[brianwawok]

I think you are conflating two things. Losing weight you do outside of races. Eat less, exercise more - you lose weight.

Now during an actual race - do you want to burn sugar or fat? Your body has a TON more fat, even a skinny guy has 50k calories of fat he can burn.. whereas the same guy only has 2k calories of sugar to burn. However burning fat is less efficient, so you are unlikely to set a speed record doing it. An ironman is a 8-16 hour race that burns 5,6,10k calories, and all the successful people I know do it via eating a lot of sugar during the race. In theory you COULD do it off fat, but no one has won a race (that I am aware) doing that, so it seems to not work as well in practice.

[zimpenfish]

You can, perhaps, train yourself to burn fat more efficiently which helps.

e.g. http://home.trainingpeaks.com/blog/article/using-low-carbohy...

[IgorPartola]

Cool. Though your linked article still states that peak fat burn output is still lower than peak carb burn output and that high intensity still is best served by carb rich diet.

[DrScump]

"high intensity" is sprinting, not distance running.

[brianwawok]

Well you are burning quite a few more carbs than fat even at a fast 1/2 or full marathon. Not sure exact switchover from more carbs to more fat is, maybe 5 or 6 hours?

[DrScump]

"Well you are burning quite a few more carbs than fat even at a fast 1/2 or full marathon"

No, you aren't. Your liver can hold only so much glycogen, and you'll use that up well before the 10-mile mark.

[zimpenfish]

http://www.ncbi.nlm.nih.gov/books/NBK22417/ suggests that an elite athlete (they're talking about 2 hour marathons) could fuel about 1:20 of that with glycogen (~18 miles) but that they burn about equal glycogen/fat (which would give 13 miles of glycogen.)

I suspect your 10 mile mark is probably correct for anyone below "almost-elite".

[IgorPartola]

OK that makes sense. So then what's the point of doing the fasting + racing thing? Seems like it'll just be very slow and difficult for you to do it.

[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.