[brianwawok]

Well in theory you run out of sugar calories after 2000 calories, but you have 50-100k fat calories. If he trains a lot in a fasted state, in theory he would have a much larger bank of energy to draw from. Many animals, for example dogs, mostly burn fat.

On the other hand, burning fat is less effecient, so your peak output is lower.

[IgorPartola]

That makes sense. So basically as long as you don't run more than 20 miles or equivalent, the benefit of low carb training is less noticeable.

One more question: what about proteins as a fuel source?

[brianwawok]

So taking a step back, as you go about a day you are always getting some energy from sugar stores, and some from fat stores. Example graph:

http://www.sportsscientists.com/wp-content/uploads/2010/01/F...

Sitting still mostly fat. Normal person as you work harder, more and more come form sugar stores - as it can get you energy faster.

All this is separate from calories you are taking in. You could eat sugar (sports drink), fat (an avocado), or protein (bacon) while you exercise. Your body will turn all of these into "sugar" which will then fuel your muscles through sugar. Fat is the longer term storage mechanism in the body. So while you could fuel a race by eating avocados or bacon, most people would have stomach issues.. as your stomach starts to shut down the harder you work (blood is in muscles not in gut, etc.). So this why when we talk about fueling in a race fat vs sugar, it's not really related to the source we are taking in our calories from.