far more adaptable.
Excerpts:
"Torbjorn Sindballe dishes on how he turned his body into a fat-burning machine.
When Torbjørn Sindballe was a professional triathlete, he used the most cutting edge science to make himself the best triathlete he could be. His efforts helped him break the bike course record at the Ironman World Championship, as well as place third there in 2007.
The following is Sindballe’s personal account of how he attempted to make his body into a fat-burning machine, thereby giving his body the most efficient and limitless fuel available to him. It was originally seen in the Nov/Dec 2010 issue of Inside Triathlon magazine.
A good friend of mine once finished a six-hour ride in the mountains on nothing but pure water. No gels, no energy drinks—just water. And he was not out on a Sunday ride—he was hammering, riding hard on the ascents and flying down the descents. Can you do that? Or are you already thinking of how many gels and bars you would need to drag along for the ride?"
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"When we are very fit, our glycogen stores can fuel a six-hour hard ride in the mountains, similar to the one my friend took. But after the ride, the glycogen tank is almost empty. In comparison, even a rail thin triathlete stores enough fat to fuel five Ironmans in a row.
Fat is an almost unlimited resource, but it comes with two problems: The human brain is a sugar lover, and the rate at which fat is burned for fuel is too slow to support a hard, fast Ironman effort. In other words, your body fuels itself with a combination of glycogen and fat (and a little protein), with fat being the source of fuel that lasts but which cannot be tapped quickly enough to keep you moving fast.
The problem of your brain loving sugar can be solved by taking in enough carbohydrates during exercise. And the fat burning problem can be abated by teaching your body to use fat at a faster rate—thus staving off the depletion of the glycogen tank and allowing you to go faster longer. (Once the glycogen is gone, your body can only tap into its fat for fuel, thus forcing you to slow down or bonk.)
The easiest way to improve your ability to oxidize fat—turn fat into energy—is to train for long hours on the trails or in the saddle at a relatively slow pace. Generally, you don’t want to go much faster than your Ironman pace if you’re trying to stimulate your fat oxidation capabilities. While most athletes are well aware of this, there are several diet and training tricks out there that claim to increase the quality of the training stimulus these rides and runs provide. I have researched and tried most of these tricks myself while I was an Ironman pro and now have an understanding of what does and doesn’t work."
Read more here:
How To Tap Into Fat For Fuel